Influence of Salmon Abundance and Ocean Conditions on Body Size of Pacific Salmon

摘要

After the North Pacific ocean climate change in 1976-77, most species of Pacific salmon (Oncorhyn-chus spp.) in North America and Asia increased in abundance and declined in body size up until the early 1990s. Several authors attributed this decline in body size of chum salmon (O. keta) to increasing population density of chum salmon in the ocean. In the mid-1990s, the body size of adult chum salmon increased in several streams in North America in spite of high population numbers. To determine if these increases in body size were restricted to local areas or more widespread geographically in North America, we examined data on the abundance and mean body size of salmon from commercial catches in waters from northern Alaska south to the state of Oregon among three time periods (1960-76,1977-94,1995-2006). Trends in body size indicate that northern and southern populations of chum, pink, and sockeye, and coho in Washington and Oregon experienced increased body size in the mid-1990s. In correlation analyses, chum, pink (O. gorbuscha), and sockeye salmon (O. nerka) body size was, in many cases, negatively related to abundance, and more negatively correlated during the 1977-94 period. For the 1960-76 period, the abundance of Pacific salmon was low, and the effect of density-dependence on the fish was the lowest. For the 1977-94 period, salmon were numerous and the effect of density on the body size of salmon was significant in many cases. For the 1995-2006 period, the abundance of salmon remained high, however, the body size of the salmon was not commonly related to population density. The 1995-2006 period appears most favorable for salmon, in that ocean resources supported salmon of large body size and large population numbers. We conclude that the carrying capacity of the North Pacific Ocean for producing Pacific salmon is not a constant value and varies with changing environmental and biological factors.