Climate change is altering northern coastal aquatic habitats, especially in fjords. Data on current ecosystem structure and biodiversity in many northern fjord and coastal ecosystems, especially for Greenland, are lacking. We used acoustic telemetry combined with stable isotope analyses in a southwest Greenland fjord to investigate marine migrations, marine, and freshwater thermal habitat use, and the marine feeding habits of 80 acoustically tagged Arctic charr over one year. During summer, most Arctic charr occupied the inner fjord. Models of Arctic charr thermal habitat use suggested higher experienced water temperatures in the inner compared to outer fjord (estimated 1.59^◦ C difference) during tagged charr mean 70-day (SD = 14 days) residencies. During February and March, non-migratory individuals used warmer waters (+0.56^◦ C higher) than fish that ultimately migrated to sea, suggesting that over-wintering habitat use patterns influenced migration tactics. Stable isotope mixing model analysis indicated that Arctic charr fed mainly on capelin, marine gammarids, and three-spine sticklebacks. The results provide a contemporary baseline for assessing predictions of potential changes in the ecology of Arctic charr in SW Greenland fjords.

1. Billions of animals undertake migratory journeys every year, with powerful consequences for ecosystem dynamics. Key behaviours that enable successful migration are often guided by the visual system. The amount and quality of information that animals can extract from visual scenes are directly related to structural eye size—larger eyes can house larger pupils, enhancing light-gathering capacity and vision by improving visual acuity and contrast sensitivity.
2. Migration should exert strong demands on individual visual performance, for example via foraging, antipredator benefits or navigational requirements. Yet, it remains elusive whether variations in eye morphology and corresponding visual capabilities are associated with migratory propensity.
3. Here, we capitalize upon intra-population variation in migratory propensity (also known as partial migration) in roach, a common freshwater fish, to directly test for migration-associated variation in image-forming eyes within a species.
4. In a multi-year field study tracking the migration decisions of over 2000 individuals in two different lake systems, we found that relative pupil size was positively associated with individual migration propensity. Computational simulations of the visual ecology associated with the observed differences in pupil size show that migrants have an extended visual detection range and that the performance gain is most pronounced for viewing small targets (e.g. planktonic prey) under low-light conditions.
5. These results suggest that the larger pupils of migrants represent an adaptation for increased foraging efficiency to aid in the accumulation of critical pre-migratory energy reserves. Together, our anatomical and functional findings provide new perspectives on visual system design in relation to individual-level migratory decision-making.

Predator and prey communities are important putative drivers of phenotypic variation in consumers. However, in natural food webs, we often lack an understanding of the ecological interactions by which the species community affects consumer traits. In a comparative study, we explore how phenotypic variation of 34 Greenlandic threespine stickleback populations is affected by the presence and absence of an intraguild predator (Arctic char). In the presence of char, we find that stickleback have a larger body size but a similar head size, resulting in allometric changes (i.e., change in relative head size) that are consistent with resource mediation of ontogenetic growth trajectories observed in other fishes. Using path analysis, we show that the observed shift in the relative head size of stickleback can be partly explained by predator-mediated effects on the species composition and biomass of zooplankton. Our study suggests that top predators can indirectly affect resource limitation of consumers via changes in the prey communities of consumers, and thereby influence the allometry of consumer traits, likely via allocation tradeoffs. These results have important implications for interpreting the putative causes of allometric variation among populations and for using comparative studies to discern the ecological causes of phenotypic variation in natural populations.

The primary production of fjords across the Arctic and Subarctic is undergoing significant transformations due to the climatically driven retreat of glaciers and ice sheets. However, the implications of these changes for upper trophic levels remain largely unknown. In this study, we employ both bulk and compound-specific stable isotope analyses to investigate how shifts at the base of fjord food webs impact the carbon and energy sources of consumers. Focusing on two rapidly changing fjords in Southern Greenland, we used the migratory Arctic char as an indicator species, sampling populations along environmental gradients within the fjords, building upon the assumption that char populations feed primarily close to their natal stream, thereby integrating a dietary gradient. Our analysis of bulk stable isotopes in Arctic char tissue confirmed this premise, revealing a consistent change in resource use from the outer to the inner fjord, which nonetheless served as preferred feeding grounds. Essential amino acid analysis further indicated shifts in carbon and nitrogen sources, consistent with changes in nutrient use near glacier inputs characterized by low turbidity and high iron levels. Notably, these changes in the source of primary production were associated with shifts in trophic positions and the transfer of polyunsaturated fatty acids, with Arctic char in glacier-influenced inner fjords feeding at lower trophic level (size-corrected) and accumulating higher levels of high-quality docosahexaenoic acid (DHA). These findings highlight the usefulness of new analytical tools in revealing that glacial retreat can substantially alter food web dynamics, enhancing both carbon flow and the nutritional quality of fish in fjord ecosystems. The two Southern Greenland fjords studied could represent the future of other fjords, where retreating glaciers become land-terminating and glacial inputs decrease. Our study underscores the critical role of glacier dynamics in affecting high-level consumers, such as salmonids, with implications for fjords globally.

Rising temperatures are leading to increased prevalence of warm-affinity species in ecosystems, known as thermophilisation. However, factors influencing variation in thermophilisation rates among taxa and ecosystems, particularly freshwater communities with high diversity and high population decline, remain unclear. We analysed compositional change over time in 7123 freshwater and 6201 terrestrial, mostly temperate communities from multiple taxonomic groups. Overall, temperature change was positively linked to thermophilisation in both realms. Extirpated species had lower thermal affinities in terrestrial communities but higher affinities in freshwater communities compared to those persisting over time. Temperature change’s impact on thermophilisation varied with community body size, thermal niche breadth, species richness and baseline temperature; these interactive effects were idiosyncratic in the direction and magnitude of their impacts on thermophilisation, both across realms and taxonomic groups. While our findings emphasise the challenges in predicting the consequences of temperature change across communities, conservation strategies should consider these variable responses when attempting to mitigate climate-induced biodiversity loss.

Background: Seasonal movements of animals often result in the transfer of large amounts of energy and nutrients across ecosystem boundaries, which may have large consequences on local food webs through various pathways. While this is known for both terrestrial- and aquatic organisms, quantitative estimates on its effects on food web structure and identification of key pathways are scarce, due to the difficulty in obtaining replication on ecosystem level with negative control, i.e. comparable systems without migration. Methods: In this study, we estimate the impact of Arctic charr (Salvelinus alpinus) migration on riverine ecosystem structure, by comparing multiple streams with strictly resident populations above natural migration barriers with streams below those barriers harboring partially migratory populations. We compared density estimates and size structure between above and below populations. Diet differences were examined through the analysis of stomach contents, changes in trophic position were examined by using stable isotopes. To infer growth rate of resident individuals, back-growth calculation was performed using otoliths. Results: We find higher densities of small juveniles in partially migratory populations, where juvenile Arctic charr show initially lower growth, likely due to higher intraspecific competition. After reaching a size, where they can start feeding on eggs and smaller juveniles, which are both more frequent in partially migratory populations, growth surpasses that of resident populations. Cannibalism induced by high juvenile densities occurred almost exclusively in populations with migration and represents an altered energy pathway to the food web. The presence of large cannibalistic charr feeding on smaller ones that have a similar trophic level as charr from strictly resident populations (based on stomach content) coupled with steeper δ¹⁵N-size regression slopes illustrate the general increase of food chain length in systems with migration. Conclusions: Our results thus suggest that the consumption of migration-derived resources may result in longer food chains through middle-up rather than bottom-up effects. Furthermore, by occupying the apex of the food chain and feeding on juvenile conspecifics, resident individuals experience reduced competition with their young counterparts, which potentially balances their fitness with migratory individuals.

The loss of biodiversity in freshwater environments is becoming an increasing problem globally. As a result, many tools have been developed and improved to reduce this decline. However, there is still a need for the identification and evaluation of precise restoration measures to improve habitats and preserve sentinel freshwater species, such as brown trout. This paper provides an up-to-date viewpoint about the life history, habitat characteristics, suitability conditions, and metapopulation dynamic modelling of brown trout, aiming to identify and discuss gaps and propose possible improvements based on collating and reinterpreting literature data. Results suggest that habitat suitability curves for environmental and hydraulic variables possess some degree of universality, for spawning habitat, fry, juvenile and adult trout. Further, an improved method to estimate the amount of suitable area by including the role of stream obstacles (i.e., macro-roughness with characteristic size in the order of the mean water depth) is proposed and discussed. This approach can be integrated into advanced metapopulation models and will allow experts to evaluate the best measures towards restoring and preserving freshwater riverine environments.

In a warming world, the input of glacier meltwater to inland water ecosystems is predicted to change, potentially affecting their productivity. Meta-ecosystem theory, which posits that the nutrient availability in the recipient ecosystem can determine the extent of cross-ecosystem boundary utilization, can be useful for studying landscape-scale influences of glacier meltwater on inland waters. Here, we investigate how the input of glacier meltwater in a river system in Southern Greenland influences the utilization of marine subsidies in freshwater fish. Our study system comprised four sites, with controls for glacial meltwater and marine subsidies, harboring a partially migrating population of arctic char, meaning that some individuals migrate to the ocean and others remain in freshwaters, and two fully resident populations as a freshwater reference. We assessed the incorporation of marine carbon in freshwater resident char using both bulk and amino acid stable isotope analysis of muscle tissue. In the population with partial migration, marine subsidies were a significant resource for resident char individuals, and estimates of trophic position suggest that egg cannibalism is an important mechanism underlying the assimilation of these marine subsidies. In proglacial streams, namely those with high glacial meltwater, the total dependence on marine subsidies increased and reached 83% because char become cannibals at smaller sizes. In the configuration of our focal meta-ecosystem, our results suggest that the importance of marine subsidies to freshwater fish strengthens within increasing meltwater flux from upstream glaciers.

The increase in the construction of mega dams in tropical basins is considered a threat to freshwater fish diversity. Although difficult to detect in conventional monitoring programs, rheophilic species and those reliant on shallow habitats comprise a large proportion of fish diversity in tropical basins and are among the most sensitive species to hydropower impacts. We used Baited Remote Underwater Video (BRUV), an innovative, non-invasive sampling technique, to record the impacts caused by Belo Monte, the third largest hydropower project in the world, on fishes inhabiting fast waters in the Xingu River. BRUV were set in a river stretch of ~ 240 km for 7 years, 2 before and 5 after the Belo Monte operation. We explored the spatial and temporal variation in fish diversity (α, β, and γ) and abundance (MaxN) using generalized additive models. We also investigated the variation of environmental variables and tested how much information we gained by including them in the diversity and abundance models. Belo Monte altered the flow regime, water characteristics, and fishery yield in the Xingu, resulting in changes in the fish community structure. Temporally, we observed sharp declines in α diversity and abundance, far exceeding those from a previous study conducted with more conventional sampling methods (i.e., catch-based) in the region. γ-diversity was also significantly reduced, but we observed a non-expected increase in β diversity over time. The latter may be associated with a reduction in river connectivity and an increase in environmental heterogeneity among river sectors. Unexpected signs of recovery in diversity metrics were observed in the last years of monitoring, which may be associated with the maintenance of flow levels higher than those previously planned. These results showed that BRUV can be a useful and sensitive tool to monitor the impacts of dams and other enterprises on fish fauna from clear-water rivers. Moreover, this study enhances our comprehension of the temporal variations in freshwater fish diversity metrics and discusses the prevalent assumption that a linear continuum in fish-structure damage associated with dam impoundments may exhibit temporal non-linearity.

