Staff

Timothy John Alexander

Dr. Timothy John Alexander

Postdoc

Department Fish Ecology & Evolution

About Me

  • Postdoc in the group Biodiversity Dynamics

Research interests

I am interested in research that guides integrated approaches to conservation and natural resource management at the resolution of ecosystems, or ideally socio-ecological systems. I am currently working as a postdoc with Projet Lac.

The main objectives of the project are to:
- Assess the current ecological status of Swiss lakes with particular emphasis on fish communities.
- Use current and historical environmental data to identify key factors that influence the development and distribution of fish biodiversity.
- Establish a reference collection of fishes at the Natural History Museum of Bern as a basis for future research.

The ecological assessment aligns with efforts by other European countries to fulfill their obligations under the EU Water Framework Directive. Under this mandate, member countries are obligated to assess, and if necessary improve, the ecological status of their lakes (among other freshwater ecosystems). The assessment process is based on identifying distinct types/typologies of each ecosystem and, within the typology, comparing each waterbody to control or reference site via indicators or indices. Lacustrine fish communities form a major component of the assessment. Although Switzerland is not part of the EU, we’re conducting a comparable sampling program to contribute to Europe-wide ecological assessments and to generally better understand the ecology of fish communities in Swiss lakes.

The second objective of the project is to shed further light on the effects of current and historical anthropogenic influence on lake biodiversity and ecosystem functioning. A history of eutrophication in Swiss lakes by anthropogenically derived phosphorus has led to major changes in the ecology of these waterbodies. The problems peaked in the late 1970's and the government responded by introducing a successful nutrient control program that put many lakes on a trajectory back towards their natural state. The repercussions of eutrophication are still evident today though in places such as Lago di Lugano where, although the lake is almost 300m deep, very little lives below 30 metres due to low oxygen caused by decaying organic matter in conjunction with thermal stratification. My colleagues have identified that the lack of oxygen in deeper waters has caused the extinction of some endemic fish species and the erosion of genetic diversity by forcing previously distinct populations and species to interbreed within the most eutrophic lakes (http://www.nature.com/nature/journal/v482/n7385/abs/nature10824.html). This project aims to go deeper into such patterns for fish species across lakes representing a wide range natural and anthropogenic conditions.

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Projects

Almost two-thirds of the native fish species of Switzerland are threatened by extinction. To preserve or specifically enhance aquatic biodiversity and habitats ...

Projet Lac - on track of the fish in our lakes
We work to combat what we consider the largest stumbling block on the way to successful biodiversity management and conservation in aquatic ecosystems: the lack of quantitative baseline data

Conservation Biology
We study causes of variation in diversity of fish assemblages and we wish to understand how variation in diversity affects the lake ecosystems

Community Ecology

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Curriculum Vitae

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Publications

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Evaluating gillnetting protocols to characterize lacustrine fish communities

Ecological research and monitoring of lacustrine ecosystems often requires a whole-lake assessment of fish communities. Gillnet sampling offers an efficient means of estimating abundance, biomass and fish community composition. However the choice of gillnet sampling protocol may influence lake characterization via physical properties of the nets and allocation of sampling effort between littoral, benthic and pelagic habitats. This paper compares two commonly used, whole-lake sampling protocols applied across 17 prealpine, subalpine and alpine European lakes ranging widely in size, depth and altitude to determine their relative strength for research and management applications. Effort-corrected estimates of abundance, biomass and species richness were correlated between the protocols and both distinguished the trout-dominated alpine communities from subalpine and prealpine lakes dominated by whitefish and perch. A considerable amount of variance remained unexplained between the two protocols however, which seemed to correspond with differences in the proportion of effort among benthic and pelagic habitats. We suggest that both the European standard (CEN) and vertical (VERT) netting protocols are suitable for assessing ecological status and monitoring changes in lake fish communities through time. However the details of each protocol should be kept in mind when comparing fish communities between lakes. Mesh sizes used in CEN nets produce a more even size frequency distribution, suggesting that this protocol is most appropriate for assessing size structure of fish assemblages. The high proportion of netting effort in benthic habitats shallower than 70 m depth under the CEN protocol means that, particularly in larger lakes, outcomes will be disproportionately influenced by the ecological condition of this habitat. The VERT protocol presumably provides a more accurate estimate of whole-lake CPUE and community composition because effort, in terms of net area, is more evenly distributed across the entire volume of the lake. This is particularly important in large and deep lakes where pelagic habitats occupy a high proportion of the lake volume.
Alexander, T. J.; Vonlanthen, P.; Périat, G.; Degiorgi, F.; Raymond, J.-C.; Seehausen, O. (2015) Evaluating gillnetting protocols to characterize lacustrine fish communities, Fisheries Research, 161, 320-329, doi:10.1016/j.fishres.2014.08.009, Institutional Repository

