Abundance Estimates of Steller Sea Lions (Eumetopias jubatus) Off the Western Coast of Hokkaido, Japan

Abstract. Aerial surveys were conducted to estimate the abundance of wintering populations of Steller sea lions (SSLs; Eumetopias jubatus) in the Sea of Japan originated from Russian waters using the line transect sampling method during spring from 2005 to 2019. The survey areas covered the continental shelf off the western coast of Hokkaido. A total of 28 196 km survey effort was made through the study period under suitable sighting conditions. The numbers of SSLs in haul-out sites were counted separately. Estimations and model selection for the effective strip width were carried out using the maximum-likelihood method. The abundance estimates of SSLs with the line transect method varied annually, ranging from 284 animals (coefficient of variation [CV] = 0.732) in 2016 to 6271 (CV = 0.194) in 2014. This large variation in abundance estimates might be attributable to interannual changes in the survey coverage, spatial distribution, haul-out behaviors, and seasonal variation of wintering populations of SSLs in the waters off Hokkaido. This warrants further investigation of seasonal and annual change of distribution to adjust key information on the abundance estimates reported here for use in the conservation and management of this species.