Trophic ecology of nonanadromous rainbow trout in a post-glacial lake system: partial convergence of adfluvial and fluvial forms

Abstract

The rainbow trout (Oncorhynchus mykiss (Walbaum, 1792)) is one of many salmonid species exhibiting a gradient of life histories including fluvial (stream-resident), anadromous (ocean-migrant), and adfluvial (lake-migrant) forms, the last of which is less extensively studied than the other two. Our goal was to determine the extent of diet segregation between fluvial and adfluvial rainbow trout. We collected stomach content and stable isotope data on rainbow trout sampled in stream and lake habitats in a southwestern Alaska watershed during summer and compared them with data on sympatric stream- and lake-specialist char species, Dolly Varden (Salvelinus malma (Walbaum in Artedi, 1792)) and Arctic char (Salvelinus alpinus (Linnaeus, 1758)), respectively. Rainbow trout in streams fed largely on aquatic insects, while those in the lake ate primarily benthic snails and amphipods. The trophic segregation of stream-resident and lake-migrant rainbow trout mirrored but was less extreme than the divergence of lotic Dolly Varden and lentic Arctic char in the same system. Spawning sockeye salmon (Oncorhynchus nerka (Walbaum in Artedi, 1792)) provided a nutrient subsidy in the form of eggs that supported rainbow trout in both stream and lake (littoral) habitats, causing their isotopic signatures to converge. This study augments knowledge of partial migration and trophic divergence within populations.