Distribution of Steller sea lions Eumetopias jubatus in relation to spring-spawning fish in SE Alaska

Abstract

Energetic demands are high for Steller sea lions Eumetopias jubatus during spring, when females are pregnant and lactating and males are preparing for extended fasts on breeding territories. Therefore, we predicted that the distribution of sea lions in SE Alaska in spring would be influenced by the distribution of spring spawning aggregations of high-energy prey species (Pacific herring Clupea pallasii and eulachon Thaleichthys pacificus). The spatial distribution of sea lions during spring reflected the distribution of spawning eulachon in northern Southeast Alaska, particularly in Lynn Canal and along the Yakutat forelands. Haulouts with peak numbers of sea lions in spring were located significantly closer to eulachon spawning sites than haulouts that peaked at other times of year. Some haulouts were occupied only during the eulachon spawning period. The maximum number of sea lions at haulouts in spring was inversely correlated with the distance to the closest eulachon aggregation and was positively associated with the number of eulachon within 20 km. Aerial surveys conducted every 7 to 10 d during March through May in 2002 and 2003 revealed large numbers of sea lions in the water at herring spawning sites in 2002 and 2003; however, there were no significant relationships between the number of herring spawning sites and number of sea lions (except at distances >60 km). The number of sea lions was greater at herring spawning sites in 2003, corresponding to higher herring biomass. Seasonally aggregated, high-energy prey species influence the seasonal distribution of sea lions and may be critical to their reproductive success.