MEPS prepress abstract  -  DOI:

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

Taketoshi Kodama*, Junya Hirai, Saori Tamura, Takashi Takahashi, Yosuke Tanaka, Taiki Ishihara, Atsushi Tawa, Haruyuki Morimoto, Seiji Ohshimo


ABSTRACT: Diet compositions of larval Pacific bluefin tuna (PBT) Thunnus orientalis caught in the Sea of Japan during the summer of 2011–2015, were investigated using both microscopic (n = 149) and metagenetic (n = 120) methods to determine their prey and feeding habits. The microscopic analysis revealed that prey abundance in gut contents ranged from 0–10 ind./larva (mean ± SD: 0.89 ± 1.5 ind./larva, including larvae with empty guts), which was low, similar to levels for larval bluefin tunas in other seas except the Mediterranean. The cladoceran Podonidae family (Podon spp. and Evadne spp.) represented the most abundant prey (0.73 ± 1.4 ind./larva) 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, and the PBT larvae fed on them selectively. Copepods were abundant in the sea; however, PBT larvae fed few of them. The ability of copepods to escape predation is higher than that of Oikopleura spp. and Podonidae, and larval guts were mostly not full based on the numbers and sizes of prey.. This study successfully characterized the prey of PBT larvae in the Sea of Japan, and showed that PBT larvae were unable to make use of the abundance of copepods, a fact which may lead to high larval mortality.