Of all the great spectacles of nature, animal migration is without doubt one of the most remarkable. It has fascinated mankind for thousands of years, with reports of migratory fishes seasonally moving in and out of the Black Sea being documented over twenty centuries ago. The study of migration continues to be a major focus of contemporary biological research, and we now know that migration is a ubiquitous feature in the life cycle of an extremely diverse range of animals, from microscopic crustaceans to large, sea-dwelling mammals.
When living in an environment where alternative habitats change their relative profitability either during the season, e.g. due to temperature changes or during an animal’s ontogeny, habitat shifts carried out through migration will often be an evolutionary superior strategy over residency. However, although analyses of changes in relative costs and benefits of different habitats may well predict the timing of migration of the average individual, it has rarely been applied to individual organisms, explaining why often only a part of a population migrates (partial migration), why individuals migrate at different times or to different locations (differential migration) or maybe most interestingly, why populations differ in their migration patterns (both partial- and differential migration).
Cyprinid fish often migrates from shallower lakes into streams during winter to escape predation risk during winter, where their growth rates are low due to low temperatures and where they therefore cannot profit from the higher food abundance in the lakes. However, most often only a part of the fish populations are migrating (i.e. partial migration) and among the migrants, timing, duration and destination of migration are different (i.e. differential migration). Since the cyprinid fish are often dominant species in the lake ecosystems with high ecological impact, their temporary absence from the lakes are affecting both lower trophic levels of the lake and resident top-predators. Hence, the study system offers a novel opportunity to study the interactions between differentiation in individual life history and ecosystem structure and dynamics.