Storskarvens (Phalacrocorax carbo spp.) predation på Krankesjöns partiellt migrerande fiskpopulation

Partial migration, when only parts of a population migrate and others stay resident, is a common phenomenon in nature. The underlying explanation for the behavioural differences within a population remains a research question and several recent studies have highlighted the role of predation. The present study is based on data collected from thousands of fish, individually marked with passive integrated transponder tags over a decade. By using passive telemetry the migratory behaviour of the fish have been monitored. Data from tags were recovered from known cormorant (Phalacrocorax carbo spp.) roosting sites in Lake Krankesjön and Lake Vombsjön, located in southern Sweden. Results show that the fish stocks in Lake Krankesjön are under the influence of predation from piscivorous birds and that roach (Rutilus rutilus) avoid cormorant predation by migrating into the connecting streams. The total number of days a roach spent out of the lake was negatively related to the probability of being preyed upon by cormorants. Moreover, our results show that the cormorant, more extensively preys on common rudd (Scardinius erythrophthalmus) than on roach. Migration has an impact on the fate of individual fish as well as on the structure of whole ecosystems. Cormorant numbers increase all over Europe and therefore, also the additive effects on the predation pressure of prey fish. Hence, future studies that try to explain how predation affects migratory behaviour in fish should consider cormorant predation as an important factor