Comparison of diet overlap and prey selectivity within different habitats among exotic and native salmonids in a Lake Superior tributary

The introduction of nonnative salmonids may negatively affect native brook trout (Salvelinus fontinalis) abundance and population dynamics in tributaries of Lake Superior used for spawning and rearing habitat by all three species.  Stream salmonids mainly feed on drifting invertebrates and competition for food may impact native fauna.  Field experiments were performed to determine the composition of invertebrate drift and juvenile salmonid diet year-round across habitat types.  The invertebrate abundance in the drift was ~33% higher in low gradient reaches and 70% greater in high gradient reaches than in beaver ponds. Invertebrate drift was highest in July and August dropping by ~50% during December and January.  Diet overlap was highest (0.63) during age 1 between steelhead (Onchorhynchus mykiss) and brook trout.  Diet overlap showed greatest overlap in winter/early spring in the beaver ponds; the greatest overlap in the summer/fall was in the high and low gradient areas.  The selectivity index based on biomass showed selection for similar prey items including larval Chironomidae, adult dipterans, and Baetidae.  Our data suggests competition for food resources may exist among the different age classes within and among our study species, and it may be dependent on age and species specific seasonal movements.