Day–night alternation prevails over food availability in synchronising the activity of Piaractus brachypomus (Characidae)

Pirapatinga given submaximal rations grow faster when fed at night than when fed during daylight hours. It was hypothesised that these fish were day–night conformers, with a nocturnal acrophase, and expended more energy during daytime feeding. In order to test this hypothesis, telemetry was applied to study the behaviour of cultured pirapatinga (stocking biomass of 40 kg·m –3; 27 ± 1 °C; 6 mg O 2·L –1; 13.5 L/10.5 D) under three feeding schedules (diurnal versus nocturnal feeding [12 g·d –1·fish –1 over 10 h] versus fasting) that were evaluated in succession over 1 week each. This implied a feasibility study to test the adequacy of tagging fish with surgically implanted transmitters (ventrolateral incision, posterior to the pelvic girdle). There was no mortality or tag loss over 12 months following surgery, and abdominal incisions healed within 4 weeks. Under all three feeding schedules, activity increased at dawn, peaked during daytime, decreased at twilight and reached a minimum during the late night. Diurnal and twilight activity levels were similar under all three schedules, whereas nocturnal activity was significantly lower with daytime feeding. These observations indicate that pirapatinga have a diurnal activity acrophase, which is little influenced by food availability, and imply that the higher growth of fish fed at night does not originate from lower energetic expenditures. La croissance du pirapatinga en aquaculture est nettement plus élevée lorsque l’aliment est exclusivement distribué au cours de la scotophase. L’hypothèse testée dans cette étude est que le pirapatinga avait un type d’activité typiquement nocturne, synchronisé par l’alternance jour-nuit, et que la distribution d’aliment au cours de la photophase impliquait des dépenses énergétiques supplémentaires. Pour tester cette hypothèse, l’activité locomotrice du pirapatinga a été étudiée par télémétrie en milieu contrôlé (biomasse de 40 kg·m –3 ; 27 ± 1 °C ; 6 mg O 2·L –1 ; 13,5 L/10,5 N), dans trois contextes d’alimentation (diurne opposée à nocturne [12 g·j –1·poisson –1 pendant 10 h] versus à jeun) évalués successivement pendant une semaine chacun. L’étude a requis la mise au point et l’évaluation d’un protocole d’implantation abdominale d’émetteurs de télémétrie adapté à des poissons compressés latéralement (incision ventrolatérale, postérieure à la ceinture pelvienne). Aucune mortalité ou perte de marque n’est survenue pendant les 12 mois suivant le marquage, l’incision abdominale é