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Stomach contents of 437 age-0 year Pacific bluefin tuna Thunnus orientalis (20·3-59·4 cm fork length, L(F)) caught in the Tsushima Current and the Kuroshio regions around Japan were examined to investigate their ontogenetic diet shift. Prey compositions were diverse and different between regions. Although the seasonal growth patterns were different between regions, ontogenetic diet shifts shared a common pattern. In the Tsushima Current region (Sea of Japan), small T. orientalis (20-25 cm L(F)) preyed upon small squid (juvenile Enoploteuthis chunii), and larger ones (25-35 cm L(F)) gradually shifted their diet to mesopelagic fish (Maurolicus japonicus). In the Kuroshio region (Pacific Ocean), small T. orientalis (20-25 cm L(F)) preyed upon small zooplankton (mostly crustacean larvae), and larger ones (25-40 cm L(F)) shifted to epipelagic fishes (Etrumeus teres, Sardinops melanostictus and Engraulis japonicus). The observed data suggest that T. orientalis switch to a diet more based on fish prey items, which have more body mass and greater swimming ability than small squid and zooplankton, after they reach a L(F) of 25 cm.

The feeding habits of skipjack tuna Katsuwonus pelamis juveniles (8.5–66.8 mm standard length) were examined, collected from the tropical western Pacific in October to December 1994, and their habits were compared with those of the other tuna Thunnus spp. juveniles (9.8–55.3 mm standard length). The indices, frequency of occurrence of each food item in the total number of stomachs examined (%F), percentage of number of each food item to the total number of all food items identified (%N), percentage of wet weight of each food item to the total wet weight of all food items identified (%W), and relative importance of each food item (IRI) were estimated in the north equatorial current (NEC) and the north equatorial countercurrent (NECC) areas. The most dominant prey item of skipjack juveniles in the two areas was fish larvae. Other major prey items in the NEC area were Euphausiacea, Amphipoda, and Copepoda; whereas those in the NECC area were Copepoda, Cephalopoda, Euphausiacea, and Amphipoda. In the other tuna juveniles, the IRI of fish larvae in the two areas was remarkably high. Other prey, Euphausiacea and Cephalopoda in the NEC area only and Cephalopoda in the NECC area, were also found. These results indicate that the skipjack juvenile is primarily a piscivorous feeder although they also depend on various other prey organisms, whereas the other tuna juveniles are stronger piscivorous feeders.

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