Foraging habits of invasive three-spined sticklebacks (Gasterosteus aculeatus) – impacts on fisheries yield in Upper Lake Constance

A massive increase in the pelagic population of non-endemic three-spined sticklebacks, Gasterosteus aculeatus L. in Lake Constance has coincided with drastic declines in fishery yields. This study assesses the possible direct and indirect impact of the mass occurrence on native fish species in the lake. Laboratory foraging experiments showed that larvae of roach Rutilus rutilus L., perch Perca fluviatilis L. and whitefish Coregonus lavaretus L. are accessible to sticklebacks as food. However, distinct species effects were apparent, with whitefish showing no effective predator avoidance strategy and therefore experiencing drastically increased mortality risk compared to the other predator adapted prey species. Furthermore, in absence of larval prey, sticklebacks were shown to feed predominantly on Daphnia in the field, indicating a strong interspecific food competition with whitefish. The results suggest that sticklebacks, acting as both an invasive species and a predator in the pelagic zone where prior no predator or neozoon existed, create a unique challenge to the unadapted autochthonous fish, and thus threaten the ecological resilience of the lake. Impacts on recruitment, e.g. through larval predation and interspecific competition for zooplankton could explain recent drastic declines in fishing yields.