Feeding ecology of juvenile Pacific bluefin tuna Thunnus orientalis in the Sea of Japan

The feeding ecology of juvenile Pacific bluefin tuna (PBF) Thunnus orientalis in the Sea of Japan was examined to better understand the survival process before fishery recruitment. Stomach content analysis showed that the main prey items were fish, especially Japanese anchovy Engraulis japonicus, cephalopods, mainly small squid Enoploteuthis chunii, and copepods. In addition, although both fish prey and cephalopods were important for large juveniles (100–300-mm fork length, FL), the stomach contents of small juveniles (40–99 mm FL) mainly included fish prey. The strong possibility of cannibalism was also suggested, although its contribution to total food consumption was limited. The size of fish prey increased significantly with the FL of PBF. Nitrogen isotope ratios (i.e. δ15N) also increased with increasing FL (12–227 mm), suggesting that the trophic level increases rapidly during the juvenile stage because of the active feeding on fish by PBF. Such feeding habits, especially feeding on fish, would be essential to achieve a high growth rate and would therefore be associated with the survival of juvenile PBF before recruitment in the Sea of Japan. In addition, PBF exhibited a decrease in stomach fullness during the night-time, and their estimated gastric evacuation rate was 0.36 h–1.