Untersuchung der Fischpopulation im Lago di Poschiavo und im Silsersee

Um unsere Gewässer effizient zu bewirtschaften und zu schützen, muss der Ist-Zustand bekannt sein. Im „Projet Lac“ wird die Fischartenzusammensetzung in den alpinen Seen zum ersten Mal überhaupt standardisiert erhoben. Dieser Bericht fasst die Resultate für den Lago Poschiavo und den Silsersee zusammen. Beide Seen sind organisch wenig belastet und nährstoffarm. Die physikalischen und chemischen Eigenschaften des Lago di Poschiavo sind jedoch schon heute stark durch die Kraftwerksnutzung geprägt. Der Silsersee weist entlang der gesamten Uferlinie schöne, naturnahe Uferhabitate auf, wie sie in keinem der tiefer gelegenen tiefen Alpenseen vorkommen. Beide Seen weisen eine für die Fischerei attraktive Salmonidengemeinschaft auf. Die historische Artenvielfalt beider Seen wurde jedoch stark durch die Einfuhr von Fischarten aus anderen Einzugsgebieten beeinflusst. Inwiefern noch eigenständige und autochthone Forellenpopulationen vorhanden sind wir derzeit noch genetisch abgeklärt. Unsere Netzfänge zeigen insgesamt ein anderes Bild als die Angelfischerfänge, da im Silsersee deutlich mehr Fische und eine grössere Biomasse (korrigiert für den Aufwand) gefangen wurde als im Lago di Poschiavo. Die Resultate der repräsentativen Abfischungen zeigen, dass die Forellen im Lago di Poschiavo hauptsächlich im Litoral des Sees gefangen werden. Die Seesaiblinge bevorzugen eine Tiefe zwischen 20-40m. Somit bevorzugen die beiden für die Fischerei wichtigsten Fischarten die Habitate , die durch eine Nutzung des Lago di Poschiavo für die Pumpspeicherung am stärksten beeinflusst würden. Es muss deshalb davon ausgegangen werden, dass die Pumpspeicherung sich stark auf die Populationsgrössen der Fische des Lago di Poschiavo auswirken wird. Im Silsersee sind die Seesaiblingsfänge in den letzten Jahren stark zusammengebrochen. Unsere Fänge zeigen, dass die Seesaiblinge heute im Silsersee eine ähnliche Dichte aufweisen wie im Lago di Poschiavo. Die Dichte der Seesaiblinge im See kann deshalb nicht als Ursache für die im Vergleich zum Lago di Poschiavo geringen Fänge im Silsersee herangezogen werden. Auch die Grösse der Seesaiblinge war in den beiden Seen vergleichbar, was nicht auf ein geringeres Wachstum schliessen lässt. Es muss deshalb davon ausgegangen werden, dass die geringen Fänge im Vergleich zu Lago di Poschiavo teilweise auf eine geringere Fangwahrscheinlichkeit zurückzuführen sind.
Vonlanthen, P.; Périat, G.; Seehausen, O.; Alexander, T.; Guillard, J.; Colon, M. (2014) Untersuchung der Fischpopulation im Lago di Poschiavo und im Silsersee, 51 p, Institutional Repository

