Comparison of Diet Selectivity in a Lake Superior Tributary

The introduction of two nonnative salmonids, steelhead trout (Oncorhynchus mykiss) and coho salmon (O. kisutch), may be affecting native brook trout (Salvelinus fontinalis) abundance and population dynamics in tributaries of Lake Superior.  These tributaries are used for spawning and rearing habitat by all three species; space and food resources are shared at various life stages.  In streams, salmonids feed mostly on drifting invertebrates and competition for food resources is commonly invoked as one method of exotic species impact on native fauna.  Field experiments were performed to determine the composition of invertebrate drift and diet for the three species, ages 0 and 1, during spring, summer and fall in beaver ponds,  and low and high gradient environments in Pictured Rocks National Lakeshore on the south shore of Lake Superior.  Diet preference was determined by calculating selectivity according to the linear food selection index (L) of Strauss (1979).  Mean selectivity was calculated in order to allow comparisons among the different fish groups.  Fish consistently avoided the same items by month and habitat, however, selected items varied both within months and habitats.  There were significantly more items avoided than selected by all fish categories in all months and all habitats (p<0.001).  Age 0 fishes (coho, steelhead, and brook trout) avoided fewer items than the age 1 fishes.  Selection often reflected temporal hatches of prey items or spatially limited prey items; in November all fishes in all habitats selected adult Philopotamidae which have a late fall hatch in our region and fishes in both the high and low gradient areas selected for coho eggs.  Fishes in the beaver pond areas did not select for coho eggs because adult coho were unable to access those areas in large enough numbers to add to the prey resource pool.  In June all fishes in all habitats selected adult Empididae but only fishes in the high gradient areas where longnose suckers (Catostomus catostomus) deposited eggs selected for the sucker eggs.  Our data suggests that there is potential for competition for food resources among the different age classes within and among our study species because they often select for similar prey items.