We examined the effects of feeding conditions and size differences on the aggressive behaviour and cannibalism in Pacific bluefin tuna (PBT) Thunnus orientalis larvae. In a 24 h experiment, restricted feeding alone was found to remarkably increase the frequency of aggressive behaviour, which was further elevated by differences in fish size. In a 4‐day rearing experiment, while aggressive behaviour was increased by restricted feeding alone, the frequency of cannibalism did not change significantly. Although the frequency of aggressive behaviour did not increase with difference in size factor, small fish in this group gradually tended to die over 4‐day period. In the restricted feeding and size difference group, large fish were observed to bite the small fish, and almost all the small fish died on the day after the start of the experiment. These results suggest that the aggressive behaviour of PBT larvae is chiefly increased by the shortage of live food; however, deaths related to cannibalism mainly occurred in small larvae and rapidly increased with food restriction and differences in fish size.

Aggressive behaviour, assessed by 24-h observations, and survival rates, determined in a 2-week rearing experiment, were examined in greater amberjack, Seriola dumerili, subjected to single and multiple factors (high density, restricted feeding and diverse size). Aggressive behaviour frequency in the group subjected to both restricted feeding and size diversity (FS) was signi¢cantly higher than that in the other groups (Tukey, n 5 4, Po0.05). In the FS group, aggressive behaviour increased with the starvation time from 19 days post hatching (dph) to 23 dph (twoway analysis of variance, Tukey, n 5 4, Po0.05), but decreased gradually thereafter to 46 dph. In the rearing experiment, the survival rates for the diverse size and FS groups were lower than those of the control or restricted feeding groups (Kaplan^Meier, log-rank test, n 5100, Po0.01). Daily mortality in all groups was the highest at 24 dph; although dead ¢sh were usually small, disappearance due to cannibalism was not observed during the rearing period. The results indicate that aggressive behaviour in greater amberjack is induced by hunger and size diversity in the early juvenile stages (23 dph). Mass mortality during seedling production was mainly caused by injury and the death of small ¢sh due to aggressive behaviour/cannibalism.