Quantifying Salmon‐Derived Nutrient Loads from the Mortality of Hatchery‐Origin Juvenile Chinook Salmon in the Snake River Basin

Abstract

AbstractHatchery supplementation of anadromous salmon is extensive across the Pacific Northwest region with millions of juvenile salmon stocked annually. The influence of hatchery‐origin fish as prey items in recipient ecosystems has been explored, but influences of these fish on broader stream nutrient dynamics has not been well‐studied. Salmon‐derived nutrients (SDN) associated with the mortality of adult anadromous salmon provide key subsidies to freshwater habitats. While a number of studies have estimated current and historic SDN loading from returning wild salmon, SDN contributions from the mortality of hatchery‐origin juveniles (many of which die in the stream prior to emigration) remains largely unknown. We conducted a mass balance analysis of SDN input and export via hatchery activities (stocking and broodstock collection) in the Snake River watershed. Using Chinook salmon Oncorhynchus tshawytscha as a model species, we accounted for yearly SDN input (via hatchery‐origin juvenile fish mortality) and export (via broodstock collections and presmolt growth) over 6 years (2002–2007) in the portion of the Snake River upstream from Lower Granite Dam accessible to anadromous fish. In the year with highest smolt mortality (2003), hatchery‐origin smolt mortality provided a net input of SDN equivalent to approximately 8,100 returning adults. In the year with lowest smolt mortality (2004), hatchery activities collectively yielded a net loss of nutrients. Although the mass of SDN from hatchery‐origin smolts may be presented in adult equivalencies, functional influences of SDN from hatchery smolt mortality are likely to differ. Salmon‐derived nutrients from hatcheries enter food webs through largely piscivorous pathways whereas SDN from adult carcasses enter food webs through multiple pathways at multiple trophic levels. The SDN from hatchery‐origin smolts probably influence different components of the food web more than do adult carcasses and have the potential to more directly affect predator populations.