Warum, wohin, wie weit und wann wandern die Fische in unseren Flüssen? Wie wirkt sich der Klimawandel mit extremen Wetterlagen, steigenden Wassertemperaturen, veränderten Abflüssen und Niedrig- oder Hochwasser auf Bewegungsmuster aus? Inwieweit beeinträchtigen künstliche Barrieren die Fischbewegungen und zudem die genetische Struktur der Arten? Diesen Fragen geht ein Forscherteam der Eawag und ETH nach, indem es Bewegungen diverser Fischarten über mehrere Jahre im Rhein-Aare-Flussnetz verfolgt.

It remains unknown how species' populations across their geographic range are constrained by multiple coincident natural and anthropogenic environmental gradients. Conservation actions are likely undermined without this knowledge because the relative importance of the multiple anthropogenic threats is not set within the context of the natural determinants of species' distributions. We introduce the concept of a species 'shadow distribution' to address this knowledge gap, using explainable artificial intelligence to deconstruct the environmental building blocks of current species distributions. We assess shadow distributions for multiple threatened freshwater fishes in Switzerland which indicated how and where species respond negatively to threats — with negative threat impacts covering 88% of locations inside species' environmental niches leading to a 25% reduction in environmental suitability. Our findings highlight that conservation of species' geographic distributions is likely insufficient when biodiversity mapping is based on species distribution models, or threat mapping, without also quantifying species' expected or shadow distributions. Overall, we show how priority actions for nature's recovery can be identified and contextualised within the multiple natural constraints on biodiversity to better meet national and international biodiversity targets.

River habitats are fragmented by barriers which impede the movement and dispersal of aquatic organisms. Restoring habitat connectivity is a primary objective of nature conservation plans with multiple efforts to strategically restore connectivity at local, regional, and global scales. However, current approaches to prioritize connectivity restoration do not typically consider how barriers spatially fragment species' populations. Additionally, we lack knowledge on biodiversity baselines to predict which species would find suitable habitat after restoring connectivity. In this paper, we asked how neglecting these biodiversity baselines in river barrier removals impacts priority setting for conservation planning. We applied a novel modelling approach combining predictions of species distributions with network connectivity models to prioritize conservation actions in rivers of the Rhine-Aare system in Switzerland. Our results show that the high number and density of barriers has reduced structural and functional connectivity across representative catchments within the system. We show that fragmentation decreases habitat suitability for species and that using expected distributions as biodiversity baselines significantly affects priority settings for connectivity restorations compared to species-agnostic metrics based on river length. This indicates that priorities for barrier removals are ranked higher within the expected distributions of species to maximize functional connectivity while barriers in unsuitable regions are given lower importance scores. Our work highlights that the joint consideration of existing barriers and species past and current distributions are critical for restoration plans to ensure the recovery and persistence of riverine fish diversity.

Metabarcoding of environmental DNA (eDNA) is a powerful tool for describing biodiversity, such as finding keystone species or detecting invasive species in environmental samples. Continuous improvements in the method and the advances in sequencing platforms over the last decade have meant this approach is now widely used in biodiversity sciences and biomonitoring. For its general use, the method hinges on a correct identification of taxa. However, past studies have shown how this crucially depends on important decisions during sampling, sample processing, and subsequent handling of sequencing data. With no clear consensus as to the best practice, particularly the latter has led to varied bioinformatic approaches and recommendations for data preparation and taxonomic identification. In this study, using a large freshwater fish eDNA sequence dataset, we compared the frequently used zero-radius Operational Taxonomic Unit (zOTU) approach of our raw reads and assigned it taxonomically (i) in combination with publicly available reference sequences (open databases) or (ii) with an OSU (Operational Sequence Units) database approach, using a curated database of reference sequences generated from specimen barcoding (closed database). We show both approaches gave comparable results for common species. However, the commonalities between the approaches decreased with read abundance and were thus less reliable and not comparable for rare species. The success of the zOTU approach depended on the suitability, rather than the size, of a reference database. Contrastingly, the OSU approach used reliable DNA sequences and thus often enabled species-level identifications, yet this resolution decreased with the recent phylogenetic age of the species. We show the need to include target group coverage, outgroups and full taxonomic annotation in reference databases to avoid misleading annotations that can occur when using short amplicon sizes as commonly used in eDNA metabarcoding studies. Finally, we make general suggestions to improve the construction and use of reference databases for metabarcoding studies in the future.

Die einzigartige Fischvielfalt in Fliessgewässer-Ökosystemen in und um die Schweiz
Aufgrund der geografischen Lage in vier der wichtigsten Wassereinzugsgebieten Europas und der grossen Vielfalt an Lebensräumen weisen die aquatischen Ökosysteme der Schweiz eine aussergewöhnlich hohe Fischvielfalt auf. Diese Vielfalt ist hauptsächlich durch zwei verschiedene Prozesse entstanden, einerseits durch die Zusammenführung bereits vorhandener Vielfalt, und andererseits durch postglaziale Evolutionsprozesse. Der letztgenannte Prozess hat zu einem hohen Grad an Endemismus vor allem in den grösseren Seen des Alpenraums geführt. Diese einzigartige Vielfalt ist auf vielen verschiedenen Ebenen zu finden. Neben der Artenvielfalt beherbergen die verschiedenen Flüsse und Bäche auch eine Vielfalt an diversen Fischgemeinschaften, wobei sowohl die Unterschiede in der Artenzahl als auch in der Artenzusammensetzung zu unterschiedlichen Nahrungsnetzen und Ökosystemprozessen zwischen den Flüssen führen. Viele Fischarten weisen auch ein hohes Mass an intraspezifischer Vielfalt auf, z. B. in Bezug auf Morphologie, Physiologie, Ökologie, Lebensgeschichte und Genom. Diese intraspezifische Vielfalt ist insofern von Bedeutung, dass verschiedene Populationen möglicherweise an unterschiedliche Lebensräume angepasst sind und eine hohe genetische Vielfalt aufweisen. Letzteres ermöglicht es den einzelnen Individuen innerhalb einer Population, verschiedene Nischen zu besetzen, und es ermöglicht den Populationen, sich besser an veränderte Umweltbedingungen anzupassen. [...]

The unique fish diversity in riverine ecosystems in and around Switzerland
As a consequence of its geographic location within four of Europe’s major watersheds and a large amount of habitat diversity, aquatic ecosystems in Switzerland harbor an exceptionally high amount of fish diversity. This diversity has mainly been formed by two different processes, i.e. assembly of previously existing diversity and postglacial evolutionary processes. The latter of these processes has led to a high degree of endemism in especially the larger lakes in the Alpine region. The unique diversity can be found on many different levels. Besides the diversity of species, the different rivers and streams also harbor a diversity of fish communities, where both the variation in the number of species and the composition of species result in different food webs and ecosystem processes between rivers. Many of the fish species also show a high degree of intraspecific diversity in e.g. morphology, physiology, ecology, life history and in their genome. This intraspecific diversity is important in terms of different populations potentially being adapted to different habitats and in terms of populations harboring a high genetic diversity. The latter makes the individual fish within a population able to occupy different niches and makes the populations better able to adapt to changing environmental conditions. [...]

Die Biodiversität in Gewässern ist akut und stark bedroht. Um sie effektiv zu schützen, muss sie bekannt sein. Dazu sind flächendeckende, standardisierte Untersuchungen der Diversität notwendig. Ebenso sind vertiefte taxonomische Kenntnisse dringend erforderlich, um diese Diversität zu erkennen. Insbesondere Gewässerlebensräume für gefährdete Arten sollten besser geschützt und wiederhergestellt werden.

In partially migratory species, individuals either migrate at some point(s) in life or reside within their natal habitat throughout life. For salmonid fish, migration creates opportunities for feeding and growth, but it is also associated with increased mortality risk. Such trade-offs likely differ between the sexes, since reproductive output is more closely tied to body size in females than males. However, testing hypotheses on sex-specific migratory behaviour in would-be first-time migratory salmonids is difficult, since sexes are generally morphologically indistinguishable prior to maturation. Previous studies have evaluated the influence of sex on migration based on dissection of migratory juveniles or the sex ratio of returning adults. However, both approaches are potentially biased by differential survival during migration. Here, we utilise advances in minimally invasive genetic sex-determination methods for salmonids to investigate sex-specific, spring out-migration propensity in potamodromous brown trout (Salmo trutta) in a pre-Alpine, central European lake. We show that there are marked differences in migratory behaviour between males and females, with small (~10 cm) females being approximately twice as likely to migrate out of their natal river in spring compared to similarly sized males, which generally migrate for the first time at larger sizes (in similar proportions to larger females). This study highlights how novel genetic sex-determination techniques can provide insight into the sex- and size-specific life-history trade-offs that shape migration propensity. Moving forward, these techniques should become useful tools for ecologists and fisheries managers.

Piscivorous fish are important predators in aquatic systems and as such they can have far-reaching effects on ecosystem composition and function. These effects depend on piscivore predation rates and behaviour, and recruitment of young-of-the-year fish into piscivory can hereby govern ecosystem properties. Growth and recruitment can differ between water bodies due to e.g. general productivity, but information on variation in juvenile growth and body condition between habitats within water bodies is scant. We here evaluate growth, body condition, food occurrence and stomach contents of an important piscivore, pike (Esox lucius), over the first growth season in two contrasting and spatially separated homogenous habitat types (emergent and submerged vegetation separated by 50 m of open sand) within the same lake. Individual size and body condition in pike were higher in the submerged vegetation early in the season, whereas by the end of their first summer pike were larger and in higher body condition in the emergent vegetation, in spite of occurrence of zooplankton, macroinvertebrates and fish prey being overall higher in the submerged vegetation. Pike showed habitat-specific patterns of macroinvertebrate consumption (higher in the submerged vegetation) and date-specific patterns of zooplankton (higher early in the season), macroinvertebrate (lower late in the season) and fish (higher later in the season) consumption that were not a result of occurrence of food types, as occurrence and consumption patterns did not match. We conclude that pike that hatched in the emergent vegetation habitat were larger towards the end of the season and, hence, these pike should have a higher survival probability and possibly contribute more to pike population density and predation at older ages, but also that submerged vegetation provides an alternative and added recruitment environment for pike in shallow lake ecosystems.

