Similarities of distributions and feeding habits between Bullet tuna, Auxis rochei, and Pacific bluefin tuna, Thunnus orientalis, larvae in the southern Sea of Japan

Bullet tuna (Auxis rochei: BT) larvae and juveniles are found during summer in the Sea of Japan, where those of Pacific bluefin tuna (Thunnus orientalis: PBF) also occur. However, the ecology of BT larvae and their interactions with the PBF larvae are both unclear. To identify the differences in the spatial distributions and feeding habits between BT and PBF larvae at the larval stages, they were evaluated in the Sea of Japan during the summer. The distributions were assessed using generalized additive models (GAMs) in 2016, 2017, and 2018, and the feeding habits of both species were investigated using a metagenetic technique in 2017 and a morphological technique in 2017 and 2018. The BT and PBF larvae were the only members of the family Scombridae found during the study. The GAMs indicated that BT larvae occur in warm water (> 24.3 ?), and PBF larvae are found at similar temperatures (> 23.5 ?). BT larvae occurred in more saline conditions than PBF larvae. The ecological niche breadth was estimated with a diversity of environments at which the target species are present, and that of BT was estimated to be greater than that of PBF. Morphologically identified gut contents of BT larvae consisted mainly of copepods and cladocerans, and appendicularians, with sagitta being added to this list through metagenetic identification, and the PBF larvae had the same prey. The breadth of the feeding niche, the diversity of the feeding contents, of the BT larvae were wider than those of the PBF, and their feeding niches highly overlapped. The number of different species in the gut of BT larva was significantly lower than that in PBF larva, suggesting feeding behavior of PBF larvae is superior to that of BT larvae. These results show that PBF larvae are the competitors of BT larvae and may have a competitive advantage; however, environmental characteristics that are unique to the BT larvae are present. These interspecies interactions keep the coexistence of BT and PBF at the juvenile period in the Sea of Japan.

Automated summary

Bullet tuna and Pacific bluefin tuna larvae coexist in the Sea of Japan, but the former has unique ecological characteristics that allow for their coexistence despite competition with the latter.