oa Horizontal and vertical movements of the whale shark, Rhincodon typus, in the southern waters of Japan

Background Conservation of whale sharks is making remarkable advances through many studies ongoing in tropical and sub-tropical oceans. However, there have been few studies in the North Pacific Ocean. Some whale shark appearances, as well as by-catch by set-net and purse seining in the southern waters of Japan are reported every year. These waters are important for the conservation ecology of whale sharks because they are the northern limit of the whale shark's distribution and have high productivity influenced by the Kuroshio and Oyashio Currents. However, the habitat and movement of whale sharks around Japan in the North Pacific Ocean are poorly understood. Approach To investigate the habitat and movement of the whale shark, two whale sharks were tagged with PAT-tags (MK-10, Wildlife) and released off the coast of Kouchi, Japan in October 2013 (WS13: 4.2 m in total length) and June 2014 (WS14: 7.2 m). For biological studies and rearing trials, WS13 had been reared for 6 years and 4 months in the Osaka Aquarium of KAIYUKAN, Osaka, and WS14 had been reared for 4 months in the Osaka Aquarium Biological Research Institute of Iburi Center (OBIC) of KAIYUKAN, Kochi, Japan before tagging. The PAT-tags recorded ambient temperature, swimming depth and light level data for 30 and 90 days, respectively, until the detached from the whale shark. After the PAT-tag detachment, the tag floats to the surface and transmits the sampled data via the Argos satellite system. Geolocations of tagged sharks were estimated using light measurement data from the tag and sea surface temperature from the satellite. The horizontal and vertical movements were examined in relation to oceanographic conditions. Results The two sharks moved in the same direction to the east along the Kuroshio Current after being released, despite being released in different seasons. However, WS13 remained in the eddy in the south of the Kuroshio Current, while WS14 crossed the Kuroshio Current to north and remained in the eddy near the Kuroshio-Oyashio transitional area. The total distance WS13 traveled was 1,407 km over 30 days (48.5 km/day), and WS14 traveled 3,022 km over 90 days (29.0 km/day). The mean daily speed while moving along the Kuroshio Current was higher than while remaining in the eddy for both sharks. The primary swimming depth was surface mixed water where the temperature was above 20 °C. The two sharks spent most of their time near the surface above 10 m depth. Diving over the mesopelagic zone (>200 m depth) was rarely observed while the two sharks were traveling along the Kuroshio Current, and wasn't observed while the sharks remained in the eddy. The maximum swimming depth for WS13 was 632 m, and for WS14 was 1560 m, and lowest temperature recorded was 7.1 °C for WS13 and 2.4 °C for WS14 when the sharks were at their maximum depth. Conclusions In southern waters of Japan, horizontal movements of whale sharks are influenced by the Kuroshio Current and the eddy. Their vertical movements are regulated by the thermocline and the threshold temperature was around 20 °C