Ecological Effects of Live Salmon Exceed Those of Carcasses During an Annual Spawning Migration

We tested the hypothesis that the carcasses of anadromous Pacific salmon (Oncorhynchus spp.) constitute a significant source of nutrients in the nutrient-poor freshwaters where these fish migrate, spawn, senesce, and die. In a 110 m-long stream reach in Southeast Alaska, we retained nearly 3000 salmon carcasses and compared streamwater nitrogen (N), phosphorus (P), and the biomass of benthic biofilm in this reach with an upstream reference reach. The study spanned 5 months, bracketed the entire salmon run, and encompassed significant seasonal variation in abiotic stream conditions. Concentrations of dissolved and particulate N and P followed distinctly unimodal patterns through time, which tracked the abundance of live salmon, and we observed strong predictive relationships between live-salmon abundance and streamwater-nutrient concentrations. In contrast, we did not observe clear relationships between salmon carcasses and streamwater nutrients. Biofilm biomass within our study reaches seemed to more closely track the abundance of live salmon than the abundance of carcasses. The experimental retention of carcasses had a minor or undetectable influence on nutrient concentrations and biofilm within the study reach as compared to the reference reach. We conclude that physical factors such as temperature, discharge, nutrient limitation, and irradiance vary seasonally in ways that maximize the influence of nutrients provisioned by live salmon and minimize the influence of carcass-derived nutrients on the aspects of stream ecosystems that we examined. Overall, our results promote a new perspective on the ecological role of salmon in freshwaters, and contribute to a more mechanistic understanding of how migratory fishes can influence aquatic ecosystems.