1. Lakes are recipients of allochthonous organic matter and nutrients. However, the importance of these subsidies for food webs and how they vary with lake trophic status remains unclear, especially for large lakes.
2. We assessed the source and fate of organic matter and nutrients in seven perialpine lakes across a gradient of trophic status. We measured carbon and nitrogen stable isotopes of amino acids of lake-residing Atlantic trout, Salmo trutta, to determine the source of primary production (i.e., how carbon is fixed in the ecosystem) and how it is transferred through food webs, respectively. Based on essential amino acid carbon fingerprinting, we estimated the probability of organic carbon originating from autochthonous (algal), allochthonous (terrestrial plant), and recycled (bacterial) sources. In addition, we used amino acid δ¹⁵N to track how this primary production is transferred to consumers in general, and by using different trophic amino acids (glutamic acid and alanine), identify the trophic pathways involving either metazoan or protozoans.
3. We found a high likelihood of autochthonous origin of organic carbon (86 ± 9%) in trout that contrasted with allochthonous origins of particulate organic matter and some sediments. We showed that those estimates are good proxies of source reliance. Our results also highlighted the importance of bacterial origin of organic carbon in fish (12%). The likely autochthonous origin of this carbon was supported by trophic markers (Ala δ¹⁵N) that suggest the role of protists in transferring recycled organic carbon up the food web. While the sources of nitrogen sustaining food webs varied among lakes, we found a conserved carbon fingerprinting of fish. Overall, this suggests an uncoupling between the source of nutrients and organic carbon in large perialpine lakes.
4. Across a wide range of trophic status (c. 2 orders of magnitude range of phosphorus concentration), several lines of evidence suggested that perialpine lake food webs shared a common reliance on autochthonous and bacterial production.
5. Our study is the first to quantify the dependence on allochthonous organic carbon in lake food webs based on new amino acid stable isotope markers (carbon fingerprinting and Ala δ¹⁵N) and shows promise for estimating the source of carbon fixation in ecosystems. Our results support previous suggestions that terrestrial organic carbon is a relatively minor source for aquatic consumers despite contributing to the pool of organic matter, and more importantly, its contribution does not vary substantially with trophic status in perialpine lakes.

Arctic charr (Salvelinus alpinus [L.] complex) has been widely used as a model system for studies in evolutionary ecology because of its diversity in feeding ecology, habitat use, life history forms, and associated morphologies observed in matured individuals. However, we still know relatively little about traits exhibited early in life of the species, although the trait diversity of matured individuals may largely be shaped during development. Egg size is a key determinant of various traits exhibited early in life. Therefore, describing egg size variation within- and between-individuals as well as the link between egg size and adult traits will be a useful step in understanding the early life trait diversity of Arctic charr. Here, using Greenlandic Arctic charr, which includes alternative life history forms (i.e. anadromous and resident) and spawning habitat use (i.e. lake and river spawner), we described egg size variation (i.e. clutch-mean egg diameter and within-clutch variation) and explored the link between egg size variation and female body length, life history form, and spawning habitats. As in many other fishes, clutch-mean egg diameter increased with female body length. No significant effect of other female traits on clutch mean-egg diameter was detected, suggesting that female body size variation could be a direct cause of early life history trait variation. On the other hand, we found that the degree of within-clutch variation of the anadromous life history form was higher than that of the resident life history form. The pattern could be interpreted in an adaptive context. For instance, given that the anadromous life history form tends to be semelparous, anadromous females could decrease the likelihood of complete reproductive failure by producing variable-sized offspring within a clutch since at least some offspring are expected to be matched to the prevailing environment.

1. Climate warming imposes a severe threat to freshwater ecosystems, which are dominated by ectotherms such as fish and aquatic insects. To better predict the effects of climate warming on thermally sensitive ecosystems, information on how temperature affects individual traits within populations is fundamental.
2. Patterns of intraspecific variation in thermal reaction norms along geographic thermal gradients provide valuable information. Immediate temperature effects on individual traits can be inferred from the shape of the thermal reaction norm. The way that temperature and associated environmental conditions (to which populations have been exposed over generations) affect individual traits through transgenerational plasticity in, and/or natural selection on, these traits can also be inferred from patterns of trait variation along a geographic thermal gradient.
3. Many studies have documented patterns of intraspecific variation in thermal reaction norms along geographic thermal gradients. However, most previous studies cannot exclude the possibility that the observed geographic patterns are solely explained by random processes, such as isolation by distance, due to the lack of replication of geographic gradients. Here, we show consistent patterns in intraspecific trait variation along geographic thermal gradients using Salmo trutta (brown trout), which is an ecologically and economically important fish in alpine streams.
4. We kept trout embryos collected from 52 families from 14 populations along wide and replicated elevational gradients from three Alpine drainages (Danube, Po, and Rhine) in two temperature treatments (3.2 and 6.2°C). Timing and body size at emergence from the nest, which are key early life-history traits of trout affecting their early growth and survival, were measured.
5. Besides faster embryonic development at warmer temperatures, we found that offspring from low-elevation parents took very slightly longer to reach emergence from fertilisation and were larger than offspring from high-elevation parents. Importantly, this was evident for all three drainages. Further analyses found that the longer duration until- and larger body length at- emergence of low-elevation trout were mainly due to the larger eggs laid by low-elevation females, which had larger body size than high-elevation females.
6. Trout female body size, which is positively correlated with egg size, is susceptible to temperatures and associated environmental conditions. Consequently, climate warming may not only immediately accelerate development rate but also shift timing and size at emergence through egg size-mediated maternal effects within a relatively short time scale.

While aquatic (blue) and terrestrial (green) food webs are parts of the same landscape, it remains unclear whether they respond similarly to shared environmental gradients. We use empirical community data from hundreds of sites across Switzerland and a synthesis of interaction information in the form of a metaweb to show that inferred blue and green food webs have different structural and ecological properties along elevation and among various land-use types. Specifically, in green food webs, their modular structure increases with elevation and the overlap of consumers’ diet niche decreases, while the opposite pattern is observed in blue food webs. Such differences between blue and green food webs are particularly pronounced in farmland-dominated habitats, indicating that anthropogenic habitat modification modulates the climatic effects on food webs but differently in blue versus green systems. These findings indicate general structural differences between blue and green food webs and suggest their potential divergent future alterations through land-use or climatic changes.

1. Animal migration is one of the most spectacular and visible behavioural phenomena in nature with profound implications for a range of ecological and evolutionary processes. Successful migration hinges on the ability to exploit temporary resources (e.g. food) and evade threats (e.g. predators) as they arise, and thus the timing of migration is often regarded as a dominant predictor of individual migratory success.
2. However, with the exception of intensively studied taxa (mainly birds), relatively few studies have investigated inter-individual annual and seasonal variation in migratory timing and performance, or tested predictions on how migration across high and low predation-risk habitats may exert selection on migratory timing. In particular, studies that assess the survival consequences of variation in migratory timing remain rare, which is most likely due to the logistical challenges associated with monitoring survival success and population-level characteristics simultaneously.
3. Here, we address the above-mentioned questions using roach Rutilus rutilus, a fish that migrates from lakes characterised by high predation risk into low-risk streams during winter. Specifically, we used individual-based tracking of roach in two European lake systems over multiple migration periods (9 and 7 years respectively), to obtain highly detailed (year-round scheduling, repeat journeys and the fate of individuals) data on the variability/synchrony of migratory timing in spring and autumn respectively.
4. We report seasonal differences in the variability of migratory timing, with lower variance and higher migration synchrony in spring lake arrival timing as compared to autumn lake departure timing. Furthermore, the timing of autumn migration is more variable across years than the timing of spring migration. Second, we find that later arrival to the lake habitat is positively associated with apparent survival from 1 year to the next, whereas we found no effect of lake departure timing on survival probability.
5. These findings represent rare evidence showing how intraspecific variation in timing in migratory fish differs across years and seasons, and how variation in timing can translate into survival consequences for prey in systems characterised by high predation risk.

Quaternary climate fluctuations can affect speciation in regional biodiversity assembly in two non-mutually exclusive ways: a glacial species pump, where isolation in glacial refugia accelerates allopatric speciation, and adaptive radiation in underused adaptive zones during ice-free periods. We detected biogeographic and genetic signatures associated with both mechanisms in the assembly of the biota of the European Alps. Age distributions of endemic and widespread species within aquatic and terrestrial taxa (amphipods, fishes, amphibians, butterflies and flowering plants) revealed that endemic fish evolved only in lakes, are highly sympatric, and mainly of Holocene age, consistent with adaptive radiation. Endemic amphipods are ancient, suggesting preglacial radiation with limited range expansion and local Pleistocene survival, perhaps facilitated by a groundwater-dwelling lifestyle. Terrestrial endemics are mostly of Pleistocene age and are thus more consistent with the glacial species pump. The lack of evidence for Holocene adaptive radiation in the terrestrial biome is consistent with faster recolonization through range expansion of these taxa after glacial retreats. More stable and less seasonal ecological conditions in lakes during the Holocene may also have contributed to Holocene speciation in lakes. The high proportion of young, endemic species makes the Alpine biota vulnerable to climate change, but the mechanisms and consequences of species loss will likely differ between biomes because of their distinct evolutionary histories.

Trait expression of natural populations often jointly depends on prevailing abiotic environmental conditions and predation risk. Copepods, for example, can vary their expression of compounds that confer protection against ultraviolet radiation (UVR), such as astaxanthin and mycosporine-like amino acids (MAAs), in relation to predation risk. Despite ample evidence that copepods accumulate less astaxanthin in the presence of predators, little is known about how the community composition of planktivorous fish can affect the overall expression of photoprotective compounds. Here, we investigate how the (co-)occurrence of Arctic charr (Salvelinus alpinus) and threespine stickleback (Gasterosteus aculeatus) affects the photoprotective phenotype of the copepod Leptodiaptomus minutus in lake ecosystems in southern Greenland. We found that average astaxanthin and MAA contents were lowest in lakes with stickleback, but we found no evidence that these photoprotective compounds were affected by the presence of charr. Furthermore, variance in astaxanthin among individual copepods was greatest in the presence of stickleback and the astaxanthin content of copepods was negatively correlated with increasing stickleback density. Overall, we show that the presence and density of stickleback jointly affect the content of photoprotective compounds by copepods, illustrating how the community composition of predators in an ecosystem can determine the expression of prey traits that are also influenced by abiotic stressors.

In der Schweiz gibt es verschiedene Fisch-Monitoring-Programme. Nun wurden sie erstmals gemeinsam ausgewertet, mit dem Ziel, statistische Zusammenhänge zwischen vom Menschen beeinflussten Umweltfaktoren (Temperatur, landwirtschaftliche Landnutzung, Hydromorphologie, Wanderhindernisse) und dem Vorkommen von Fischen bzw. deren Dichte zu identifizieren. Die Analyse weist darauf hin, dass alle häufigeren Fischgattungen durch diese Umweltfaktoren in ihrem Vorkommen beeinflusst werden, und zwar auf unterschiedliche Weise.

