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

We used a multi-stock comparison to identify spatial and temporal characteristics 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 among indices of survival rate (residuals from the best-fit stock-recruitment curve) and found positive covariation among Fraser River sockeye stocks (southern B.C.) and, to a greater extent, among Bristol Bay stocks (western Alaska) but no evidence of covariation between these two regions or with stocks of other regions in B.C. and Alaska. This indicates that important environmental processes affecting variation in sockeye survival rate from spawners to recruits operate at regional spatial scales, rather than at the larger, 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 dramatic and persistent increase in survival rates coinciding with the abrupt changes in the North Pacific environment in the mid-1970s; however, there was little evidence of a similar response for Fraser River stocks.