ABSTRACT
The Pacific bluefin tuna (PBT), Thunnus orientalis (Scombridae), is widely distributed in the Pacific Ocean from the subarctic to tropical areas and over the equator to temperate areas of the Southern Hemisphere (Collette and Nauen 1983). It is known to be a highly migratory fish species (Inagake et al. 2001; Itoh et al. 2003a; Itoh 2004). Despite this wide distribution range, reproductive activity is believed to occur only in the western North Pacific Ocean, particularly in the waters around the Ryukyu Islands and in the Sea of Japan (Itoh 2004). These two areas are recognized as the spawning grounds of PBT as these mature fish migrate here during the spawning season. The spawning seasons are from April to June around the Ryukyu Islands (Chen et al. 2006) and from June to August in the Sea of Japan (Tanaka et al. 2007). The waters around the Ryukyu Islands are the major spawning grounds, and they contribute to more than 70% of the PBT recruitment around Japan (Itoh 2004). These aggregating spawning 124groups have been targeted by long-line fisheries. This spawning aggregation is not only economically valuable as a fishery resource but also ecologically important for reproduction as a support to the majority of the PBT population. However, ecological information related to the spawning aggregation of PBT and fishery ground formations in this area is poorly understood.
