Seasonal Differences in Prey Availability Around a Steller Sea Lion Haulout and Rookery in the Gulf of Alaska

Abundance and distribution of fish biomass were surveyed around a Steller sea lion (Eumetopias jubatus) haulout (nonbreeding) and rookery (breeding) site in the Gulf of Alaska to test the hypothesis that seasonal occupation of either site was related to the availability of prey. The haulout and rookery are located 30 nmi (55.56 km) apart at Long Island and Marmot Island in the Central Gulf of Alaska region where the Steller sea lion population is slowly recovering from a severe decline. Surveys conducted in May and November of 2002 (just before and after the breeding season) showed significantly higher prey energy density (total fish biomass density × energy content; kJ nmi) around the Long Island haulout than around the Marmot Island rookery. A survey conducted in July of 2002 (during breeding season) showed prey energy densities that were not significantly different between Long Island and Marmot Island but that were more concentrated in a single area by Marmot Island. Major prey species groups in all surveys were arrowtooth flounder, walleye pollock, cod, and soles; all are known prey of Steller sea lions in this area. Steller sea lion counts at Long Island during nonbreeding seasons from 2000 to 2004 correlated significantly with midwater prey energy densities. Steller sea lion counts at Marmot Island over the same period did not correlate with midwater prey energy densities in either breeding or nonbreeding seasons. The results of the study indicate that prey availability may be an important factor in the choice of haulout sites by Steller sea lions, and the higher prey availability at rookery sites provides some advantage.