Sockeye Spawning Presence and Stream Temperature Interact to Control Juvenile Coho Growth and Condition in a SW Alaska Watershed

Robust returns of Pacific salmon (Oncorhynchus spp.) to spawning grounds in SW Alaska provide an enormous resource subsidy to consumers in freshwater ecosystems. However, variation in stream thermal regimes, in addition to patchiness in spawning activity across the landscape, constrains the ability of ectothermic consumers such as resident fishes to consume and assimilate eggs and carcasses.  We investigated how spatial variation in sockeye salmon (O. nerka) spawning and stream temperature affect growth, energetic status, and fatty acid composition of juvenile coho (O. kisutsh), a known egg predator, in 7 streams in the Wood River watershed, SW Alaska. We compared mean body size of juvenile coho in late summer among multiple years and found that positive temperature effects on growth were mediated by sockeye spawning presence, wherein the largest juvenile coho were found in warm streams where sockeye spawned. Individuals that consumed more salmon eggs (estimated from stable isotope ratios), were larger (p < 0.001, R² = 0.13), and had altered fatty acid composition, but there were no significant effects on condition factor or total fat accumulation. These results indicate that benefits of salmon subsidies to aquatic consumers depend strongly both on temperature and on individual variation in energy allocation.