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Fine-scale horizontal distributions of multiple species of larval tuna off the Nansei Islands

Atsushi Tawa*, Taketoshi Kodama, Kay Sakuma, Taiki Ishihara, Seiji Ohshimo

*Corresponding author:

ABSTRACT: To quantitatively evaluate the distribution of tuna larvae related to oceanographic conditions, we conducted investigations off the Nansei Islands in the western North Pacific in June from 2015 to 2017. Five species, Pacific bluefin tuna (Thunnus orientalis, PBF), yellowfin tuna (T. albacares, YFT), skipjack tuna (Katsuwonus pelamis, SKJ), frigate tuna (Auxis thazard), and bullet tuna (A. rochei, BT), were collected consistently. The most dominant species was BT throughout the three year period, followed by SKJ in 2015 and YFT in 2016 and 2017. The horizontal larval distributions of the five species were largely influenced by the Kuroshio: those of two Auxis species were distributed in the Kuroshio and the Kuroshio inshore waters, whereas those of the other species in the Kuroshio offshore waters. These differences are consistent with the differences in spawner distributions of the tunas. The generalized additive models (GAMs) indicate that the larval densities of tunas were affected by the sea surface height anomaly (SSHA) and they were not always accumulated by horizontal transport. Sea surface temperature (SST) and salinity possibly influenced the larval physiology and survival, thereby determining their densities. In the GAMs, PBF and YFT had similar responses to SST, and YFT and SKJ similarly responded to salinity. To avoid their overlapping the ecological niches, the larvae of three species (PBF, YFT and SKJ) are expected to differ in their other habitats such as vertical distribution and feeding habit.