Mesoscale movement patterns of common bottlenose dolphins along Kuroshio Current and its bifurcation

In the waters off Japan, the patterns of the Kuroshio Current (KC) and its bifurcations are highly variable, which significantly affect the physical and biological environments. Knowledge about species' habitats of marine apex predators such as cetaceans and their variability is fundamental to understand not only their population structures but also their ecosystem structures related through bottom-up and top-down trophic linkages. In dynamic ocean environments, however, such crucial information has not been provided adequately by snapshot-based conventional approaches, such as sighting surveys. Spatiotemporal habitat utilization patterns of the common bottlenose dolphin (Tursiops truncatus) were investigated using fin-mounted Advanced Research and Global Observation Satellite (ARGOS) tags. Behavioral states at each tracking location were categorized into transiting- or area-restricted searching (ARS) modes based on Bayesian state-space model. Twelve satellite-tracked dolphins showed different movement patterns. Their habitat ranges were estimated by minimum convex polygons (MCPs), and movement patterns were classified into three types (four animals each of eastward, offshore, and westward movements) based on MCP overlapping among tagged dolphins. The animals that restricted their habitat to the coastward side of the KC (CSKC) tended to show ARS modes more frequently. Estimated locations of these animals corresponded well with cold coastal waters. On the other hand, offshore movements corresponded well with clockwise flow generated by the bifurcation of the KC, and those dolphins were likely straying into less productive waters. These patterns suggest that common bottlenose dolphins off the southern coast of Japan mainly utilize productive CSKC waters but occasionally move to offshore waters.