Effects of food type, habitat, and fish predation on the relative abundance of two amphipod species, Gammarus fasciatus and Echinogammarus ischnus

We evaluated the abundance patterns of Gammarus fasciatus and Echinogammarus ischnus in dreissenid and macrophyte areas in Hatchery Bay, Lake Erie before (1997) and after round goby (2001, 2002) invaded the area. Total amphipod abundance was higher before round goby invasion in both habitats. In mussel beds, E. ischnus abundance was similar or significantly higher than G. fasciatus. In macrophytes, G. fasciatus was significantly more abundant than E. ischnus. In laboratory experiments, we compared amphipod survivorship and growth when fed mussel feces and pseudofeces (F+P) or macrophytes with epiphytes (M+E). Gammarus fasciatus survivorship and growth were higher when fed F+P than M+E. Echinogammarus ischnus showed similar survivorship under both diets, but significantly higher growth when fed M+E than F+P. Therefore inter-habitat differences in food resources cannot explain the abundance patterns observed in the lake. We also estimated the relative vulnerability of G. fasciatus and E. ischnus to yellow perch (Perca flavescens) and round goby (Neogobius melanostomus) predation in laboratory feeding trials using mussel colonies or macrophyte beds as substrate. Both fish strongly preferred E. ischnus in macrophytes, but consumed relatively more G. fasciatus than E. ischnus in dreissenid habitats. Our results suggest that dreissenid establishment may have facilitated the invasion of E. ischnus. However, habitat-specific differences in vulnerability to fish predation may mediate the coexistence of G. fasciatus and E. ischnus by minimizing expansion of E. ischnus to macrophyte areas. Our results also suggest that round goby invasion can alter amphipod abundance patterns in Lake Erie.