Horizontal distribution and habitat of Pacific bluefin tuna, Thunnus orientalis, larvae in the waters around Japan

摘要

Pacific bluefin tuna (PBF), Thunnus orientalis (Temminck and Schlegel, 1844), is highly migratory and capable of traversing large distances throughout the North Pacific Ocean. The majority of spawning activity has been reported only from the Sea of Japan and western parts of the Pacific Ocean. We have conducted larval surveys in these areas since 2009 ("late period"), and used generalized additive models (GAM) with presence/absence data to analyze larval distribution with respect to oceanographic conditions, such as sea surface temperature and chlorophyll-a concentration. Here, we compare our survey data with some historical data collected from 1979 to 1988 ("early period"). In total, 9192 individual PBF larvae were collected from 1979 to 2015, and body lengths ranged from 2 to 11 mm. In the early period, the relatively higher probability area for the presence of PBF larvae in the Pacific Ocean was wider than that in the Sea of Japan. However, in the late period, the relatively higher probability area for presence in the Sea of Japan was wider than in the early period. The spline function of sea surface temperature for PBF larvae was lower in the early period than in the late period, though large uncertainty in the Pacific Ocean in the late period was observed. These results suggest that the change in the distribution pattern of PBF larvae may be caused by ocean warming.