Dependency of Japanese harbor seals (Phoca vitulina) on salmon set nets at Cape Erimo, Hokkaido, Japan

Abstract

In this study, we observed the behavior of two age groups of harbor seal (Phoca vitulina richardii), i.e., yearlings, which often become bycaught and individuals 2 yr or older, during their stays at salmon set nets at Cape Erimo, Japan. From July to November 2011–2015, acoustic receivers that track tagging seals were attached to three set nets located near the haul-out site. Our observations showed that the mean visit interval and the mean stay of seals at the set nets during fishing operations were significantly higher than before operations. A cluster analysis, where Cluster 1 consisted of seals that spent a long time at the set nets and Cluster 2 consisted of the other seals, showed that seals in Cluster 1 visited the set nets every day, while Cluster 2 seals visited once every 3–5 d. In addition, age 2 + seals in Cluster 1 stayed longer at regular time frames (at night), suggesting that individuals of this age group in Cluster 1 are highly dependent on the salmon set nets as foraging sites at night. It is especially clear that the set nets near the haul-out site have influenced the foraging ecology of some seals.