Seasonal distribution of Steller sea lions (Eumetopias jubatus) in relation to high-quality ephemeral prey species in southeastern Alaska

Abstract

Energetic demands are high for sea lions during spring when females are pregnant and lactating and males are preparing for extended fasting during the breeding season. Therefore, I predicted that the distribution of sea lions in spring would be influenced by the distribution of spring-spawning aggregations of high-energy Pacific herring (Clupeapallasii) and eulachon (Thaleichthys pacificus) in southeastern Alaska. Monthly aerial surveys at 23 Steller sea lions haul outs revealed that haulout use was seasonally dynamic. Some sea lion haulouts were only occupied during spring. Other haulouts exhibited pronounced increases in the number of sea lions during certain seasons. Sea lion haulouts with peak numbers of sea lions in spring were significantly closer to forage fish aggregations than haulouts with peak numbers of sea lions at other times of year. From March through May 2002, I used aerial surveys to monitor the number of Steller sea lions at spring spawning aggregations of Pacific herring and eulachon. The maximal numbers of sea lions observed were 949 at a eulachon-spawning site and 252 at a herring-spawning site. Seasonal pulses of high-energy food resources may be critical to the reproductive success of individual Steller sea lions.