Feeding a high-density pellet improves the size homogeneity of juvenile tambaqui (Colossoma macropomum)

This study aimed to evaluate the influence of bulk feed density on feeding behavior, aggressiveness, growth performance, and homogeneity of juvenile tambaqui (Colossoma macropomum). It consisted of three experiments. The first experiment assessed the extrusion process and physical characteristics of diets with different extrusion settings, resulting in high-density pellets (HDP), medium-density pellets (MDP), and low-density pellets (LDP). The second experiment evaluated the growth performance and homogeneity of juvenile tambaqui (23.3 ± 1.7 g, 11.4 ± 0.3 cm) fed these diets. A total of 225 fish were distributed across 15 tanks (500 L, 15 fish/tank) and fed four times daily for nine weeks. The third experiment observed behavioral interactions, including feeding and aggression, by monitoring fish for 360 minutes over three days. Results showed that pellet physical characteristics were significantly influenced by extrusion settings, except for water solubility and absorption. In experiment two, the feed density (HDP, MDP, or LDP) did not affect weight gain, but the coefficient of variation values indicated that fish fed HDP were more homogeneous, with a coefficient of variation of 5.76 %. In experiment three, fish fed MDP exhibited more aggressiveness and social instability. In conclusion, tambaqui fed HDP showed a more stable social hierarchy, less aggression, and smaller size variation, suggesting that HDP may lead to more homogeneous batches during the grow-out stage.