Impacts of environmental variability and global warming scenario on Pacific bluefin tuna (Thunnus orientalis) spawning grounds and recruitment habitat

Pacific bluefin tuna (Thunnus orientalis) have spawning grounds in waters stretching from south of Okinawa to east of Taiwan. This species is typical of fishes that spawn seasonally in small, limited areas. Any marked change in the marine environment of the spawning grounds is likely to have a direct impact on larval survival and growth. We conducted rearing experiments on larvae to investigate these impacts and found that a reduction in temperature resulted in poor growth during the juvenile stage, even if larval survival rates did not change. In the wild, this reduced growth rate is likely to reduce survival rates because smaller juveniles have poorer swimming ability and their ability to avoid predators is also reduced. This is especially important since the Kuroshio current, which connects the spawning grounds to the nursery grounds, transports larvae rapidly because of its faster surface current (the western boundary current), thus larvae arriving too quickly in coastal waters can be exposed to very cold temperatures. An ocean model (MIROC) simulation under a climate warming scenario (IPCC, 2007) predicted that the temperature in the spawning ground would be 3 °C higher in 2100 than in present time, while the transport to nursery grounds would also be faster. In this case, the combination of several mechanisms would control the recruitment of juvenile bluefin. On the spawning grounds, high temperatures exceeding the optimal range would increase larvae mortality and any surviving larvae would reach the nursery grounds more quickly, but warmer coastal waters would have less negative impact on their growth. We forced a model of larval drift with MIROC output fields to study the complex response of bluefin tuna recruitment. As a result, the predicted survival rates of larvae arriving in Japanese coastal waters in 2100 would decline to 36% of present recruitment levels.