Day-night alternation prevails over food availability in synchronising theactivity of Piaractus brachypomus (Characidae)

Pirapatinga given submaximal rations grow faster when fed at night than whe n fed during daylight hours. It was hypothesised that these fish were day-n ight conformers, with a nocturnal acrophase, and expended more energy durin g 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 degrees 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 i mplied a feasibility study to test the adequacy of tagging fish with surgic ally implanted transmitters (ventrolateral incision, posterior to the pelvi c girdle). There was no mortality or tag loss over 12 months following surg ery, 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 i ndicate 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. (C) 2000 Ifremer/Cnrs/Inra/Ird/Cemagref/Editions scientifiques et medicales Elsevie r SAS.