The restricted spawning area and season of Pacific bluefin tuna facilitate use of nursery areas: A modeling approach to larval and juvenile dispersal processes

To examine the larval transport processes of this species and help determine why the Pacific bluefin tuna (Thunnus orientalis: PBT) spawns in a restricted area and during a limited season compared to other tropical tuna species, we used survey data of larval distributions, Lagrangian numerical simulations, and SST data analysis. A high density area of larvae was detected in the waters east of Taiwan, where larvae were likely entrained in anticyclonic eddies after hatching. The numerical simulations indicated that spawning areas east or south of Taiwan could supply PBF larvae to the estimated nursery area located in Japanese coastal waters, irrespective of annual fluctuations in oceanographic conditions. The simulation also showed that particles released to the east of the spawning area were not successfully transported to the nursery area. The trajectories of the successful particles indicated that larvae can be transported with a low energetic cost to cooler temperate waters by utilizing the strong current of the Kuroshio or the Ryukyu Current followed by the Kuroshio. The ambient temperatures around the nursery were found to be favorable with a similar temperature range as the larval distribution area only during August to September. If they do not spawn during the period April to July, the larvae will be exposed to unfavorable water temperature conditions. We hypothesize that PBT spawn in a restricted region during a limited period (April to July) to maximize the chances of their larvae entering the nursery regions in temperate waters with low energetic costs, which ensures optimal thermal conditions for growth and survival, although the spawning area may be unfavorable for the adults due to warm water temperatures.