Ecological interactions among the 'exotic triad' Eurasian ruffe (Gymnocephalus cernuus), round gobies (Neogobius melanostomus), and zebra mussels (Dreissena polymorpha), and their effects on native yellow perch (Perca flavescens)

Eurasian ruffe, round gobies, and zebra mussels (the ‘exotic triad’) are recent invaders of the Great Lakes nearshore ecosystem. The goal of this study was to elucidate the potential interactions among these species and to determine their effects on native yellow perch. Laboratory experiments were used to investigate the (1) effect of zebra mussels on fish growth, (2) strength of interspecific competition among perch, gobies, and ruffe, (3) relative vulnerability of these fishes to predation, and (4) food and habitat preferences of perch, gobies, and ruffe. Field surveys assessed current food and habitat overlap among these fishes in lakes Superior, Huron, and Michigan. Four major predictions were advanced. First, fish growth would decline in mussel habitats because of increased habitat heterogeneity. Second, gobies would have a competitive advantage over perch and ruffe in the presence of live mussels because gobies consume mussels. Third, gobies would be the preferred prey of piscivorous fish, thereby decreasing their potential effects on perch and ruffe. Fourth, perch, gobies, and ruffe would overlap in food use, but habitat use would differ. Fish growth in laboratory experiments was generally unaffected by the presence of mussels, except that goby growth tended to be positively affected by mussels. Overall, fish growth declined at low food levels representative of an invasion scenario, but ruffe growth declined more than goby growth when the species co-occurred. Perch growth was similar to goby growth when sympatric, but declined at low food levels in the presence of all three invaders. Smallmouth bass consumed fewer gobies than either perch or ruffe. Thus, piscivores could exacerbate the negative effects of the ‘exotic triad’ on perch through sustained predation pressure. Laboratory experiments and field observations suggest that perch can co-occur with gobies and ruffe in macrophyte and sand habitats where their ranges overlap, and that they consume similar soft-bodied benthic invertebrate prey. As gobies and ruffe spread, they may increasingly compete with yellow perch for food and habitat, thereby reducing perch growth rates in zebra mussel-colonized nearshore areas. Thus, yellow perch populations may be sustained only by stocking, decreased fishing pressure, and control of nonindigenous species.