Survival rates of coho (Oncorhynchus kisutch) and chinook salmon (O. tshawytscha) released from hatcheries on the U.S. and Canadian Pacific coast 1972-1998, with respect to climate and habitat effects

Smolt-to-adult survival rates were estimated for 18 659 coho and chinook coded wire tag (CWT) groups released in 1972–1998 from 206 hatcheries on the U.S. and Canadian Pacific coast. Survival rates of 153 wild CWT groups showed similar trends as those of hatchery fish. The long-term trend or both coho and chinook was a decline in all regions south of Alaska, while survival rates increased in Alaska. Regional and annual variation explained 46% of the total variation for coho, 34% for fall chinook, and 42% for spring chinook. Regression analysis was used to explore the relationship between survival rate and climate during the year of release, and the variable that showed the strongest relationship was summer sea surface temperature (SST) at the place where the fish reach the ocean. The estimated relationship is quadratic in log space, with an optimum around 13°C for coho (95% confidence interval: 12.87°C–13.08°C) and fall chinook (12.44°C–13.42°C), but such an optimum could not be accurately determined for spring chinook (2.56°C–12.24°C). The SST variable alone explained 41% of the regional and annual variation of coho survival rates, only 12% for fall chinook, but 44% for spring chinook due to low survival rates at high SST values. Little is known about the ecological dynamics that link SST and survival rate, but SST is highly correlated with a suite of physical and biological factors in the ocean. There has been a long-term increase in SST from the early 1970s to the late 1990s, corresponding to the declining survival rates south of Alaska and increasing survival rates in Alaska. During a cooler period in the mid 1980s, survival rates increased for some years south of Alaska, but decreased in Alaska. These results suggest that the decline in wild salmon abundance in the 1990s was due in considerable part to changes in ocean conditions and increases in wild stock abundance may be expected if ocean conditions change.