 

Climate warming imposes a serious threat, especially to freshwater ecosystems in temperate and (sub)polar regions, which are often dominated by cold-adapted ectotherms. Although relatively intense warming during winter is common across the climatic regions, comparably little focus has been put on the organismal impacts of winter warming. Embryonic development, which is exceptionally susceptible to ambient temperature, occurs during winter in various freshwater ectotherms. Yet, our knowledge of the effects of increased temperature during embryogenesis on later life stages is limited. Using brown trout (Salmo trutta), we examined how a 1.5°C temperature increase from fertilization to hatching affects various traits at the onset of the free-swimming stage (i.e. a comparison between 3.5 and 5.0°C treatments). Although all hatchlings were kept at the same temperature (7.0°C) from hatching to the onset of the free-swimming stage for about two months, the temperature increase during embryogenesis substantially reduced key ecological behaviours, i.e. activity and exploration levels, at the onset of the free-swimming stage despite only marginal temperature effects on morphological and physiological traits at this stage. Given the importance of behavioural traits in early growth and survival, our study suggests a likely pathway through which subtle changes in mean winter temperature affect early fitness.

Statistical species distribution models (SDMs) are widely used to quantify how taxa respond to environmental conditions and to predict their distribution. However, the application of SDMs to freshwater fish taxa is complicated by the active dispersal of fish taxa through river networks, and the species- and habitat-dependent observation process (i.e., the sampling method and effort) required to accurately sample their distributions. Many studies have applied presence-absence models (PAMs) to fish taxa, while more recent studies have proposed zero-inflated models (ZIMs) to account for count observations with many zeroes. However, relatively few studies have incorporated the observation process into the model structure, which would facilitate the combination of data from various monitoring programs that differ in their observation process. In this study, we use conceptual models to identify potentially dominant natural and anthropogenic environmental conditions with a direct, mechanistic effect on the distributions of freshwater fish taxa in Switzerland, a region with a large range of environmental conditions, from alpine streams that are mainly affected by hydromorphological alterations to lowland streams in densely populated areas with intensive agricultural land use. Moreover, numerous barriers impede fish migration along the entire river network. Using combined data from two fish monitoring programs in Switzerland, we applied an exhaustive cross-validation procedure to select a set of environmental variables with the highest (out-of-sample) predictive performance for the PAM and ZIM for fish density (individuals/m²) of the seven most prevalent fish taxa (Salmo spp., Cottus spp., Squalius spp., Barbatula spp., Barbus spp., Phoxinus spp., Gobio spp.). We used these variables to develop a PAM and ZIM for each taxon that accounts for differences in sampling methods and sampling effort. We quantified the quality of fit during calibration using all samples and predictive performance during 5-fold cross-validation of each model.
Results show that stream temperature and stream morphology within the accessible habitat commonly appear among the best predictive presence-absence models for multiple taxa. Spatial variables that account for migration barriers and quantify morphological conditions within the accessible habitat were selected for 6 out of 7 taxa. The selected PAMs performed well for all taxa with an intermediate prevalence (10–40%), with an explanatory power () of between 0.32 - 0.37 during calibration using all samples and only minor decreases in explanatory power during cross-validation (= 0.34 – 0.44). As expected, the PAM for the highly prevalent Salmo spp. (91%) failed to predict the few absence data points. By contrast, the ZIM model performed best for Salmo spp., with a standardized likelihood ratio of 1.56. For all other taxa besides Barbus spp. the ZIM models also had likelihood ratios above one, indicating a better predictive performance than the null model. We hope this study stimulates the development and application of fish species distribution models based on prior knowledge of causally linked environmental variables and incorporating observation errors to improve their predictive performance. This can facilitate learning from biomonitoring data to support management.

Parasite infracommunities tend to be stochastic in nature, although environmental characteristics such as the type of water source in streams and host traits can have an effect on the biotic assemblages and by extension the parasite fauna. We examined the effect of water source and the rate of adult fish migration on the metazoan parasite infracommunities of conspecific juvenile brown trout, Salmo trutta L. among streams flowing into Lake Lucerne (Switzerland). Juvenile (1 to 2-year old) fish harboured higher parasite species richness in groundwater-fed than in surface water-fed streams, whereas the rate of fish migration did not affect infracommunity richness. Heteroxenous species were more common in groundwater-fed streams with high and medium rates of trout migration, whereas infracommunities in surface water-fed streams and streams with low rates of fish migration were dominated by one monoxenous parasite or lacked infections. Similarity in the parasite infracommunity composition of juvenile trout across streams was explained by the interaction between type of water source and adult migration rates. Our conclusions support that similarity in the parasite composition of resident freshwater conspecifics can be predicted by the local environmental settings and host migratory behaviour, whereas parasite richness is mainly influenced by the environmental characteristics.

While PIT-tag tracking using mobile antennas is being increasingly used to study fish movement and survival in streams, little is known about the limitations of the method, especially over longer periods of time and under different environmental settings. We used 6 years of data combining tagging, mobile antenna tracking and recaptures of Salmo trutta in multiple small streams in the Lake Lucerne drainage area in Switzerland to evaluate the relative importance of different environmental and intrinsic factors affecting the efficiency of the method. Our study system and experimental design allowed us to accurately verify the continuous presence and survival of recaptured fish in the stream after tracking, which meant that we could estimate detection probability with high confidence. The mean detection probability of tagged trout was 43%, but we found that fish length had a strong negative effect on detection probability, especially in males. Multivariate axes of stream environmental features did not predict efficiency but stream width alone was significantly positively correlated with efficiency. Additionally, stream temperature when tracking had a positive effect on fish detectability. Tag loss at recapture was globally rare (<8%) but common in large postspawn females (>30%). Based on the escape response of fish after detection, we could estimate the proportion of ghost tags, which reached a plateau of around 80% 2 years after tagging. We finally showed that our models of tag loss, fish detection and escape response are needed to interpret detection events. Our results highlight that individual variation in detection probability and tag loss is high and has to be considered for analysis.

Patterns of animal migration and the ecological forces that shape them have been studied for centuries. Yet ecological impacts caused by the migration, such as altered predator–prey interactions and effects on community structure, remain poorly understood. This is to a large extent due to the scarcity of naturally replicated migration systems with negative controls, that is, ecosystems without migration. In this study, we tested whether partial migration of certain species within the overall prey community affects foraging ecology of top predators and thereby alters energy pathways in food webs. We carried out the study in independent replicated freshwater lake systems, four with and four without opportunity for prey migration. Specifically, we compared predator foraging mode in lakes where cyprinid prey fish perform seasonal partial migrations into connected streams with lakes lacking migratory opportunities for prey fish. We found clear seasonal bottom-up effects of prey migration on predators, including changes in size structure and total biomass of ingested prey, size-specific changes in littoral versus pelagic origin of diet, and a higher degree of feast-and-famine for predators in systems with migratory prey. Our analyses further showed that partially migratory prey species constitute a larger part of the prey community in systems that allow migration. Hence, prey migrations have important implications for predator foraging ecology and may cause seasonal shifts in the importance of their supporting energy pathways. We suggest that such bottom-up effects of partial migration may be a widespread phenomenon both in aquatic and in terrestrial ecosystems.

1. Different migratory species have evolved distinct migratory characteristics that improve fitness in their particular ecological niches. However, when such species hybridize, migratory traits from parental species can combine maladaptively and cause hybrids to fall between parental fitness peaks, with potential consequences for hybrid viability and species integrity.
2. Here, we take advantage of a natural cross‐breeding incident to study migratory behaviour in naturally occurring hybrids as well as in their parental species and explore links between migratory traits and predation risk.
3. To achieve this, we used electronic tags and passive telemetry to record detailed individual migration patterns (timing and number of migratory trips) in two common freshwater fish species, roach Rutilus rutilus, common bream Abramis brama as well as their hybrids. Next, we scanned for tags regurgitated by a key avian predator (great cormorant Phalacrocorax carbo) at nearby roosting sites, allowing us to directly link migratory behaviour to predation risk in the wild.
4. We found that hybrid individuals showed a higher number of short, multi‐trip movements between lake and stream habitats as compared to both parental species. The mean date of first lake departure differed between bream and roach by more than 10 days, while hybrids departed in two distinct peaks that overlapped with the parental species' averages. Moreover, the probability of cormorant predation increased with multi‐trip movement frequency across species and was higher for hybrids.
5. Our data provide novel insights into hybrid viability, with links to predator‐mediated ecological selection. Increased exposure to predators via maladaptive migratory behaviour reduces hybrid survival and can thereby reinforce species integrity.

Passive integrated transponder (PIT)‐tagging is commonly used in behavioural studies of fish, although long‐term evaluations of effects from tagging under natural conditions are scarce. We PIT‐tagged common bream Abramis brama, European perch Perca fluviatilis, pike Esox lucius and roach Rutilus rutilus, released them in their lakes of origin and recaptured them after 103-3269 days. Overall, tagged fish did not differ in condition from non‐tagged fish, except for small R. rutilus that had a lower length‐specific body mass in one lake in 1 year. We conclude that PIT‐tagging in general has negligible long‐term effects on fish condition.

The recent focus on a common movement ecology paradigm has increased awareness about the importance of distinguishing different movement patterns for understanding their specific impact on animal ecology and evolution. One specific form of movement is nomadism, where animals move between habitats in an arrhythmic fashion. Although not yet described in detail for freshwater fish, nomadism between lakes within drainage systems could affect local lake food webs in an otherwise unpredictable fashion. In this study, we used passive telemetry to describe inter‐lake movements of 1,280 individually tagged adult benthivorous bream (Abramis brama) over nine contiguous years in a drainage system with two paired lakes. The movements can best be described as partial nomadism, where a part of the population is neither resident nor migratory, but instead moves between lakes in a variable, arrhythmic fashion, often with long residency in one lake before moving to the other. We found extensive between‐year variation in the proportion and direction of bream moving, with higher movement propensity being partly associated with lower somatic condition. Notably, movements out of lakes were significantly more in direction of the neighbouring lake than away from the paired lakes, suggesting that at least direction was not random. Since bream are important ecosystem engineers, fluctuating densities of adult bream could affect ecological stability in lakes. Our study suggests that fish populations in interconnected lakes within drainages should not be treated as isolated units, and we discuss this in relation to lake ecology and the importance for lake ecosystem management.

We tested for phenotype‐to‐habitat associations in brown trout Salmo trutta populations from two ecologically different habitat types; i.e., groundwater and surface‐water‐fed streams. Additionally, we raised captive offspring from two such populations under standardised conditions to test whether potential phenotypic differentiation would be passed on to offspring. We found analogous differentiation by habitat in multiple wild populations. Some of these morphological differences were at least partially inherited by offspring. We suggest that this could have implications for both scientists and fisheries authorities studying or managing trout populations.

