Many species experience predictable fine-scale bottlenecks early in life, limiting population-scale viability. Juvenile anadromous salmonids (Oncorhynchus spp.) in temperate streams face a strong bottleneck in early summer after emergence and the onset of exogenous feeding due to constraints in both space and resources, spurring density-dependent growth limitations and emigration in rearing streams. Rearing streams were historically subsidized by the carcasses of multiple anadromous fish species with specific spawning phenologies. Theory suggests that increasing resources during this period could increase rearing density and growth rates and decrease intraspecific aggression and emigration. We tested whether a high-quality trophic resource from a historic marine nutrient subsidy could increase YOY chinook salmon (O. tshawytscha) growth and density while decreasing intraspecific aggression and emigration using a flow-through mesocosm system. Mesocosm channels simulated pool habitat and received either one post-spawn Pacific lamprey (Entosphenus tridentatus) carcass or were held as controls. Juvenile salmon were stocked at a known density and we tracked growth, behavior, and emigration for four weeks of carcass decay.
Results/Conclusions
We found that the presence of lamprey carcasses had weak overall effects on intraspecific aggression and feeding rates, but significantly decreased emigration rates during the first three weeks of carcass breakdown. Juvenile salmon growth rates were similar in control and treatment conditions and little change in overall body morphology was detected. This work represents some of the first to explore the role of a historic summer-timed carcass resource on juvenile salmon growth, behavior, and emigration. Recovery of Pacific lamprey in the range of ESA-listed Pacific salmon species may alter fine-scale salmon rearing densities, downstream dispersal rates, and growth during early summer. Understanding role of resource phenology and fine-scale constraints on early life history bottlenecks can improve conservation and restoration of species experiencing strong density-dependence. Restoring historic resources with specific phenologies may support or stabilize consumers facing life history-specific bottlenecks.