California Sea Grant College Program

The genus Thunnus of the family Scombridae includes three species of bluefin tunas (Atlantic bluefin tuna – T. thynnus, Pacific bluefin tuna - T. orientalis and southern bluefin tuna - T. maccoyii). The bluefin tunas were first recognized as two independent species (Northern and Southern bluefin) based on subtle differences in morphological characters. Northern bluefin tunas are now recognized as morphologically, geographically and genetically separate species located in the Atlantic and Pacific oceans.

The Pacific bluefin (T. orientalis) is the only species which remains unmanaged; this lack of management persists despite intensive fisheries on both sides of the Pacific. The current life history model indicates that these fish spawn in the western Pacific (Sea of Japan, Philippine Sea and East China Sea). Either late in the first year or early in the second year, a portion of the population migrates to the western coast of the United States and Mexico, a journey of over 8700 km (Bayliff et al., 1991). The young fish that have migrated into the eastern Pacific are thought to remain there for several years, feeding on sardines and anchovies in regions of intense upwelling (Bayliff et al., 1991, Bayliff, 1993). While these tuna are fished only seasonally off California and Mexico, they may be a year-round resident (Bayliff, 1991). The migrants then travel back to the western Pacific to spawn. Why some bluefin remain in the western Pacific while others migrate across the ocean basin is unresolved. How long they stay in the eastern Pacific, what habitats are most important, what triggers their return to the west is unclear.