Artenvielfalt und Zusammensetzung der Fischpopulation im Hallwilersee

Um unsere Gewässer effizient zu bewirtschaften und zu schützen, muss der Ist-Zustand bekannt sein. Im „Projet Lac“ wird die Fischartenzusammensetzung in den alpinen Seen zum ersten Mal überhaupt standardisiert erhoben. Dieser Bericht fasst die Resultate für den Hallwilersee zusammen. Über fünf Tage wurden im Herbst 2012 im Hallwilersee insgesamt 160 standardisierte und reproduzierbare Befischungsaktionen durchgeführt. Diese Resultate können in Zukunft als Vergleich für die Entwicklung der Fischgemeinschaft im Hallwilersee herangezogen werden. Darüber hinaus können folgende Rückschlüsse gezogen werden: Der Hallwilersee ist heute ein mässig mit Nährstoffen belasteter und warmer See. Dies führt zu einer hohen Primärproduktion, einer Sauerstoffübersättigung im Metalimnion und einem Sauerstoffdefizit im Hypolimnion. Zur Unterstützung der Sanierung wird der See seit Jahren im Sommer mit Sauerstoff versorgt und die die Zirkulation im Winter durch Luftzufuhrt unterstützt. Trotzdem sind die Fische heute immer noch starken anthropogenen Umweltveränderungen ausgesetzt, die sich auf die Fischartenzusammensetzung auswirken. So stehen beispielsweise den Felchen im Sommer und Frühherbst kaum Lebensräume zur Verfügung, die genügend aber nicht zu viel Sauerstoff enthalten und nicht zu warm sind. Ebenfalls ist keine typische Tiefseefischfauna vorhanden. Demgegenüber ist nur ein kleiner Teil des Ufers vom Hallwilersee stark verbaut. Die Verbauungen befinden sich insbesondere in Siedlungsnähe oder bei Bootanlegestellen. Der grössere Teil des Litorals besteht aus strukturarmem Feinsediment, das für Fische ein wenig attraktives Habitat darstellt. Die Starke Sedimentierung kann wiederum zumindest teilweise auf die Eutrophierung zurückgeführt werden. Für die Fischerei zeigen die Resultate des „Projet Lac“, dass der Hallwilerseee im Vergleich mit anderen alpinen Seen ein produktiver See ist. Gerade aber die für die Fischerei wichtigsten Felchen finden im Hallwilersee die zum Überleben notwendigen Bedingungen nur teilweise. Um die Fischerei im Hallwilersee zu fördern, muss die Nährstoffbelastung noch weiter reduziert werden. Insbesondere die für die Fischerei wichtigen Felchen würden von einer solchen Entwicklung profitieren. Weiter sind invasive Arten im See stark verbreitet. Erwähnenswert ist diesbezüglich der Kaulbarsch der im Hallwilersee sehr häufig ist.
Vonlanthen, P.; Périat, G.; Seehausen, O.; Alexander, T. (2014) Artenvielfalt und Zusammensetzung der Fischpopulation im Hallwilersee, 37 p, Institutional Repository

Long-term trends in invertebrate-habitat relationships under protected and fished conditions

Few studies examine the long-term effects of changing predator size and abundance on the habitat associations of resident organisms despite that this knowledge is critical to understand the ecosystem effects of fishing. Marine reserves offer the opportunity to determine ecosystem-level effects of manipulated predator densities, while parallel monitoring of adjacent fished areas allows separating these effects from regional-scale change. Relationships between two measures of benthic habitat structure (reef architecture and topographic complexity) and key invertebrate species were followed over 17 years at fished and protected subtidal rocky reefs associated with two southern Australian marine reserves. Two commercially harvested species, the southern rock lobster (Jasus edwardsii) and blacklip abalone (Haliotis rubra) were initially weakly associated with habitat structure across all fished and protected sites. The strength of association with habitat for both species increased markedly at protected sites 2 years after marine reserve declaration, and then gradually weakened over subsequent years. The increasing size of rock lobster within reserves apparently reduced their dependency on reef shelters as refuges from predation. Rising predation by fish and rock lobster in the reserves corresponded with weakening invertebrate–habitat relationships for H. rubra and sea urchins (Heliocidaris erythrogramma). These results emphasise that animal–habitat relationships are not necessarily stable through time and highlight the value of marine reserves as reference sites. Our work shows that fishery closures to enhance populations of commercially important and keystone species should be in areas with a range of habitat features to accommodate shifting ecological requirements with ontogenesis.
Alexander, T. J.; Johnson, C. R.; Haddon, M.; Barrett, N. S.; Edgar, G. J. (2014) Long-term trends in invertebrate-habitat relationships under protected and fished conditions, Marine Biology, 161(8), 1799-1808, doi:10.1007/s00227-014-2462-2, Institutional Repository