Background: Seasonal spatio-temporal variation in habitat quality and abiotic conditions leads to animals migrating between different environments around the world. Whereas mean population timing of migration is often fairly well understood, explanations for variation in migratory timing within populations are often lacking. Condition-dependent tradeoffs may be an understudied mechanism that can explain this differential migration. While fixed condition-specific thresholds have been identified in earlier work on ontogenetic niche shifts, they are rare in differential migration, suggesting that thresholds in such systems can shift based on temporally variable environmental conditions.
Methods: We introduced a model based on size-specific tradeoffs between migration and growth in seasonal environments. We focused on optimal migratory timing for first-time migrants with no knowledge of an alternative habitat, which is a crucial stage in the life history of migratory salmonids. We predicted that optimal timing would occur when individuals move from their natal habitats based on a seasonally variable ratio of predation and growth. When the ratio becomes slightly more favorable in the alternative habitat, migratory movement can occur. As it keeps shifting throughout the season, the threshold for migration is variable, allowing smaller individuals to move at later dates. We compared our model predictions to empirical data on 3 years of migratory movement of more than 800 juvenile trout of varying size from natal to feeding habitat.
Results: Both our model and empirical data showed that large individuals, which are assumed to have a lower predation risk in the migratory habitat, move earlier in the season than smaller individuals, whose predicted predation-to-growth ratio shifted to being favorable only later in the migratory season. Our model also predicted that the observed difference in migratory timing between large and small migrants occurred most often at low values of growth differential between the two habitats, suggesting that it was not merely high growth potential but rather the tradeoff between predation and growth that shaped differential migration patterns.
Conclusions: We showed the importance of considering condition-specific tradeoffs for understanding temporal population dynamics in spatially structured landscapes. Rather than assuming a fixed threshold, which appears to be absent based on previous work on salmonids, we showed that the body-size threshold for migration changed temporally throughout the season. This allowed increasingly smaller individuals to migrate when growth conditions peaked in the migratory habitat. Our model illuminates an understudied aspect of predation as part of a condition-dependent tradeoff that shapes migratory patterns, and our empirical data back patterns predicted by this model.

 

Ecological opportunity is considered a crucial factor for adaptive radiation. Here, we combine genetic, morphological and ecological data to assess species and ecomorphological diversity of Artic charr in six lakes of a catchment in southernmost Greenland, where only charr and stickleback occur. Because the diversity of habitats and resources increases with lake size, we predict a positive association between lake size and the extent of ecomorphological diversity. The largest lake of the catchment harbours the largest Arctic charr assemblage known today. It consists of six genetically differentiated species belonging to five ecomorphs (anadromous, littoral benthic, profundal dwarf, planktivorous, piscivorous), of which the latter comprises two ecomorphologically extremely similar species. Lakes of intermediate size contain two ecomorphologically and genetically distinct species. Small lakes harbour one genetically homogeneous, yet sometimes ecomorphologically variable population. Supporting our prediction, lake size is positively correlated with the extent of ecomorphological specialization towards profundal, pelagic and piscivorous lifestyle. Furthermore, assemblage-wide morphospace increases sharply when more than one genetic cluster is present. Our data suggest that ecological opportunity and speciation jointly determine phenotypic expansion in this charr radiation.

The foraging activity of large-bodied benthivorous fish has been suggested to be of key importance for maintaining shallow lakes in a turbid state. Since especially the spring ecosystem dynamics determines the successive state of shallow lakes, such impact could depend on seasonality in benthivore activity patterns. However, we do not yet know to what extent the activity of large-bodied benthivorous fish affects lake turbidity across the year. In order to investigate seasonal dynamics in bream activity and its impact on water turbidity under natural conditions, bream daily activity was studied in a small (39 ha) shallow Danish lake using passive biotelemetry technology, i.e. a modified Passive Integrated Transponder (PIT)-tag antenna system. We tracked the activity of 448 benthivorous bream over a period of four years (2012 to 2016) and during the same period wind conditions, water turbidity and temperature was measured. Results showed a clear relationship between bream activity and water turbidity at water temperature below 15°C indicating that winter season activity of benthivorous bream may play an important role for maintaining lake ecosystems in a turbid state. Also wind speed and wind direction affected water turbidity, suggesting that wind induced resuspension can be important even in small shallow lakes. This is to our knowledge the first full-scale study under natural conditions to describe how bream activity influence lake turbidity on a day-to-day basis. Our findings also add a seasonal component to previous findings by showing that benthivorous feeding bream have the potential to increase water turbidity also in the winter season and thereby, ultimately, impact ecosystem functioning within shallow lakes.

The fate of dissolved organic carbon (DOC) is partly determined by its availability to microbial degradation. Organisms at upper trophic levels could influence the bioavailability of DOC via cascading effects on primary producers and bacteria. Here we experimentally tested whether the presence of fish in aquatic food webs can indirectly affect the composition of the DOC pool. We found that fish had strong positive effects on phytoplankton biomass that affected the dynamics of DOC composition. Specifically, fish increased protein-like, algae-derived DOC mid-experiment, concurrent with the strongest fish-induced increase in phytoplankton biomass. Fish also increased bacterial abundance, altered the community composition and diversity of bacteria, and temporarily increased DOC compounds with fluorescence properties indicative of microbially-reprocessed organic matter. Overall, our experiment revealed that fish can positively influence the substrate (algae-produced DOC) and the key players (bacteria) of the microbial carbon pump. Consequently, fish could contribute to carbon sequestration by stimulating both the production of bioavailable DOC and the microbial degradation of bioavailable to persistent DOC. We propose this as a novel mechanism whereby the loss of predators from global ecosystems could alter carbon cycling.

Removal of cyprinid fish is a widely used biomanipulation tool to transform turbid shallow eutrophic lakes in north temperate regions into a clear water state. We here evaluate the removal of cyprinids from streams as a supplement to lake fishing. Since cyprinids often aggregate in high densities in lake inlet/outlet streams during winter migration, removal of fish in this space-confined habitat may be cost-efficient as compared to fish removal in the lake habitat. In two consecutive years, we annually removed up to 35% of the dominant cyprinids from an inlet stream to a lake and argue that this could easily be increased with a more targeted fishing effort. Concurrently, we monitored species- and length-specific variation in migration propensity, to explore how this relates to efficient fish removal. Smaller planktivores generally had a much higher migratory propensity than larger benthivores. Hence, stream fishing specifically targets species and size groups that are less efficiently controlled with traditional lake fishing methods. As a rule of thumb, stream fishing is most efficient when water temperature is 2–6°C. Prior to implementing fish removals from streams, the potential evolutionary consequences of the targeted removal of migratory phenotypes should be considered.

There is increasing evidence that phenotypic variation can strongly impact community structure and ecosystem functions. Alewife Alosa pseudoharengus is a planktivorous fish species that strongly impact lake ecosystems. It has previously been demonstrated that phenotypic variation related to differences in life history among landlocked and anadromous alewife populations alters the strength of interactions with other species, potentially modifying its role in the community. The migration between freshwater and marine ecosystems by anadromous alewife creates seasonal differences in alewife densities, which causes lake zooplankton communities to alternate between large-body size and higher densities in the spring, and small-body size and low densities in the summer and fall. In lakes with resident (landlocked) alewife, predation from alewife modifies the zooplankton community to having low zooplankton densities and mainly small-bodied zooplankton year-round. The strong effects of phenotypic variation in alewife on zooplankton may be important for coexisting species that rely on zooplankton as a resource. Here we use estimates of growth, and direct diet and stable isotope analyses to ask if the presence- and phenotypic variation of alewife alters the ontogenetic trajectory of young-of-the-year (YOY) largemouth bass Micropterus salmoides, which depend on zooplankton in the early life stages. We found that both the presence- and phenotypic variation of alewife affects growth, trophic position, and diet of largemouth bass. YOY largemouth bass from lakes without alewife grew faster, switched to piscivory earlier, and reached higher trophic positions than in alewife lakes. In lakes with landlocked alewife largemouth bass grew slower and obtained a lower trophic position than those in lakes with anadromous alewife. These divergences can be explained by the strong effects of alewife on zooplankton community structure. Our results demonstrate how the strong effects of phenotypic variation can propagate through natural food webs to influence important life history transitions in other species.

The value of biodiversity is widely appreciated, but we are only beginning to understand the interplay of processes that generate biodiversity and their consequences for coevolutionary interactions. Whereas predator–prey coevolution is most often analyzed in the context of evolutionary arms races, much less has been written about how predators are affected by, and respond to, evolutionary diversification in their prey. We hypothesize here that adaptive radiation of prey may lead to diversification and potentially speciation in predators, a process that we call an upwards adaptive radiation cascade. In this paper we lay out the conceptual basis for upwards adaptive radiation cascades, explore evidence for such cascades, and finally advocate for intensified research.

The concept of the niche has long been a central pillar in ecological theory, with a traditional focus on quantifying niches at the species or population level. However, the importance of individual-level niche variation is increasingly being recognised, with a strong focus on individual specialisation. While examples illustrating the contribution of the individual niche to whole population niche structure are accumulating rapidly, surprisingly little is known about the conditions that shape the differences between these two potentially divergent components. Though theory predicts that stability should influence the extent of such intra-specific specialisation, we know of no previous study that has investigated its role in individual specialisation, and the differentiation between individual- and population niches. Here, we studied the diet of individuals from multiple populations of an aquatic top-predator, Salmo trutta, inhabiting contrasting stable, groundwater fed and unstable, surface water fed pre-alpine streams. Based on stomach content analysis, we found that individuals living in stable environments displayed a higher degree of specialisation than those in unstable environments, with the between individual component of niche width being approximately twice as high in the former. We subsequently validated these results by evidence gained from stable isotope analysis of muscle tissue. As such, we reveal that environmental stability can signifi cantly infl uence individual niches within populations, leading to increased specialisation.

Underutilized ecological niches provide ecological opportunity and thereby promote diversification. Postglacial lakes are known to provide such favourable conditions for evolutionary diversification and adaptive radiations of fishes. Several postglacial radiations of Arctic charr Salvelinus alpinus (L.), whitefish Coregonus sp. and stickleback Gasterosteus aculeatus (L.) are described in the literature. The Arctic charr is known to frequently exhibit two to four sympatric morphotypes. In this study we investigated the patterns of diversity of Arctic charr within a drainage system on the Vatnahverfi peninsula in Southern Greenland. We found exceptionally high morphological diversity, with up to five sympatrically occurring morphotypes. In the largest lakes, a planktivore, a littoral benthic, a profundal benthic, an anadromous and a piscivorous type could be distinguished. These morphotypes did not only differ in their morphology, but were also ecologically and genetically distinct. Moreover, one morphotype was subdivided into two sympatric, genetically clearly distinct clusters that also tended to differ in morphology, and partly in diet. Considering the clear distinctiveness of the ecomorphs in the large lakes I suggest regarding them as separate species. Different morphotypes could also be distinguished in one of the small and isolated lakes but, in contrast to the large lakes, the morphotypes were not genetically distinct and indicated rather individual specialization and ontogenetic niche shifts other than ecological divergence. Consistent with predictions of island biogeography theory, the highest charr diversity was observed in the largest and deepest and presumably also least isolated lake at low altitude.

