Vertical distribution of larval Pacific bluefin tuna, Thunnus orientalis, in the Japan Sea

Abstract

To better understand habitats of larval Pacific bluefin tuna (PBT; Thunnus orientalis) in the nursery grounds, the vertical distribution of larval PBT was collected using a 1-m2 Tucker trawl and a 3.14-m2 ring net at depths of 0, 10, 20, and 30 m during the spawning seasons of 2016 and 2017. Most of the larvae in the Japan Sea (756 of 938 individuals) were collected from the surface layer using a ring net, while similar numbers were collected from 0 to 20 m depth in the Nansei area. The temperature at ≥20 m depths in the Japan Sea was cooler than the optima temperature, and the thermocline was steep at 10 m depth, and the cold subsurface water was suggested to make the PBT larvae aggregate near the surface. A weak ontogenetic and diel migration was observed; the body size at 10 m depth during the day and 0 m at night were significantly larger than those at 0 m during the day in the Japan Sea. These results suggested that some small larvae sink to 10 m and/or large larvae move to the surface at night. Our results showed that the larval PBT habitat is strongly controlled by water temperature, and that global warming would have a positive effect on PBT larvae in the Japan Sea, which was evidenced by the higher density of PBT larvae after 2010 compared to before 1990.