Expansion of cultured Pacific salmon into marine ecosystems

Abstract

Global harvest of salmon has increased steadily since the mid-1970's. Record harvest levels are currently being established largely from production of farmed and ranched salmon. Farmed salmon are hold captive throughout their lives, whereas reached salmon are released as juveniles into natural ecosystems where they mix with wild salmon. Observation of a nonlinear adult-to-smolt relationship in ranched salmon can be interpreted as evidence that marine waters have been overstocked, provided the ratio of adults to smolts declines as the number of smolts increases. Three ranching programs — Oregon coho, Japan chum, and Alaska pink salmon — are examined here for evidence that ocean mortality from natural causes is directly dependent on abudance of smolts. Interpretation of the data does not support the hypothesis that marine waters have been overstocked with hatchery smolts to a level where survival of salmon has been reduced. However, the possibility that industrial production of ranched salmon smolts is reducing, or may soon reduce, survival of fishery resources should not be ignored.