Mass production of fertilized eggs by artificial insemination from captive-reared Pacific bluefin tuna (Thunnus orientalis)

Artificial insemination (AI) techniques for obtaining fertilized eggs from captive-reared ~ 3- to 4-year-old Pacific bluefin tuna (Thunnus orientalis) were developed. Gametes for AI were acquired from fish harvested from commercial offshore sea cages. Sperm was successfully obtained from almost all male fish, whereas the occurrence of ovulated eggs was relatively low. Therefore, the AI was dependent on the frequency of fish carrying ovulated eggs, which peaked at 6.09% of harvested fish between July 21 to 31, 2012 (17 of 279 fish examined). Using the collected sperm and ovulated eggs, AI was conducted seven times. In five cases the egg buoyancy and fertilization rates were > 85 and 70%, respectively. The total number of fertilized eggs obtained in each of these five AI trials ranged from 1.80 to 3.03 million. By contrast, a low AI success rate was observed in the other two cases, in which the fertilization rate was < 40%. Furthermore, the total number of fertilized eggs in these two trials was less than one million. The buoyant eggs obtained were transported to hatcheries for larval rearing. The hatching rate ranged from 43.6 to 64.3%, except for one case, in which the hatching rate was 26.1%. From 0.25 to 2.00 million larvae were acquired in each trial. These larvae were subsequently reared in large-scale larviculture tanks, resulting in the successful transfer of 1630-4410 fingerlings to offshore sea cages in each rearing trial. These results demonstrate that the fertilized eggs produced by this AI method are practicable for the production of Pacific bluefin fingerlings for commercial tuna aquaculture. Furthermore, this AI method will help streamline selective breeding programs for bluefin tuna.