A review of the broodstock management and larviculture of the Pacific northern bluefin tuna in Japan

We reviewed research on the broodstock management and larviculture of the Pacific northern bluefin tuna (PBT) Thunnus orientalis in Japan. Japan has been at the forefront of PBT research since 1970 due to the participation by federal and prefectural governments and universities in a national project aimed to optimize productivity of the sea around Japan. In 1979, scientists at Kinki University succeeded in the first spontaneous spawning in captivity by the broodstock of 5year-old PBT. Successful spawning was also performed in 1980 and 1982, but no spawning then occurred until 1993, when Maruha Nichiro Holdings, Inc. (MNH) and Nippon Formula Feed Manufacturing Company, Ltd. (NFFMC) became involved in tuna farming and succeeded in the spawning of four-year-old broodstock. Since then, successful spawning of PBT in captivity has been reported from several sites as well as spawning in Kinki Univ. since 1994. With the successful spawning of PBT, the Fisheries Research Agency (FRA; formerly, Japan Sea Farming Association) of Kinki University, in conjunction with MNH, NFFMC, and Takuyo Ltd., has actively carried out research on and development of tuna larviculture technology. Thus, knowledge about broodstock management and larviculture has accumulated in Japan, but technical problems with larviculture still remain to be solved. There are 9 sites of successful spawning in net pens in Japan so far. At 4 of these sites in regions around Amami Island, yearly spawning has stably occurred. We have been accumulating data about the period of maturation, environmental key factors triggering the spawning, the age of onset of spawning, and the pattern of spawning through measurement of the ambient environment, gonad morphometry, endocrinology, mitochondrial DNA analysis, and daily careful observation of broodstock. Research on PBT larviculture at Kinki University, FRA, MNH, NFFMC, and Takuyo have succeeded in producing tens of thousands of hatchery-raised juveniles. As a result, Kinki University succeeded in establishing the full life cycle of PBT in captivity, and also achieved its aquaculture life cycle. However, solutions are needed for the remaining technological issues of PBT larviculture, including sinking syndrome, where larvae die on the bottom of the tank during the early phase of larviculture, the search for appropriate food (species, size, and nutrition) around the transition stage from larva to juvenile, cannibalism and collision against walls in the juvenile, and malformation and viral diseases in the young stage.