Vertical movements of Pacific bluefin tuna (Thunnus orientalis) and dolphinfish (Coryphaena hippurus) relative to the thermocline in the northern East China Sea

Pacific bluefin tuna, Thunnus orientalis, and dolphinfish, Coryphaena hippurus, are temporally sympatric top predators in the north East China Sea. To clarify their vertical habits in relation to the thermocline, we analyzed swimming depth and ambient temperature data for Pacific bluefin tuna that were obtained from animal-borne data-loggers in April, May, and November 2007. We also reanalyzed published electronic tagging data for free-ranging dolphinfish in the Tsushima Straight. When the vertical temperature structure was homogeneous, Pacific bluefin tuna made vertical excursions to the bottom layer in early May and November. When the thermocline developed in late spring, bluefin tuna made frequent dives below the thermocline, and their main distribution depth shifted to above the thermocline from the surface. During the same period, dolphinfish in this area remained near the surface and did not dive across the thermocline. However, swimming data for dolphinfish that were obtained in September and October showed that dolphinfish extended their vertical depth ranges as the thermocline depth increased.