Estimation of feeding strategies of spotted seals (Phoca largha) migrating to three regions in Hokkaido, Japan

Abstract

AbstractTo understand the current feeding habits of spotted seals, this study estimates feeding strategies based on region, season, growth stage, and sex through the results of stomach content analyses, and compares its findings with feeding habits of the past. As a result of permutational multivariate analysis of variance (PERMANOVA), region, season, growth stage, and sex were detected as significant. Non‐metric multidimensional scaling (nMDS), hierarchical cluster analysis, and the growth stages were categorized into age classes: Age 0 to age 4 years were classified as young, and age 5 and older were classified as adult. For the seasons, November and December were classified as winter, and February to May was classified as spring. And clustering with flexible mixture modeling, samples were divided into eight feeding strategies. In Rebun, the feeding strategy consisting of Octopodidae species showed a high total energy value and appeared at a high occurrence rate for both young and adult. In Saroma, Osmeridae species in winter and Clupeidae species in spring showed a high proportion. Both Osmeridae and Clupeidae species showed a high proportion in both seasons, all growth stages, and both sexes. In Rausu, adult was particularly dominant in Alaska pollock (Gadus chalcogramma) in spring, while young appeared at a high rate in Ammodytidae species. The present study has shown that seals have become particularly dependent on fish in all layers of the marine environment. In addition to drastic changes in their environment, the increase in the number of individuals of a competing species that is Steller sea lions in Rebun, and a decrease in fishery resources have made it impossible for spotted seals to take advantage of their high‐energy feeding strategy mainly composed of Alaska pollock both Rebun and Rausu. As a result, regional differences in feeding strategies for both Rebun and Rausu have become more clearly differentiated than in the past, and the seals have flexibly changed their feeding strategies to accommodate the availability of prey species in their particular region. This study has succeeded in accurately classifying the feeding strategies of spotted seals in three disparate regions of Hokkaido and has revealed significant temporal and regional differences.