Mass balance analysis of salmon-derived nutrients in a managed river system: Nutrient subsidies from spawning adults and hatchery smolts

Anadromous salmon represent a potentially important influx of nutrients to the streams in which they spawn.  To date the majority of studies quantifying the net load of salmon-derived nutrients (SDN) to inland streams have focused on wild salmon populations with little consideration of effects that stocking may have on annual input or export of SDN.  We conducted a mass balance analysis of SDN for the upper Snake River watershed from 2002 to 2007. Using Chinook salmon as a model species, we account for yearly SDN input from 1) returning adult salmon and 2) mortality of stocked smolts.  We account for yearly export or loss of potential SDN via 1) emigration of wild smolts, 2) growth and subsequent emigration of hatchery origin fall Chinook smolts, 3) adult broodstock collection, and 4) harvest.  Mortality of stocked smolts contributed substantially to the net SDN load in five of the six years, accounting for up to 20% of the total load of SDN in 2005, 2006, and 2007, when adult returns were particularly low. The mass balance analysis does not, however, account for where and how SDN is added.  The influence of SDN from smolt mortality may therefore differ from SDN from adult mortality.