Diet composition and feeding habits of larval Pacific bluefin tuna Thunnus orientalis in the Sea of Japan: integrated morphological and metagenetic analysis

Diet compositions of larval Pacific bluefin tuna (PBT) Thunnus orientalis caught in the Sea of Japan during the summers of 2011–2015 were investigated using both microscopic (n = 149) and metagenetic (n = 120) methods to determine the species’ prey and feeding habits. The microscopic analysis revealed that prey abundance in gut contents ranged from 0–10 ind. larva−1 (mean ± SD: 0.89 ± 1.5 ind. larva−1, including larvae with empty guts), which was low, similar to levels for larval bluefin tunas in other seas except the Mediterranean. The cladoceran family Podonidae (Podon spp. and Evadne spp.) represented the most abundant prey (0.73 ± 1.4 ind. larva−1) observed microscopically. Metagenetic analysis of the 18S V9 region detected a taxonomically wide range of prey. In addition to Podonidae, morphologically unidentified taxa were detected as major prey, including Copepoda (Paracalanus parvus s.l. and Labidocera sp.), Appendicularia (Oikopleura spp.), and Doliolida (Doliolum sp.). Compared with plankton communities in the sea, the proportions of Oikopleura spp. and Podonidae in the guts of the larvae were high, indicating that PBT larvae fed on them selectively. Although copepods were abundant in the sea, PBT larvae only consumed a small number of them. The ability of copepods to escape predation is higher than that of Oikopleura spp. and Podonidae, and the guts of the PBT larvae generally contained a low number of copepods considering their abundance and size in the region. This study successfully characterized the prey of PBT larvae in the Sea of Japan, and showed that they were unable to exploit the abundance of copepods, a fact which may lead to high larval mortality.