Aim We used a novel approach to infer foraging areas of a central-place forager, the Steller sea lion (Eumetopias jubatus), by assessing changes in the temporal and spatial distribution patterns of sea lions at terrestrial sites. Specifically, our objectives were (1) to classify seasonal distribution patterns of Steller sea lions and (2) to determine to what extent the seasonal distribution of Steller sea lions is explained by seasonal concentrations of prey.
Location Southeast Alaska, USA.
Methods Steller sea lions of all age classes were counted monthly (2001–04) by aerial surveys at 28 terrestrial sites. Hierarchical cluster analysis and principal components analysis were used to classify seasonal distribution patterns of Steller sea lions at these terrestrial sites. We estimated the proportion of sea lions in the study area that were associated with each seasonal distribution pattern.
Results Multivariate ordination techniques revealed four distinct seasonal distributional patterns. During December, 55% of the sea lions in the study area were found at Type 1 sites, located near over-wintering herring aggregations. During May, 56% of sea lions were found at Type 2 sites, near aggregations of spring-spawning forage fish. In July, 78% of sea lions were found at Type 3 sites, near summer migratory corridors of salmon. During September, 44% of sea lions were found at Type 4 sites, near autumn migratory corridors of salmon.
Main conclusions Seasonal attendance patterns of sea lions were commonly associated with the seasonal availability of prey species near terrestrial sites and reflected seasonal foraging patterns of Steller sea lions in Southeast Alaska. A reasonable annual foraging strategy for Steller sea lions is to forage on herring (Clupea pallasii) aggregations in winter, spawning aggregations of forage fish in spring, salmon (Oncorhynchus spp.) in summer and autumn, and pollock (Theragra chalcogramma) and Pacific hake (Merluccius productus) throughout the year. The seasonal use of haulouts by sea lions and ultimately haulout-specific foraging patterns of Steller sea lions depend in part upon seasonally available prey species in each region.