L‐<scp>TRP</scp> supplementation alters the aggressive behaviour of yellowtail lambari (<scp><i>Astyanax altiparanae</i></scp>)

The yellowtail lambari (Astyanax altiparanae) is a fish species with considerable commercial potential that commonly exhibits aggressive behaviour toward conspecifics. Although body size is a key factor in the establishment of a social hierarchy in other species, its influence on aggression in lambari remains unclear. Moreover, dietary supplementation with L-tryptophan (L-TRP) has shown potential in modulating aggression in fish, warranting investigation in aquaculture contexts. This study aimed to generate an ethogram of aggressive behaviours in yellowtail lambari and assess the effects of body size and L-TRP supplementation on aggression. In the first experiment, direct confrontations between larger and smaller fish revealed that larger individuals exhibited higher frequencies of aggressive behaviour. In the second experiment, fish were fed either a control diet or diets supplemented with L-TRP (0.69 or 1.38 g/100 g) for 10 days, after which they were subjected to direct confrontation with an intruder. L-TRP supplementation resulted in a reduction in the frequency of some aggressive behaviours. These findings enhance our understanding of aggression in yellowtail lambari and provide insights into improving management and welfare in aquaculture.