A review of size trends among North Pacific salmon (Oncorhynchus spp)

The abundance of North Pacific salmon (Oncorhynchus spp.) has nearly doubled during the period 1975-1993. As salmon population numbers have increased, there have been corresponding decreases in average adult size at return (maturity). As nearly all of the growth of Pacific salmon occurs in the ocean, the ocean plays an important role in determining salmon abundance. We found that 45 of 47 North Pacific salmon populations, comprising five species from North America and Asia, are decreasing in average body size. Total salmon production correlated well with environmental trends between 1925 and 1989, but the inverse relationship between population abundance and average size during the period 1975-1993 indicates that there is a limitation to the salmon-sustaining resources of the ocean. The increased ocean survivorship and expansion of enhancement programs in the 1980s and early 1990s are probable factors in the ocean-wide reduced size of salmon. If these trends continue, the productivity of salmon populations may decrease as fecundity, egg size, and age at maturity change in response.