Size-dependent agonistic behaviour and mortality in juvenile black seabream, Acanthopagrus berda during nursery rearing: Implications of dentition-linked dominance

Agonistic behaviour during early developmental stages poses a significant challenge in marine fishes. In Acanthopagrus berda , observations revealed size-specific mortality clustered between 1.5 and 2.5 cm standard length (SL), prompting behavioural investigations. Controlled experiments were conducted to evaluate the effects of crowding (100, 200, and 300 fish / m³) and size variation (segregated and unsegregated groups) on growth, survival, and aggression in 100 L seawater aquaria. While crowding had no significant effect (p > 0.05) on final length or growth, size segregation significantly (p < 0.05) affected survival and growth. Despite having greater size variations; unsegregated (mixed) groups exhibited higher survival and growth than segregated (uniform) groups with higher mortality. Morphological analysis revealed that juveniles above 2.5 cm SL possess prominent canine-like teeth, enabling targeted attacks on smaller conspecifics. Across all trials, mortalities were consistently recorded in individuals < 2.5 cm SL, indicating a critical size threshold for vulnerability. These results highlight a behavioural size dominance hierarchy in A. berda juveniles, likely mediated by ontogenetic dentition development, and underscore the importance of periodic culling off of individuals above 2.5 cm from the nursery tanks to mitigate aggression-induced mortality. This study offers novel insights into intra-specific aggression patterns during early development and provides practical guidelines for behavioural management in hatchery systems.