Estimation of Environmental Factors That Influence Migration Timing and Distribution of Pacific White-Sided Dolphins around Hokkaido, Japan

Hydrographic, hydroacoustic, and cetacean visual surveys were conducted using a quantitative echosounder to estimate environmental factors influencing migration timing and distribution of Pacific white-sided dolphins (Lagenorhynchus obliquidens) in the Tsugaru Strait (nonbreeding area) and Funka Bay (breeding area) near Hokkaido, Japan. Higher numbers of dolphins were observed in the Tsugaru Strait during May and June and in Funka Bay during June, July, and August. Potential prey were observed in the Tsugaru Strait in May, June, and August, but they may not have been of a suitable size in August, which may explain absence of the dolphins in that month. In Funka Bay, potential prey were abundant in May, but dolphins may have been absent because of small prey size and low water temperature. In a smaller-scale analysis, the relationship between dolphins and prey was different in the two areas, with dolphin distribution more closely associated with prey distribution in the Funka Bay breeding area. This difference may have resulted from mothers needing to feed more frequently during lactation. These data should serve as an important foundation for estimating environmental factors impacting small cetaceans around Japan as well as changing environmental factors during the cetacean life cycle.