Diel and ontogenetic changes in larval body density related to swim bladder volume were investigated in Paci¢c blue¢n tuna, Thunnus orientalis, to determine the causality of larval mortality^adhesion to the water surface and contact with the tank bottom during seedling production. The density of larvae with de£ated swim bladders increased with total length and days post hatch. Diel density change was observed after day 2 post hatch; owing to daytime de-£ation and night-time in£ation of the swim bladder, the density was relatively higher during the daytime. Increased swim bladder volumes clearly reduced larval density during the night-time after day 9 post hatch. However, the density of larvae with in£ated swim bladders was greater than rearing water density (Dr40.0099). The small density di¡erence between larvae and rearing water (Dr 5 0.0022 À 0.0100) until day 4 post hatch may have caused larval mortality by adhesion to the water surface because larvae can be easily transported to the water surface by aeration-driven upwelling in rearing tanks. Density increased noticeably from day 5 to day 9 post hatch. The increased density di¡erence (Dr 5 0.0065 À0.0209) in larvae and rearing water possibly induced mortality by contact with the tank bottom because larvae sink particularly during the night-time on ceasing swimming.

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Tuna (genus Thunnus), particularly Pacific bluefin tuna (T. orientalis; PBT), are commercially important fish in the aquaculture industry worldwide. The objective of this study was to investigate sexual dimorphism in the growth performance of aquaculture-produced PBT and develop techniques for its sex manipulation, for the first time in tuna. A comparison of the body size between sexes revealed that male-cultured PBTs were larger than females at harvest. We also confirmed that cyp19a1a (encoding a gonadal aromatase) expression increased specifically in the genotypic female PBT gonads during sex differentiation. This suggested that aromatase plays an important role in ovarian differentiation and that the suppression of aromatase activity may effectively induce masculinization in genotypic females. Therefore, we administered letrozole—an aromatase inhibitor (AI)—into sexually undifferentiated PBT through the oral route. AI administration resulted in a 100% sex reversal of genotypic females into phenotypic males at the molecular level. Our results provide the basis for future studies on the establishment of mono-sex male production technology in PBT, which would help improve the productivity of closed-cycle PBT aquaculture. Furthermore, this study offers important insights into the understanding of the sex-wise growth of tuna species in aquacultural settings, and developing sex manipulation techniques.

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