Potential Forage Competition Between Introduced Pacific Salmonids And Native Brook Trout As Determined Through a Diet Comparison of Juveniles in a Lake Superior Tributary

To assess the potential for forage competition, a diet comparison of native and introduced salmonid species in a tributary of Lake Superior in Pictured Rocks National Lakeshore was performed using stomach contents obtained using non-lethal gastric lavage.  The three target species included native brook trout Salvelinus fontinalis, who are currently the subject of a rehabilitation project, steelhead trout  Oncorhynchus mykiss and coho salmon Oncorhynchus kisutch, which were removed from the stream during the 2008-2010 sampling seasons. Stomach contents were assessed for organisms selected from the drift and the benthos and compared to available forage based on seasonal and habitat specific collections. It was expected that evidence of interspecific competition between brook trout and the Pacific salmonids would be strong due to the lack of shared evolutionary histories. All species primarily fed on invertebrates in the drift; however, benthic feeding was suggested under specific conditions.  Drift was dominated year round by chironomid midge larvae while the summer months had high proportion of dipteran adults. Winter drift samples showed a large number of plecopterans and ephemerellid mayflies.  Benthic feeding focused on gastropods and oligochaetes.  Our data suggests that there is a potential for diet overlap among juvenile salmonids and that exotic species may be significantly impacting native brook trout foraging.