Construction of small hydropower plants (<10 megawatts) is booming worldwide, exacerbating ongoing habitat fragmentation and degradation, and further fueling biodiversity loss. A systematic approach for selecting hydropower sites within river networks may help to minimize the detrimental effects of small hydropower on biodiversity. In addition, a better understanding of reach-and basin-scale impacts is key for designing planning tools. We synthesize the available information about (1) reach-scale and (2) basin-scale impacts of small hydropower plants on biodiversity and ecosystem function, and (3) interactions with other anthropogenic stressors. We then discuss state-of-the-art, spatially explicit planning tools and suggest how improved knowledge of the ecological and evolutionary impacts of hydropower can be incorporated into project development. Such tools can be used to balance the benefits of hydropower production with the maintenance of ecosystem services and biodiversity conservation. Adequate planning tools that consider basin-scale effects and interactions with other stressors, such as climate change, can maximize long-term conservation.

Interactions among multiple anthropogenic stressors threaten freshwater fish and pose challenges for fisheries management and conservation. Previous studies of multiple‐stressor effects on freshwater fish suggest a prevalence of antagonistic interactions. However, taxonomy, life stage and/or environmental context likely modify the magnitude and direction of fish responses to multiple stressors. Stressor intensity, impact mechanism, exposure time and ecosystem size may further affect interaction outcomes. Large‐scale studies quantifying how these variables moderate stressor interactions are lacking. To address this knowledge gap, we performed a meta‐analysis of 29 factorial multiple‐stressor experiments to examine the influence of seven potential moderator variables on the magnitude and direction of stressor interactions. Using weighted random‐effects meta‐analytic models, we demonstrate the importance of taxonomic identity and life stage for interaction outcomes. In particular, Cypriniformes showed stronger antagonisms than Salmoniformes, as did larval fish compared to juveniles. Interaction outcomes also varied among the measured fish responses with survival yielding stronger antagonisms than biomass. Increasing experimental duration and volume of the experimental units both drove interactions towards synergisms, supporting findings from previous studies that synergisms take time and space to develop. In an era when the number of stressors affecting freshwater systems is increasing rapidly, our study provides a vital step towards identifying generalities in multiple‐stressor outcomes and thus improved predictions of multiple‐stressor impacts. Furthermore, our meta‐analysis complements studies in real streams, rivers and lakes by providing an experimentally derived context for the growing number of multiple‐stressor assessments in research, management and conservation of freshwater fish.

Nach erfolgreichem Abschluss der interdisziplinären Vorgängerprojekte «Rhone-Thur», «Integrales Flussgebietsmanagement» und «Geschiebeund Habitatsdynamik» wurde im Rahmen des durch das Bundesamt für Umwelt (BAFU) unterstützten Forschungsprogramms «Wasserbau und Ökologie» ein neues Projekt gestartet, welches sich auf die Sedimentdynamik und Vernetzung in Fliessgewässern konzentriert. Das Ziel des Projekts ist es, das im Rahmen der Vorgängerprojekte erarbeitete Wissen weiter zu vertiefen und auszubauen. Dabei stehen wiederum konkrete Fragen und Antworten zu Hochwasserschutz- und Revitalisierungsprojekten im Vordergrund. Im vorliegenden Beitrag werden die Zielsetzung sowie die Projektschwerpunkte mit den daraus resultierenden einzelnen Teilprojekten vorgestellt.

Studies of predator-mediated selection on behaviour are critical for our understanding of the evolution and maintenance of behavioural diversity in natural populations. Consistent individual differences in prey behaviour, especially in the propensity to take risks ("boldness"), are widespread in the animal kingdom. Theory predicts that individual behavioural types differ in a cost-benefit trade-off where bolder individuals benefit from greater access to resources while paying higher predation-risk costs. However, explicitly linking predation events to individual behaviour under natural conditions is challenging and there is currently little data from the wild. We assayed individual behaviour and electronically tagged hundreds of fish (roach, Rutilus rutilus) before releasing them into their lake of origin, thereby exposing them to predation risk from avian apex predators (cormorants, Phalacrocorax carbo). Scanning for regurgitated tags at the cormorant roosting site provided data on individual predation events. We found that fish with higher boldness have a greater susceptibility to cormorant predation compared to relatively shy, risk-averse individuals. Our findings hereby provide unique and direct evidence of behavioural type-dependent predation vulnerability in the wild, i.e. that there is a predation cost to boldness, which is critical for our understanding of the evolution and maintenance of behavioural diversity in natural populations.

Species integrity can be challenged, and even eroded, if closely related species can hybridize and produce fertile offspring of comparable fitness to that of parental species. The maintenance of newly diverged or closely related species therefore hinges on the establishment and effectiveness of pre- and/or post-zygotic reproductive barriers. Ecological selection, including predation, is often presumed to contribute to reduced hybrid fitness, but field evidence for a predation cost to hybridization remains elusive. Here we provide proof-of-concept for predation on hybrids being a postzygotic barrier to gene flow in the wild. Cyprinid fishes commonly produce fertile, viable hybrid offspring and therefore make excellent study organisms to investigate ecological costs to hybrids. We electronically tagged two freshwater cyprinid fish species (roach Rutilus rutilus and bream Abramis brama) and their hybrids in 2005. Tagged fish were returned to their lake of origin, exposing them to natural predation risk from apex avian predators (great cormorant, Phalacrocorax carbo). Scanning for regurgitated tags under cormorant roosts 3–4 years later identified cormorant-killed individual fish and allowed us to directly test for a predation cost to hybrids in the wild. Hybrid individuals were found significantly more susceptible to cormorant predation than individuals from either parental species. Such ecological selection against hybrids contributes to species integrity, and can enhance species diversification.

Intraspecific phenotypic variation can strongly impact community and ecosystem dynamics. Effects of intraspecific variation in keystone species have been shown to propagate down through the food web by altering the adaptive landscape for other species and creating a cascade of ecological and evolutionary change. However, similar bottom-up eco-evolutionary effects are poorly described. Here we show that life history diversification in a keystone prey species, the alewife (Alosa pseudoharengus), propagates up through the food web to promote phenotypic diversification in its native top predator, the chain pickerel (Esox niger), on contemporary timescales. The landlocking of alewife by human dam construction has repeatedly created a stable open water prey resource, novel to coastal lakes, that has promoted the parallel emergence of a habitat polymorphism in chain pickerel. Understanding how strong interactions propagate through food webs to influence diversification across multiple trophic levels is critical to understand eco-evolutionary interactions in complex natural ecosystems.

1. Migration is a widespread phenomenon, with powerful ecological and evolutionary consequences. Morphological adaptations to reduce the energetic costs associated with migratory transport are commonly documented for migratory species. However, few studies have investigated whether variation in body morphology can be explained by variation in migratory strategy within a species.
2. We address this question in roach Rutilus rutilus, a partially migratory freshwater fish that migrates from lakes into streams during winter. We both compare body shape between populations that differ in migratory opportunity (open vs. closed lakes), and between individuals from a single population that vary in migratory propensity (migrants and residents from a partially migratory population). Following hydrodynamic theory, we posit that migrants should have a more shallow body depth, to reduce the costs associated with migrating into streams with higher flow conditions than the lakes the residents occupy all year round.
3. We find evidence both across and within populations to support our prediction, with individuals from open lakes and migrants from the partially migratory population having a more slender, shallow-bodied morphology than fish from closed lakes and all-year residents.
4. Our data suggest that a shallow body morphology is beneficial to migratory individuals and our study is one of the first to link migratory strategy and intraspecific variation in body shape.

Swiss rivers have a large potential for energy production from hydropower and there is currently a push towards to construction of small hydropower plants (SHPs, <10 MW). Since the early 20s there were about 7000 SHPs running and the potential of further SHPs expansion is estimated in an annual increase of about 2,200GWh which is interesting for the planning of the “Energiestrategie 2050” to cover the loss of energy production from opting out of the nuclear energy program. However, the impact of SHPs on biodiversity and the provisioning of ecosystem services is not well known. In this study I sought to investigate whether SHPs have an impact on Swiss alpine river ecosystems, with special focus on the macroinvertebrate community and brown trout (Salmo trutta) foraging ecology, by studying near-natural and impacted stream reaches of five SHPs in Graubünden (Switzerland). I analyzed environmental data, benthic macroinvertebrate community composition and brown trout somatic condition and stomach contents. I found no significant differences but trends for benthic macroinvertebrate communities in residual reaches which might reflect a negative influence of SHPs and could be explained by environmental variables. Residual flow reaches had a more stable river channels, lower current velocities, significantly lower discharges and lower organic matter concentrations. Trout had a significantly lower somatic conditions in residual reaches compared to downstream reaches and it also tended to be lower than in upstream regions. Trout from residual reaches fed disproportionally more on EPT taxa than there were available in the benthic invertebrate community. This feeding pattern might have come at the cost of a higher energy expenditure and therefore led to reduced body condition. My study shows that SHPs had substantial impacts on stream habitat and ecosystem processes. Further, I could show that the reduced trout somatic condition in residual flow reaches may be a result of shifts in invertebrate prey availability.

Although migratory plasticity is increasingly documented, the ecological drivers of plasticity are not well understood. Predation risk can influence migratory dynamics, but whether seasonal migrants can adjust their migratory behaviour according to perceived risk is unknown. We used electronic tags to record the migration of individual roach (Rutilus rutilus), a partially migratory fish, in the wild following exposure to manipulation of direct (predator presence/absence) and indirect (high/low roach density) perceived predation risk in experimental mesocosms. Following exposure, we released fish in their lake summer habitat and monitored individual migration to connected streams over an entire season. Individuals exposed to increased perceived direct predation risk (i.e. a live predator) showed a higher migratory propensity but no change in migratory timing, while indirect risk (i.e. roach density) affected timing but not propensity showing that elevated risk carried over to alter migratory behaviour in the wild. Our key finding demonstrates predator-driven migratory plasticity, highlighting the powerful role of predation risk for migratory decision-making and dynamics.

Adaptive radiation is an important evolutionary process to shape biological diversity. To understand the initial stages and early adaption processes is a major problem in evolutionary biology. The Greenland freshwater drainage systems are very young and the structure of the ecosystems is relatively simple. Due to that the Greenland study system can be regarded as a great opportunity to address questions of evolutionary biology. Threespine stickleback Gasterosteus aculeatus is a proven study organism to investigate these processes. Using existing knowledge of stickleback ecology and evolutionary history investigated in different regions, freshwater stickleback populations in southwest Greenland have been investigated for the first time. Body shape analysis of eighteen populations was conducted to investigate patterns of local adaption. It was found that stickleback populations in adjacent lake and streams were differed in two out of four cases. It was also found that present phenotypes where different between many lakes. In addition to that, morphology of the very diverse water bodies has also been assessed. Properties of the water bodies have proved to explain phenotypic variation among stickleback populations. Using relatively simple methods, this work gives an impression of the phenotypic diversity of Greenland stickleback populations and can be seen as a signpost for further research projects in this area.

