Since agonistic interaction has been a major obstacle for fish hatchery and represents great economic losses for fish farmers, aggressiveness during the different post-hatching stages of the matrinxã, Brycon amazonicus, is analyzed and compared. Each group of three larvae was observed at six different sessions: 12, 24, 36, 48, 60 and 72. ±. 2. h after hatching (HAH) for 20. min, with 10 repetitions for each treatment. Aggressive behavior and motor activity were described by an ethogram: approach, attack, chase, flight, frontal display, mouth fight, mouth opening, threat, circular and linear swimming, contact and Ω-posture. Biometric measurements have indicated an increase in total length and mass (P<. 0.001). Number of aggressive behaviors (12 HAH: 95.80. ±. 14.68; 24 HAH: 53.40. ±. 20.99; 36 HAH: 12.40. ±. 9.44; 48 HAH: 22.80. ±. 18.89; 60 HAH: 35.20. ±. 17.57 and 72 HAH: 92.40. ±. 28.54, P<. 0.001) and duration of motor activity (12 HAH: 1446.70. ±. 160.70; 24 HAH: 453.10. ±. 184.95; 36 HAH: 100.50. ±. 42.18; 48 HAH: 22.8. ±. 110.30; 60 HAH: 99.10. ±. 23.83 and 72 HAH: 822.90. ±. 361.28, P<. 0.001) in the group was higher at 12 and 72 HAH. Results suggest that lower motor activity reduced the probability of encounters between larvae and, therefore, social interactions between group members of B. amazonicus. © 2014 Elsevier B.V.
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