Juvenile Diet Selectivity of Three Salmonid Species in a Lake Superior Tributary

Sympatric native and exotic juvenile salmonid diets were assessed to determine the potential for interspecific competition and niche differentiation. All species primarily fed on drift invertebrates; however, benthic feeding (increased relative abundances of oligochaetes and gastropods in diet) was suggested under specific conditions, possibly indicating niche differentiation. From May-November of 2010 benthic samplers, drift nets, and gastric lavage were used to collect samples to calculate diet selectivity indices for benthic and drift prey items in the diet of native brook trout Salvelinus fontinalis and exotic steelhead Oncorhynchus mykiss and coho salmon Oncorhynchus kisutch in a tributary of Lake Superior in Pictured Rocks National Lakeshore.   Stomach contents and available forage samples were assessed for abundance and biomass of drift and benthic organisms.  Available forage was compared to prey items based on seasonal and habitat specific collections. Strong evidence of interspecific competition  between brook trout and Pacific salmonids was expected due to lack of shared evolutionary histories.  Drift was dominated year round by larval chironomids while the summer showed a high proportion of dipteran adults. Winter drift showed a large number of larval plecopterans and ephemeropterans.  Benthic feeding focused on gastropods and oligochaetes.  Evidence of species-specific benthic feeding may point to a shift in foraging tactics due niche differentiation caused by the historic introduction of exotic salmonids to a stream with an established native salmonid population of the same feeding guild.