Effects of stocking density and rearing factors on aggressive behaviour and cannibalism in the Pacific bluefin tuna Thunnus orientalis (Temminck & Schlegel) larvae

We studied the effects of stocking density and multiple factors on the aggression and cannibalism of Pacific bluefin tuna, Thunnus orientails (Temminck & Schlegel) larvae. In feeding experiment, if the appropriate food was abundant and the fish a uniform size, the frequency of aggressive behaviour was unaffected by the increase in stocking density from low-(0.1 fish/L) to high-density (0.5 fish/L). However, high-density conditions significantly increased aggressive behaviour when food was scarce. Aggressive behaviour increased only slightly due to fish size differences, and there was no increase due to high-density conditions. However, when fish of different size were present under a restricted feeding regime, aggressive behaviour significantly increased compared to the case of restricted feeding alone and was further amplified at high-density. Cannibalistic mortality was more likely for small-size fish, which was significantly increased by high-densities. In particular, when fish size differences and feeding shortages overlapped, cannibalism increased sharply as does aggressive behaviour. Even if the fish are reared at high-density, if the shortage of appropriate food and the spread of fish size difference are prevented, the aggressive behaviour is suppressed, as the result, it is possible to rear at least 0.5 fish/L with thorough food management.