P1: The stickleback in Upper Lake Constance

The massive increase of the non-native three-spined stickleback (Gasterosteus aculeatus) in Lake Constance, as well as the nutrient decline within the course of lake re-oligotrophication, has strong negative effects on the native fish community as well as local fisheries. After more than 50 years of inconspicuous existence, sticklebacks now represent more than 80% of the fish stock in Upper Lake Constance (96% in open water (pelagic zone), the habitat of the originally dominant whitefish). The invasive stickleback is a direct competitor for food, particularly for whitefish. It also feeds on eggs and larvae of other fish species, which might cause recruitment of native fish species to decrease. Such a development displays a unique special case in fisheries biology for a large, nutrient-poor (oligotrophic) lake like Lake Constance. The reasons for the mass occurrence of the three-spined stickleback are unknown.

In this research project, we will investigate the autecology of the three-spined stickleback in Lake Constance. Acoustic surveys with parallel trawl nets, to verify the hydroacoustic data obtained, will be carried out with the aim of distinguishing sticklebacks from other fish species in the lake and, thus, obtaining a reliable stock assessment. We will also conduct gut content analysis of the three-spined stickleback, in order to evaluate niche overlap and potential direct damage by interfering with the recruitment of native fish species. In addition, we will investigate the seasonal infestation with the parasitic tapeworm Schistocephalus solidus. The age of the three-spined stickleback will be determined, to gain insights into the growth of this species in Lake Constance. We will also investigate the possibility of different types of Lake Constance sticklebacks using geometric morphometry (landmarks). Furthermore, targeted fishing (gill and trawl net, electric fishing) in the littoral and pelagic zone will be used to examine the spatial spawning behaviour as well as the habitat overlap, and to estimate abundance.