Differences in larval growth of Pacific bluefin tuna (Thunnus orientalis) between two spawning areas, and an evaluation of the growth-dependent mortality hypothesis

Abstract

The larval growth of Pacific bluefin tuna (Thunnus orientalis) was compared between two main spawning areas, the Sea of Japan (SOJ) and the western North Pacific (WNP). Growth trajectories indicate that the larval survival depends on their growth in both areas. Until the flexion stage, larvae of the SOJ showed lower growth than those of the WNP, but after the post-flexion stage, there was no significant differences in larval growth between two areas. The observed growth differences were likely driven by the sea surface temperature (SST): the lower SST in the SOJ negatively affected the larvae until they reached to the flexion stage. Survival over the flexion stage is one of the major ontogenetic milestones during the larval development in the SOJ. Chlorophyll-a concentrations were negatively correlated with SST and tended to be higher in the SOJ than in the WNP. Prey availability was presumed to be higher in the SOJ. As larval swimming and feeding abilities improved with flexion, larvae would have been able to capitalize on the favorable feeding environment in the SOJ, thus eliminating the temperature mediated growth difference in pre-flexion larvae.