Catch Rates, composition and fish size from reefs managed with periodically-harvested closures

Periodically-harvested closures are commonly employed within co-management frameworks to help manage small-scale, multi-species fisheries in the Indo-Pacific. Despite their widespread use, the benefits of periodic harvesting strategies for multi-species fisheries have, to date, been largely untested. We examine catch and effort data from four periodically-harvested reef areas and 55 continuously-fished reefs in Solomon Islands. We test the hypothesis that fishing in periodically-harvested closures would yield: (a) higher catch rates, (b) proportionally more short lived, fast growing, sedentary taxa, and (c) larger finfish and invertebrates, compared to catches from reefs continuously open to fishing. Our study showed that catch rates were significantly higher from periodically-harvested closures for gleaning of invertebrates, but not for line and spear fishing. The family level composition of catches did not vary significantly between open reefs and periodically-harvested closures. Fish captured from periodically-harvested closures were slightly larger, but Trochus niloticus were significantly smaller than those from continuously open reefs. In one case of intense and prolonged harvesting, gleaning catch rates significantly declined, suggesting invertebrate stocks were substantially depleted in the early stages of the open period. Our study suggests periodically-harvested closures can have some short term benefits via increasing harvesting efficiency. However, we did not find evidence that the strategy had substantially benefited multi-species fin-fisheries.
Cohen, P. J.; Alexander, T. J. (2013) Catch Rates, composition and fish size from reefs managed with periodically-harvested closures, PLoS One, 8(9), e73383 (12 pp.), doi:10.1371/journal.pone.0073383, Institutional Repository

Estimating whole-lake fish catch per unit effort

The European standard for gillnet sampling to characterize lake fish communities stratifies sampling effort (i.e., number of nets) within depth strata. Nets to sample benthic habitats are randomly distributed throughout the lake within each depth strata. Pelagic nets are also stratified by depth, but are set only at the deepest point of the lake. Multiple authors have suggested that this design under-represents pelagic habitats, resulting in estimates of whole-lake CPUE and community composition which are disproportionately influenced by ecological conditions of littoral and benthic habitats. To address this issue, researchers have proposed estimating whole-lake CPUE by weighting the catch rate in each depth-compartment by the proportion of the volume of the lake contributed by the compartment. Our study aimed to assess the effectiveness of volume-weighting by applying it to fish communities sampled according to the European standard (CEN), and by a second whole-lake gillnetting protocol (VERT), which prescribes additional fishing effort in pelagic habitats. We assume that convergence between the protocols indicates that volume-weighting provides a more accurate estimate of whole-lake catch rate and community composition. Our results indicate that volume-weighting improves agreement between the protocols for whole-lake total CPUE, estimated proportion of perch and roach and the overall fish community composition. Discrepancies between the protocols remaining after volume-weighting may be because sampling under the CEN protocol overlooks horizontal variation in pelagic fish communities. Analyses based on multiple pelagic-set VERT nets identified gradients in the density and biomass of pelagic fish communities in almost half the lakes that corresponded with the depth of water at net-setting location and distance along the length of a lake. Additional CEN pelagic sampling effort allocated across water depths and distributed throughout the lake would therefore help to reconcile differences between the sampling protocols and, in combination with volume-weighting, converge on a more accurate estimate of whole-lake fish communities.
Alexander, T. J.; Vonlanthen, P.; Périat, G.; Degiorgi, F.; Raymond, J. C.; Seehausen, O. (2015) Estimating whole-lake fish catch per unit effort, Fisheries Research, 172, 287-303, doi:10.1016/j.fishres.2015.07.024, Institutional Repository

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Address

E-Mail: timothy.alexander@eawag.ch
Phone: ---
Fax: +41 58 765 2168
Address: Eawag
Seestrasse 79
6047 Kastanienbaum
Office: CA B12
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Research Group

Biodiversity dynamics

Projet Lac

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