Foraging effort of juvenile Steller sea lions Eumetopias jubatus with respect to heterogeneity of sea surface temperature

Abstract

Among many other factors, the decline of the western distinct population segment of Steller sea lions Eumetopias jubatus in Alaska (USA) has been attributed to changes in the distribu- tion or abundance of prey due to the cumulative effects of fisheries and large-scale climate change. However, the depletion of localized prey resources due to small-scale environmental variability and perturbations may be impeding recovery, resulting in the need to understand how the environment currently affects this species on smaller spatial and temporal scales. The objective of this study, there- fore, was to assess how Steller sea lions respond to changes in localized environmental features. Satellite-relayed data loggers were deployed on juvenile Steller sea lions (n = 24) from July 2002 to May 2004 in the Aleutian Islands and Gulf of Alaska. Weekly indices of foraging effort (mean and maximum trip duration, diving activity) of Steller sea lions were examined with respect to corre- sponding patterns of sea surface temperature (SST) data obtained from the moderate resolution imaging spectroradiometer. An assortment of landscape metrics was used to characterize the hetero- geneity of frontal features derived from SST gradients because it has been suggested that Steller sea lions depend on prey patches associated with these features. Multivariate analyses indicated that fractal dimension and patch density of frontal features were significant factors for predicting differ- ent aspects of foraging effort (p < 0.05; n = 6 models). Overall, results suggested that aggregated frontal features associated with small-scale temperature gradients were probably conducive to forag- ing effort of Steller sea lions, but additional mechanisms should be investigated further.