2004. Evidence that reduced early marine growth is associated with lower marine survival on coho salmon

Abstract.—Coho salmon Oncorhynchus kisutch from the Strait of Georgia were used to test the hypothesis that slower growing fish in their first ocean year had lower survival over the late fall and winter than faster growing fish. The Strait of Georgia provided a suitable area for this study because it is a semi-enclosed rearing area for juvenile Pacific salmon that is distinct from the open marine rearing areas off the west coast. Coho salmon that survived the winter had significantly larger spacing between circuli on scales, indicating that brood year strength is related to growth in the first marine year. Other studies have shown that smaller fish of a cohort are less able to survive periods of energy deficit than larger fish. Thus, size-related mortality in the first marine fall and winter may be an important determinant of brood year strength of some coho salmon stocks and stocks of other species of Pacific salmon. In previous publications we proposed that most natural marine mortality of Pacific salmon Onco-rhynchus spp. in general and coho salmon O. kis-utch in particular occurred in two major episodes (Beamish and Mahnken 1999, 2001). The first ep