Seasonal changes in pelagic fish biomass around the Chiswell Island Steller sea lion rookery in 2003

Fisheries acoustics surveys were conducted around the Chiswell Island rookery in the northern Gulf of Alaska at night in April and August 2003 to assess seasonal changes in prey available to Steller sea lions (Eumetopias jubatus) foraging around the rookery. Adult walleye pollock (Theragra chalcogramma) ≥28 cm fork length was the dominant biomass in the upper 50 m of the water column in both months, increasing from 122.8 kg/nmi² in April to 457.9 kg/nmi² in August. A similar pattern was observed for Pacific herring (Clupea pallasii), which averaged 2.8 and 65.6 kg/nmi² in April and August, respectively. Incidental trawl catch suggested the appearance of age-0 pollock and juvenile salmonids (Oncorhynchus spp.) around the rookery in August as well. The increased biomass of these key prey species is linked to increased foraging trip durations by lactating sea lions from Chiswell Island, and supports the general view that sea lions in the northern Gulf of Alaska are not food limited during summer months.