Effect of ambient temperature on the vertical distribution and movement of Pacific bluefin tuna Thunnus thynnus orientalis

Immature Pacific bluefin tuna Thunnus thynnus orientalis, marked with an archival tag, were released near Tsushima Island in the eastern East China Sea (58 individuals on 7 to 14 December 1995, 47 individuals on 29 November 1996). Of these, 15 were recovered by the following summer period. From the tags retrieved, we analyzed time-series data on swimming depth and ambient temperature recorded every 128 s. Our main objectives were to determine diurnal and seasonal changes in the swimming depth of bluefin tuna and examine the effect of ambient temperature on their vertical distribution and movement. In winter, the bluefin swam within the surface mixed layer at shallower depths during the night than during the day; swimming depth displayed diel periodicity. Some individuals displaying marked diel periodicity tended to migrate further offshore (direction southwest), where greater temperature gradients in the vertical water column were observed. This indicates that diel vertical migration of bluefin tuna is a direct response to changes in the thermocline. However, as the thermocline developed further in summer, the bluefin spent most of their time at the surface, suggesting that bluefin avoid rapid temperature change at the thermocline by making behavioral thermoregulation. During the summer, however, the fish made frequent dives through the thermocline during the day, and diel periodicity of swimming depth was more prominent than that in winter. The purpose of this movement is considered to be foraging. In conclusion, spatial and seasonal changes of the vertical structure of ambient water temperature have a great effect on the vertical distribution and movement of the bluefin tuna.