IMPACT OF CRAYFISH PREDATION ON EXOTIC ZEBRA MUSSELS AND NATIVE INVERTEBRATES IN A LAKE-OUTLET STREAM

To test the hypothesis that Orconectes spp. crayfishes could be a sign ificant source of zebra mussel (Dreissena polymorpha) mortality in str eams, we conducted a laboratory experiment and a field experiment. In the laboratory experiment, maximum zebra mussel size consumed increase d with increasing crayfish size for Orconectes rusticus, Orconectes pr opinquus, and Orconectes virilis. Slopes differed among crayfish speci es but not between O. rusticus sexes. To quantify crayfish impacts on seminatural zebra mussel populations, we placed 16 enclosures (0.2 m(2 )) lined with artificial cobble substrata in a midwestern lake-outlet stream (mean width 17 m). Zebra mussels were stocked in each cage (20 000 individuals/m(2)) 14 days before male O. rusticus (20 individuals/ m(2)) were stocked in each of eight enclosures. Densities of all sizes of zebra mussels were reduced in enclosures, with a total density red uction of 31% over the 28 days of the experiment, relative to the excl osures. Crayfish also reduced gastropod densities by 54%, but did not significantly affect other invertebrates. Laboratory and field results suggest that predation by Orconectes crayfishes may significantly red uce zebra mussel populations in streams.