Seasonal Prey Resource Partitioning Among Salmonids In The Bois Brule River, Wisconsin

Abstract

Over the past century, intentional introductions of non-native Salmonids into Lake Superior and its tributaries have raised concern about possible interactions with native Brook Trout (Salvelinus fontinalis), which have historically been subject to local extirpations. The Upper Bois Brule River, Wisconsin currently supports a diverse and abundant assemblage of Salmonid species, including Brook Trout, Brown Trout (Salmo trutta), Rainbow Trout (Oncorhynchus mykiss), Coho Salmon (Oncorhynchus kisutch), and Chinook Salmon (Oncorhynchus tshawyscha). To investigate underlying reasons for apparent co-existence of potentially competing species, I documented the seasonal diet characteristics among coexisting Salmonids and compared stomach content data to seasonal prey abundances (macroinvertebrates) in the benthos and stream drift. The primary objective was to determine if niche partitioning reduces competitive interactions between Brook Trout, Brown Trout, Rainbow Trout, and juvenile Coho Salmon in the Bois Brule River, Wisconsin. Results indicated that Brook Trout and Brown Trout had significant diet overlap during spring, summer, and fall sampling dates, while there were no instances of significant dietary overlap between the non-native species. Several positive relationships between diet overlap and the abundances of different prey categories suggest prey resource partitioning in response to competition may underlie the co-occupation of Salmond species in my study area. All Salmonid species showed positive selection for Trichoptera larvae during spring, summer, and fall. In addition, Brook Trout and Brown Trout showed positive selection for Gastropods during every season. Brook Trout were the only species with strong preferences for Crustaceans, while Rainbow Trout were the only species with preferences for Ephemeroptera nymphs. Brook Trout and Rainbow Trout exhibited opportunistic diets, while Brown Trout tended to feed primarily from the benthos and Coho Salmon fed mainly on surface-oriented prey. It’s likely that the persistence of Brook Trout in the presence of non-native Salmonids is attributed to their flexibility in foraging modes, utilization of increased invertebrate drift rates, and consumption of less preferred prey items. This study provides new information on resource utilization, niche partitioning, and resource overlap among multiple coexisting Salmonid species and may be helpful for future research and management with sympatric populations of native and non-native Salmonid species.