DECLINES IN CHINOOK SALMON CATCHES IN THE STRAIT OF GEORGIA IN RELATION TO SHIFTS IN THE MARINE-ENVIRONMENT

Chinook, Oncorhynchus tshawytscha, catches in the Strait of Georgia in creased in the 1970s and reached maximum levels from 1976 to 1978. Cat ches then declined until they stabilized through regulation at levers approximately one-quarter of the 1976 to 1978 levels. The timing of th e decline in catch was synchronous with an increase in the mean temper ature of the Strait of Georgia, a decline in annual Fraser River flows , and an abrupt decrease in the marine survival of hatchery-reared chi nook released into the Strait of Georgia. Surprisingly, the number of young chinook salmon (smelts) more than doubled over the period of dec lining catches compared with the number produced during the period of high catches. The increase in smolt abundance was a consequence of the production from hatcheries that was approximately equal to wild produ ction. We conclude that there was a change in the carrying capacity fo r chinook salmon in the Strait of Georgia in the late 1970s that contr ibuted to the declines in abundance and that rebuilding stocks to the high abundance of the late 1970s is unlikely until the carrying capaci ty for chinook salmon changes either naturally or through manipulation . Although we did not separate density-dependent and density-independe nt effects on marine survival, the current total number of chinook sme lts produced appears larger than required for the current marine carry ing capacity.