The intraspecific diversity of a species can give interesting insights into evolutionary mechanisms. This genotypic and phenotypic diversity allows species to adapt to different environments and to display favourable adaptive traits, which can lead to divergence between populations and with time, to the formation of lineages or even new species. Brown trout in Central Europe has split into five lineages (Salmo trutta species complex), mostly due to the allopatric separation of drainage systems. The Engadin, a region in the Swiss Alps, is part of the Danube drainage system and harbours brown trout populations with individuals from several lineages. The aim of my study was to assess the genotypic and phenotypic diversity of trout, and possible relationships between them. This was primarily done with trout from the Engadin but also by including populations from adjacent drainage systems. A genetic approach was used where I identified lineage-related mtDNA haplotypes and applied 13 microsatellite markers. Trout phenotypic data were obtained through measurements of skin colour, spot patterns and morphology. The results revealed that the Atlantic and Danube trout lineages in the Engadin still exhibit genetic differences, but also that these were admixed to a large extent. This suggests a scenario of extensive interbreeding between the lineages in this region. Additionally, I found that the variation in skin colour and spot patterns was enhanced in the Engadin compared to the other drainage systems. A correlation between genetic and phenotypic data was not evident. The large phenotypic variation in the Engadin could, at least in part, be the result of phenotypic plasticity altered by gene flow between phenotypically diverged lineages.

The juvenile trout of streams around Lake Lucerne show different life history patterns in terms of variation in migration behaviors; some of them migrate to the lake after hatching whereas some decide to stay in the river as a resident. Juvenile trout from the tributary called Klosterbach (Uri, Switzerland) were investigated upon difference in their morphology, colors and spot pattern in relation to their decision to migrate or reside in their natal river. Also, the timing of migration was studied in relation to the length distribution within the migrants. The aim was to find certain correlations between phenotypical characteristics and the migration type and time. The research revealed that larger trout migrate earlier in the season. Possibly, this is because they have an elevated metabolic rate and escape restrained resource conditions by migrating to the lake. Moreover, no phenotypical trait was found to be significantly varying between migrants and residents. This due to the absence of phenotypical differentiation prior to migration or weaknesses in the work process of this specific project.

Migration is an important event in many animal life histories, but the degree to which individual animals participate in seasonal migrations often varies within populations. The powerful ecological and evolutionary consequences of such partial migration are now well documented, but the underlying mechanisms are still heavily debated. One potential mechanism of partial migration is between-individual variation in body condition, where animals in poor condition cannot pay the costs of migration and hence adopt a resident strategy. However, underlying intrinsic traits may overrule such environmental influence, dictating individual consistency in migratory patterns. Unfortunately, field tests of individual consistency compared to the importance of individual condition on migratory propensity are rare. Here we analyse 6 years of field data on roach migration, gathered by tagging almost 3000 individual fish and monitoring their seasonal migrations over extended periods of time. Our aims were to provide a field test of the role of condition in wild fish for migratory decisions, and also to assess individual consistency in migratory tendency. Our analyses reveal that (1) migratory strategy, in terms of migration/residency, is highly consistent within individuals over time and (2) there is a positive relationship between condition and the probability of migration, but only in individuals that adopt a migratory strategy at some point during their lives. However, life-long residents do not differ in condition to migrants, hence body condition is only a good predictor of migratory tendency in fish with migratory phenotypes and not a more general determinant of migratory tendency for the population. As resident individuals can achieve very high body condition and still remain resident, we suggest that our data provides some of the first field evidence to show that both facultative and obligate strategies can co-exist within populations of migratory animals.

While ecological monitoring and biodiversity assessment programs are widely implemented and relatively well developed to survey and monitor the structure and dynamics of populations and communities in many ecosystems, quantitative assessment and monitoring of genetic and phenotypic diversity that is important to understand evolutionary dynamics is only rarely integrated. As a consequence, monitoring programs often fail to detect changes in these key components of biodiversity until after major loss of diversity has occurred. The extensive efforts in ecological monitoring have generated large data sets of unique value to macro-scale and long-term ecological research, but the insights gained from such data sets could be multiplied by the inclusion of evolutionary biological approaches. We argue that the lack of process-based evolutionary thinking in ecological monitoring means a significant loss of opportunity for research and conservation. Assessment of genetic and phenotypic variation within and between species needs to be fully integrated to safeguard biodiversity and the ecological and evolutionary dynamics in natural ecosystems. We illustrate our case with examples from fishes and conclude with examples of ongoing monitoring programs and provide suggestions on how to improve future quantitative diversity surveys.

Animal migration is an amazing phenomenon that has fascinated humans for long. Many freshwater fishes also show remarkable migrations, whereof the spectacular mass migrations of salmonids from the spawning streams are the most well known and well studied. However, recent studies have shown that migration occurs in a range of freshwater fish taxa from many different habitats. In this review we focus on the causes and consequences of migration in freshwater fishes. We start with an introduction of concepts and categories of migration, and then address the evolutionary causes that drive individuals to make these migratory journeys. The basis for the decision of an individual fish to migrate or stay resident is an evaluation of the costs and benefits of different strategies to maximize its lifetime reproductive effort. We provide examples by discussing our own work on the causes behind seasonal migration in a cyprinid fish, roach (Rutilus rutilus (L., 1758)), within this framework. We then highlight different adaptations that allow fish to migrate over sometimes vast journeys across space, including capacity for orientation, osmoregulation, and efficient energy expenditure. Following this we consider the consequences of migration in freshwater fish from ecological, evolutionary, and conservation perspectives, and finally, we detail some of the recent developments in the methodologies used to collect data on fish migration and how these could be used in future research.

Habitat stability is an important factor that strongly influences individuals and their interactions. Both the stability of a habitat and the most prevalent conditions in it can determine ecology and phenotype of individuals that inhabit it. This should be reflected by both the niche they occupy in the habitat and their phenotype. There is still a lack of studies showing causes and effects of individual specialization on individual and population niches. To study the influence of habitat stability on these factors, I analyzed the individual diets of and conducted phenotypic analyses on trout from environmentally stable groundwater fed and unstable surface water fed streams in the Lake Lucerne drainage. There were clear ecological differences between stream types. Individuals from the stable habitat showed a higher degree of specialization than those from unstable habitats. The population niche in the stable habitat, hence, consisted of many smaller, specialized individual niches, with most of the variation in the between-individual component. In the unstable habitat, it consisted of overlapping, generalist niches with a high within-individual component to variation. Overall, my study showed that habitat stability can be an important factor in determining the niche of individuals in the habitat. I also used microsatellites to show that each stream could be considered a separate replicate. I found statistically significant differences in phenotype, both color and shape.

This study evaluated a technique to allow the long-term monitoring of individual fishes of known sex in the wild using sex confirmation in close proximity to the reproductive period combined with individual tagging. Hundreds of partially migratory roach Rutilus rutilus were tagged with passive integrated transponders (PIT) following sex determination in spring and various performance measures were compared with fish tagged outside the reproductive period in autumn. Short-term survival was >95% for R. rutilus sexed and tagged under natural field conditions. Total length (L_T) did not affect the probability of survival within the size range tagged (119–280 mm), nor were there differences in timing of migration the following season between individuals sexed and tagged in spring and individuals tagged in autumn (i.e. outside the reproductive period). Also, a similar per cent of R. rutilus sexed and tagged in spring and tagged in autumn migrated the following season (34·5 and 34·7%). Moreover, long-term recapture data revealed no significant differences in body condition between R. rutilus individuals sexed and tagged in spring, individuals tagged in autumn and unmanipulated individuals. The observed sex ratio of recaptured fish did not differ from the expected values of equal recapture rates between males and females. Hence, there is no observable evidence for an adverse effect of tagging close to the reproductive period and therefore this method is suitable for studying intersexual differences and other phenotypic traits temporarily expressed during reproduction at the individual level in fishes.

Phenotypic divergence in natural populations is often a result of ecologically-based divergent selection which might arise from intra- or interspecific competition. Intraspecific competition promotes phenotypic divergence by driving the population to exploit different resources. Whereas interspecific competition inhibits diversification, the release from interspecific competition creates ecological opportunities that permit phenotypic divergence. Phenotypic divergence is generally an important initial step towards further population divergence. In freshwater fish species of postglacial lakes, phenotypic divergence is often based on trophic polymorphism and associated with limnetic and benthic habitats. In this study, I used perch populations from 12 different pre-alpine lakes to evaluate whether these populations are able to differentiate into more specialized niches and which mechanisms driving it. I found individual specialization in perch populations in trophic level and dietary resources independent of ontogenetic niche shifts. Further, I found individual variation in fin coloration, with fin coloration ranging from bright red to bright yellow. The degree of fin color variation and the strength of the correlation between fin coloration and trophic level were strongly correlated with the density of perch. In lakes with high densities of perch, I found evidence for population divergence based on fin coloration and associated ecology. These results suggest that perch populations can specialize on more specific food resources in lakes with high densities of perch and that fin coloration may be an important trait in intraspecific communication.

Das Bundesamt für Umwelt (BAFU) hat vor mehr als 10 Jahren das interdisziplinäre Forschungsprogramm «Wasserbau und Ökologie» lanciert. Ziel des Programms ist es, wissenschaftliche Grundlagen zur Beantwortung aktueller Fragen im Fliessgewässermanagement zu erarbeiten und umsetzungsgerecht aufzubereiten. Am Programm beteiligen sich Ökologen und Flussbauingenieure der vier Institutionen VAW (ETHZ), LCH (EPFL), EAWAG und WSL des ETHBereichs sowie weitere Partner aus Praxis und Wissenschaft. Das im letzten Jahr lancierte Forschungsprojekt «Geschiebe- und Habitatsdynamik» konzentriert sich nun auf die zwei Schwerpunkte «Geschiebereaktivierung und Gewässerdynamisierung» sowie «Revitalisierung von Auenlandschaften». Wie in den vorangehenden Projekten sollen die Ergebnisse der Praxis einem breiten Publikum zugänglich gemacht werden.

Migration has evolved as a strategy to maximise individual fitness in response to seasonally changing ecological and environmental conditions. However, migration can also incur costs, and quantifying these costs can provide important clues to the ultimate ecological forces that underpin migratory behaviour. A key emerging model to explain migration in many systems posits that migration is driven by seasonal changes to a predation/growth potential (p/g) trade-off that a wide range of animals face. In this study we assess a key assumption of this model for a common cyprinid partial migrant, the roach Rutilus rutilus, which migrates from shallow lakes to streams during winter. By sampling fish from stream and lake habitats in the autumn and spring and measuring their stomach fullness and diet composition, we tested if migrating roach pay a cost of reduced foraging when migrating. Resident fish had fuller stomachs containing more high quality prey items than migrant fish. Hence, we document a feeding cost to migration in roach, which adds additional support for the validity of the p/g model of migration in freshwater systems.

