Salmon-mediated nutrient flux in Snake River sockeye salmon nursery lakes: the influence of depressed population size and hatchery supplementation

Evans ML, Kohler AE, Griswold RG, Tardy KA, Eaton KR, Ebel JD. Salmon-mediated nutrient flux in Snake River sockeye salmon nursery lakes: the influence of depressed population size and hatchery supplementation. Lake Reserv Manage. XX:XX–XX.

Since the 1970s, Snake River sockeye salmon (Oncorhynchus nerka) have returned in low numbers to nursery lakes in central Idaho, consequently diminishing marine-derived nutrient subsidies to freshwater spawning and rearing environments. In healthy Pacific salmon populations, returning adults generally import more nutrients than juveniles export, resulting in net positive salmon-mediated nutrient fluxes to freshwater ecosystems. To mitigate for declining sockeye salmon returns, Snake River nursery lakes have been stocked with embryos, with captive-reared juveniles and adults, and fertilized with inorganic nutrients. Here, we examine sockeye salmon-mediated nutrient flux in 3 nursery lakes, Alturas, Pettit, and Redfish, across 20 yr of study (1998–2017). In Redfish Lake, sockeye salmon-mediated nutrient flux was positive in all years when imports from the captive-rearing program were considered in our estimates. Without captive-rearing program inputs, Redfish Lake nutrient flux was positive in only 40% and 45% of years for phosphorus and nitrogen, respectively. In Alturas and Pettit lakes, nutrient export by juveniles exceeded nutrient import by adults in 67% and 56% of years for phosphorus and nitrogen, respectively. Overall, our findings suggest that a sufficient number of adult recruits relative to juvenile migrants, corresponding to a smolt-to-adult return rate > 0.78%, is needed to achieve positive salmon-mediated nutrient flux within lakes and that this will depend on both adequate population productivity and enhanced marine and freshwater habitat connectivity.