Patterns of covariation in survival rates of British Columbian and Alaskansockeye salmon (Oncorhynchus nerka) stocks

We used a multi-stock comparison to identify spatial and temporal character istics of environmentally driven sources of variability across four decades in the productivity of 29 sockeye salmon (Oncorhynchus nerka) stocks from British Columbia (B.C.) and Alaska. We examined patterns of covariation amo ng indices of survival rate (residuals from the best-fit stock-recruitment curve) and found positive covariation among Fraser River sockeye stocks (so uthern B.C.) and, to a greater extent, among Bristol Bay stocks (western Al aska) but no evidence of covariation between these two regions or with stoc ks of other regions in B.C. and Alaska. This indicates that important envir onmental processes affecting variation in sockeye survival rate from spawne rs to recruits operate at regional spatial scales, rather than at the large r, ocean-basin scale. The observed covariation in survival rates of Bristol Bay stocks appears to be due to a combination of both freshwater and, to a greater degree, marine processes. Bristol Bay sockeye stocks showed a dram atic and persistent increase in survival rates coinciding with the abrupt c hanges in the North Pacific environment in the mid-1970s; however, there wa s little evidence of a similar response for Fraser River stocks.