Contemporary insights from evolutionary ecology suggest that population divergence in ecologically important traits within predators can generate diversifying ecological selection on local community structure. Many studies acknowledging these effects of intraspecific variation assume that local populations are situated in communities that are unconnected to similar communities within a shared region. Recent work from metacommunity ecology suggests that species dispersal among communities can also influence species diversity and composition but can depend upon the relative importance of the local environment. Here, we study the relative effects of intraspecific phenotypic variation in a fish predator and spatial processes related to plankton species dispersal on multitrophic lake plankton metacommunity structure. Intraspecific diversification in foraging traits and residence time of the planktivorous fish alewife (Alosa pseudoharengus) among coastal lakes yields lake metacommunities supporting three lake types which differ in the phenotype and incidence of alewife: lakes with anadromous, landlocked, or no alewives. In coastal lakes, plankton community composition was attributed to dispersal versus local environmental predictors, including intraspecific variation in alewives. Local and beta diversity of zooplankton and phytoplankton was additionally measured in response to intraspecific variation in alewives. Zooplankton communities were structured by species sorting, with a strong influence of intraspecific variation in A. pseudoharengus. Intraspecific variation altered zooplankton species richness and beta diversity, where lake communities with landlocked alewives exhibited intermediate richness between lakes with anadromous alewives and without alewives, and greater community similarity. Phytoplankton diversity, in contrast, was highest in lakes with landlocked alewives. The results indicate that plankton dispersal in the region supplied a migrant pool that was strongly structured by intraspecific variation in alewives. This is one of the first studies to demonstrate that intraspecific phenotypic variation in a predator can maintain contrasting patterns of multitrophic diversity in metacommunities.

Background and purpose Enterococci are emerging causes of severe infections, including wound and bone infections in orthopedic patients. The main purpose of this study was to determine whether there was a correlation between the incidence of enterococci in tissue samples (biopsies) from orthopedic patients and consumption of cefuroxime in the orthopedic department.
Methods and results Data were obtained from the department of clinical microbiology and the hospital pharmacy. The consumption of cefuroxime successively increased from 40 defined daily doses (DDD)/10³ bed days in 2002 to 212 DDD/10³ bed days in 2009. The incidence of patients with enterococci in tissue samples increased steadily from 1.03/10³ bed days in 2002 to 5.90/10³ bed days in 2009. Regression analysis revealed a significant correlation between the consumption of cefuroxime and the incidence of enterococci.
Interpretation Continuous surveillance of species distribution, resistance rates, and antibiotic consumption is of utmost importance for optimal antibiotic strategy in orthopedic patients.

The importance of predation risk in shaping patterns of animal migration is not well studied, mostly owing to difficulties in accurately quantifying predation risk for migratory versus resident individuals. Here, we present data from an extensive field study, which shows that migration in a freshwater fish (roach, Rutilus rutilus) that commonly migrates from lakes to streams during winter confers a significant survival benefit with respect to bird (cormorant, Phalacrocorax carbo spp.) predation. We tagged over 2000 individual fish in two Scandinavian lakes over 4 years and monitored migratory behaviour using passive telemetry. Next, we calculated the predation vulnerability of fish with differing migration strategies, by recovering data from passive integrated transponder tags of fish eaten by cormorants at communal roosts close to the lakes. We show that fish can reduce their predation risk from cormorants by migrating into streams, and that probability of being preyed upon by cormorants is positively related to the time individuals spend in the lake during winter. Our data add to the growing body of evidence that highlights the importance of predation for migratory dynamics, and, to our knowledge, is one of the first studies to directly quantify a predator avoidance benefit to migrants in the field.

Trophic niche divergence is considered to be a major process by which species coexistence is facilitated. When studying niche segregation in lake ecosystems, we tend to view the niche on a one-dimensional pelagic-littoral axis. In reality, however, the niche use may be more complex and individual fidelity to a niche may be variable both between and within populations. In order to study this complexity, relative simple systems with few species are needed. In this paper, we study how competitor presence affects the resource use of brown trout (Salmo trutta) in 11 species-poor Faroese lakes by comparing relative abundance, stable isotope ratios and diet in multiple habitats. In the presence of three-spined sticklebacks (Gasterosteus aculeatus), a higher proportion of the trout population was found in the pelagic habitat, and trout in general relied on a more pelagic diet base as compared to trout living in allopatry or in sympatry with Arctic charr (Salvelinus alpinus). Diet analyses revealed, however, that niche-segregation may be more complex than described on a one-dimensional pelagic-littoral axis. Trout from both littoral and offshore benthic habitats had in the presence of sticklebacks a less benthic diet as compared to trout living in allopatry or in sympatry with charr. Furthermore, we found individual habitat specialization between littoral/benthic and pelagic trout in deep lakes. Hence, our findings indicate that for trout populations interspecific competition can drive shifts in both habitat and niche use, but at the same time they illustrate the complexity of the ecological niche in freshwater ecosystems.

Migration is an important event in the life history of many animals, but there is considerable variation within populations in the timing and final destination. Such differential migration at the population level can be strongly determined by individuals showing different consistencies in migratory traits. By tagging individual cyprinid fish with uniquely coded electronic tags, and recording their winter migrations from lakes to streams for 6 consecutive years, we obtained highly detailed long-term information on the differential migration patterns of individuals. We found that individual migrants showed consistent site fidelities for over-wintering streams over multiple migratory seasons and that they were also consistent in their seasonal timing of migration. Our data also suggest that consistency itself can be considered as an individual trait, with migrants that exhibit consistent site fidelity also showing consistency in migratory timing. The finding of a mixture of both consistent and inconsistent individuals within a population furthers our understanding of intrapopulation variability in migration strategies, and we hypothesize that environmental variation can maintain such different strategies.

Partial migration, where only some individuals from a population migrate, has been widely reported in a diverse range of animals. In this paper, what is known about the causes and consequences of partial migration in fishes is reviewed. Firstly, the ultimate and proximate drivers of partial migration are reflected upon: what ecological factors can shape the evolution of migratory dimorphism? How is partial migration maintained over evolutionary timescales? What proximate mechanisms determine whether an individual is migratory or remains resident? Following this, the consequences of partial migration are considered, in an ecological and evolutionary context, and also in an applied sense. Here it is argued that understanding the concept of partial migration is crucial for fisheries and ecosystem managers, and can provide information for conservation strategies. The review concludes with a reflection on the future opportunities in this field, and the avenues of research that are likely to be fruitful to shed light on the enduring puzzle of partial migration in fishes.

Partial migration, where populations are composed of both migratory and resident individuals, is extremely widespread across the animal kingdom. Researchers studying fish movements have long recognized that many fishes are partial migrants, however, no detailed taxonomic review has ever been published. In addition, previous work and synthesis has been hampered by a varied lexicon associated with this phenomenon in fishes. In this review, definitions and important concepts in partial migration research are discussed, and a classification system of the different forms of partial migration in fishes introduced. Next, a detailed taxonomic overview of partial migration in this group is considered. Finally, methodological approaches that ichthyologists can use to study this fascinating phenomenon are reviewed. Partial migration is more widespread amongst fishes than previously thought, and given the array of techniques available to fish biologists to study migratory variation the future of the field looks promising.

Whereas many studies have addressed the mechanisms driving partial migration, few have focused on the consequences of partial migration on trophic dynamics, and integrated studies combining the two approaches are virtually nonexistent. Here we show that temperature affects seasonal partial migration of cyprinid fish from lakes to predation refuges in streams during winter and that this migration in combination with temperature affects the characteristics and phenology of lower trophic levels in the lake ecosystem. Specifically, our six-year study showed that the proportion of fish migrating was positively related to lake temperature during the pre-migration growth period, i.e. during summer. Migration from the lake occurred later when autumn water temperatures were high, and timing of return migration to the lake occurred earlier at higher spring water temperatures. Moreover, the winter mean size of zooplankton in the lake increased with the proportion of fish being away from the lake, likely as a consequence of decreased predation pressure. Peak biomass of phytoplankton in spring occurred earlier at higher spring water temperatures and with less fish being away from the lake. Accordingly, peak zooplankton biomass occurred earlier at higher spring water temperature, but relatively later if less fish were away from the lake. Hence, the time between phyto- and zooplankton peaks depended only on the amount of fish being away from the lake, and not on temperature. The intensity of fish migration thereby had a major effect on plankton spring dynamics. These results significantly contribute to our understanding of the interplay between partial migration and trophic dynamics, and suggest that ongoing climate change may significantly affect such dynamics.

The predicted global warming may affect freshwater systems at several organizational levels, from organism to ecosystem. Specifically, in temperate regions, the projected increase of winter temperatures may have important effects on the over-winter biology of a range of organisms and especially for fish and other ectothermic animals. However, temperature effects on organisms may be directed strongly by resource availability. Here, we investigated whether over-winter loss of biomass and lipid content of juvenile roach (Rutilus rutilus) was affected by the physiologically relatively small (2-5°C) changes of winter temperatures predicted by the Intergovernmental Panel on Climate Change (IPCC), under both natural and experimental conditions. This was investigated in combination with the effects of food availability. Finally, we explored the potential for a correlation between lake temperature and resource levels for planktivorous fish, i.e., zooplankton biomass, during five consecutive winters in a south Swedish lake. We show that small increases in temperature (+2°C) affected fish biomass loss in both presence and absence of food, but negatively and positively respectively. Temperature alone explained only a minor part of the variation when food availability was not taken into account. In contrast to other studies, lipid analyses of experimental fish suggest that critical somatic condition rather than critical lipid content determined starvation induced mortality. Our results illustrate the importance of considering not only changes in temperature when predicting organism response to climate change but also food-web interactions, such as resource availability and predation. However, as exemplified by our finding that zooplankton over-winter biomass in the lake was not related to over-winter temperature, this may not be a straightforward task.

Partial migration, whereby only a fraction of the population migrates, is thought to be the most common type of migration in the animal kingdom, and can have important ecological and evolutionary consequences. Despite this, the factors that influence which individuals migrate and which remain resident are poorly understood. Recent work has shown that consistent individual differences in personality traits in animals can be ecologically important, but field studies integrating personality traits with migratory behaviour are extremely rare. In this study, we investigate the influence of individual boldness, an important personality trait, upon the migratory propensity of roach, a freshwater fish, over two consecutive migration seasons. We assay and individually tag 460 roach and show that boldness influences migratory propensity, with bold individuals being more likely to migrate than shy fish. Our data suggest that an extremely widespread personality trait in animals can have significant ecological consequences via influencing individual-level migratory behaviour.