Prevalence of collision death in 2-year-old Pacific bluefin tuna, Thunnus orientalis, reared in land-based tanks for broodstock management

• This is the first report on causes of mortality in 2-year-old PBT reared in land-based tanks.
• Most dead fish (91%) had injuries to their skin, vertebral column and/or skull.
• Collision death occurred frequently from dusk to dawn and after transfer to the tanks.

The causes of Pacific bluefin tuna (PBT) mortality must be identified to increase survival rate in land-based tanks. One of the suggested causes of death is impact against the wall of a fish tank. However, little is known about the prevalence of this type of collision death in land-based tanks, except in juveniles. We investigated deaths of 2-year-old fish to estimate the prevalence of collision death in land-based tanks. Two-year-old fish were reared in land-based tanks for 182 days after transfer, and dead fish were examined by dissection and/or X-ray to detect injuries to the skin, vertebral column, and skull. The prevalence of these injuries was considerably high (91%), but no symptoms of bacterial, viral, or parasitic disease were observed. In addition, the condition factor of the dead fish was within that of surviving PBT in sea net pens, suggesting that the fish did not die from starvation. These results indicate that collision with the tank wall was the primary cause of PBT mortality in land-based tanks. Collision death occurred frequently from dusk to dawn and immediately after transfer. Thus, limited vision and the environmental change after transfer were responsible for collision death in 2-year-old PBT. Our results suggest that the collision death rate of 2-year-old PBT could be improved by increasing light intensity during night just after transfer.Statement of relevanceBroodstock management practices of Pacific bluefin tuna need to be developed in land-based tanks to provide a stable supply of the eggs for the tuna farming industry. However, the survival rates are low in land-based tanks. Our study identified the primary cause of the mortality. We believe that this finding is a valuable first step to improve the survival rates.