Foraging ecology of walleye and brown trout in a Great Lakes tributary

The role of alternative prey on predator diet selection and survival of juvenile (parr) Chinook salmon (Oncorhynchus tshawytscha) is not well understood in the Laurentian Great Lakes. Therefore, measures of predator foraging ecology (prey species and size selection), prey densities, and functional response relationships were determined for adult walleye (Sander vitreus) and brown trout (Salmo trutta) (hatchery-reared) feeding on parr and alternate prey in the Muskegon River, a tributary of Lake Michigan, USA, from 2004 to 2007. Walleye selected for smaller than average brown trout and rainbow trout (hatchery-reared) but walleye prey size (within-prey) was independent of predator size. In general, walleye showed neutral selection for all prey species but, in some years, showed positive selection for rainbow trout and negative selection for parr. Hatchery-reared brown trout selected the smallest parr in the environment although prey size was independent of predator size. Parr were positively selected by brown trout only in April. Functional response curves were fit to describe the consumption of parr and other prey types by walleye (type II) and brown trout (type I). Interactions among rainbow trout, walleye, and brown trout favored parr survival, i.e. the presence of alternate prey (rainbow trout) significantly influenced walleye predation on parr, while brown trout appeared to become quickly limited by size or escape ability of parr. Our results should enhance understanding of food web dynamics in Great Lakes tributary habitats.