Suppression of aggressive behaviour in juvenile Atlantic cod ( Gadus morhua) by l-tryptophan supplementation

Aggressive interactions and cannibalism of juvenile Atlantic cod may cause substantial production losses under conditions of intensive rearing. In other teleosts, chronically increased brain concentrations and turnover of the neurotransmitter serotonin (5-hydroxytryptamine, 5-HT) are associated with suppressed aggression. Further, dietary supplementation with the serotonin precursor, l-tryptophan (TRP) suppresses aggression in juvenile rainbow trout ( Onchorhyncus mykiss) and reduces cannibalism in juvenile grouper ( Epinephelus coioides). In the present study, the behavioural effect of dietary TRP supplementation was observed in pairs of juvenile Atlantic cod subjected to repeated encounters for 10 days. After 3 days, one group was given TRP-supplemented feed (28 g/kg). A significant ( P < 0.01) decrease in aggressive acts (from 40 ± 2.7/10 min to 17 ± 2.0/10 min, mean ± standard error of mean) was seen after changing to TRP-supplemented feed. The mean number of aggressive acts was also significantly ( P < 0.05) lower in the TRP-treated group (17 ± 2.0/10 min) compared to a control group not receiving TRP treatment (29 ± 3.3/10 min). A second experiment examined the effect of TRP-supplemented feed (28 g/kg) on the activity of central 5-HT, quantified as the concentration ratio between the 5-HT metabolite, 5-hydroxyindoleacetic acid (5-HIAA), and 5-HT. In this experiment, the TRP-treated group ([5-HIAA]/[5-HT] = 0.56 ± 0.04) showed elevated values ( P < 0.038) compared with the control group ([5-HIAA]/[5-HT] = 0.43 ± 0.03). In conclusion, this study shows that juvenile Atlantic cod are highly aggressive and that supplementing the feed with TRP affects central 5-HT signalling systems and reduces this behaviour.