Modeling feeding and growth in Gulf of Alaska sockeye salmon: implications for high-seas distribution and migration

I analyzed data on stomach contents of sockeye salmon Oncorhynchus nerka and oceanographic variables collected during 1956 to 1964 in order to assess the spatial and temporal patterns of salmon feeding and growth in the Gulf of Alaska. A 2-stage statistical model was used to develop a feeding index, calculated as the product of feeding probability (assessed using the presence/absence of gut contents of individuals collected at each site) and stomach fullness (prey weights transformed to account for temperature-dependent gastric evacuation and allometry). Combining these results with simple bioenergetic principles, I conclude that the reduction in salmon biomass during spring observed along the southern periphery of the Gulf can be explained by an equally sharp reduction in growth potential. This feature demarcates a region (between latitude 58 and 55degreesN) that is favorable to sockeye growth, as demonstrated by an order of magnitude change in salmon growth potential over a narrow range of sea surface temperatures (ca. 6 to 7degreesC). This pattern results from the combined effects of trends in feeding probability and stomach fullness across this temperature gradient. I documented a significant trend of larger fish in cooler water during spring, which might have been related to body size dependent competitive interactions. The trend was reversed in summer, with larger fish in warmer water, perhaps due to the availability of preferred prey (e.g. squid) and directional swimming behavior characteristic of sockeye spawning migration. Assuming salmon are growth-optimizers in this environment, my results suggest that winter residency for sub-adults is restricted to north of latitude 54degrees N. Furthermore, peaks in feeding and growth shifted from a central position in the gulf during spring toward the southeast during summer, with an attendant increase in forage quality. I hypothesize that these patterns are likely to be important in drawing maturing British Columbia salmon toward their natal rivers and providing the energy needed to fuel their costly river migration during mid-summer.