Advanced spawning in Pacific bluefin tuna, Thunnus orientalis, by a shift in consecutive photothermal regimes in a land-based tank

Abstract

High mortality of age-0 hatchery-reared Pacific bluefin tuna (PBT, Thunnus orientalis) frequently occurs in sea cages during the winter. Because the main spawning season of captive PBT broodstocks is delayed by approximately 2 months compared with that of wild populations, hatchery-reared PBT juveniles are usually smaller than wild-captured fingerlings, suggesting that body size is related to survival during winter. To produce large fingerlings with thermal tolerance to cold water and prevent mass mortality of age-0 hatchery-reared fish during winter, we developed an advanced spawning technique in PBT broodstocks by manipulating environmental cues such as water temperature and day length in a land-based tank. In November 2017, fifty-eight 2-year-old PBT were transferred to a land-based tank (diameter, 20 m; depth, 6 m; volume, 1880 m3). After the broodstocks experienced first spawning between June and August 2018 induced by normal photothermal conditions, uniformly shifted photothermal conditions corresponding to December 20 (winter solstice condition) to October 20 were applied to induce spawning between April and June 2019. The application of 2-month advanced photothermal cycles allowed PBT to spawn approximately 2 months earlier than the normal spawning season of the previous year. Moreover, parentage analyses revealed that at least two females and five males spawning at 3 years of age participated in advanced spawning events and produced viable larvae at 4 years of age. These results indicate that uniformly shifted photothermal conditions starting at winter solstice conditions are effective in accelerating the sexual maturation and spawning of PBT broodstock.