UPPER THERMAL LIMITS ON THE OCEANIC DISTRIBUTION OF PACIFIC SALMON (ONCORHYNCHUS SPP) IN THE SPRING

Pacific salmon are normally thought to be distributed throughout the Subarctic Pacific, an area where they form the dominant fish fauna. We use a series of generalized additive models to show that salmon exhibit a sharp step-function response to temperature in the oceanic eastern north Pacific in spring. The critical temperature defining the southern boundary varied by species: 10.4 degrees C for pink and chum salmon, 9.4 degrees C for coho salmon, and 8.9 degrees C for sockeye salmon. These thermal limits occur well to the north of the southern boundary of the Transition Zone, at widely separated geographic positions within the Subarctic Domain, and at temperatures much lower than the lethal upper limit for each species. The sharp decline in abundance with temperature, and the remarkably low temperatures at which the response occurs, suggests that thermal barriers form an effective limit to the offshore distribution of salmon in spring, and can limit the distribution of Pacific salmon to a relatively small area of the Subarctic Pacific. The strength of this response is presumably the direct result of strong evolutionary selection. Future temperature changes in the North Pacific could therefore have a direct impact on the production dynamics of Pacific salmon.