Diel feeding and feeding-entrainable locomotor activity rhythms in cultured greenback flounder, Rhombosolea tapirina (GuvÉv†nther 1862)

The principal aims of the present study were to examine diel feeding and feedingentrainable locomotor activity rhythms in cultured greenback flounder Rhombosolea tapirina (Gunther 1862) and to examine their applications to the culture of the species. A computerised tank-based waste pellet monitoring system was developed and used to study feeding rhythms in greenback flounder. It consisted of an infrared sensor and a recording unit capable of continuously detecting wastage with reference to time. Actual feed intake was determined as the difference between pellets presented by a mechanical belt feeder and the uneaten pellets detected by the monitoring system. Diel feeding rhythms were described by the temporal variation of actual feed intake at intervals of 30 mm. The role of light-dark (LD) cycles and food deprivation in the regulation of diel feeding rhythms in greenback flounder (75-281 g) was examined. The fish is a diurnal feeder, but is capable of feeding at night. Feeding appeared to have an endogenous circadian component. A LD 12:12 cycle was a potent environmental cue to entrain circadian feeding rhythms. Re-feeding after a single period of food deprivation of 26-57 h was capable of temporarily over-riding feeding rhythms. The effect of feeding regime (difference in temporal distribution of feed) on growth and proximate body composition was investigated. The feeding regimes had an impact on the growth, with 1 meal of 3 h during the scotophase resulting in lower growth than the other five feeding regimes (feed continuously available, 12 meals of 1 h per meal, 6 meals of 1 h per meal, 3 meals of 2 h per meal, and 1 meal of 3 h during the photophase). However, the feeding regimes did not significantly influence proximate body composition. Locomotor activity patterns were examined in relation to feeding regimes and combinations of meal size and meal duration at a single daily meal. Groups assigned to a higher frequency (and hence a longer total duration) of daily meals were less active during mealtime even though the daily ration was equivalent as percentage of body weight per day. A distinct food anticipatory activity (FAA) was observed in the groups of fish receiving a single daily meal either during photophase or during scotophase, and FAA also developed before at least one out of three daily meals but not before any meal in the regimes of six daily meals or more than six daily meals. It was further demonstrated that whether or not a single daily meal is a potent zeitgeber to synchronise daily locomotor activity pattern depends on the combination of meal size and meal duration. The fish fed at the meal sizes of 0.25 and 0.5% BW day' exhibited FAA under all the three meal durations (1, 3 and 7 h) investigated. FAA was also expressed by the fish fed at 1.5% BW day' but only combined with the meal duration of 1 h. But, weight loss is not essential for the fish to develop FAA. At each meal size, the group assigned to the 7 h meal had lower and shorter FAA than that assigned to the 3 h meal, which in turn had lower values than that assigned to the 1 h meal. There was a positive correlation between the mean durations of FAA and post-feeding activity (r = 0.87; P < 0.01; n = 7). The results suggest that the greenback flounder is capable of responding to the energetic (meal size) and temporal (meal duration) impacts of a single daily meal. The latency of FAA was about 21 days. FAA persisted up to three days during food deprivation, indicating that FAA may be mediated by a feedingentrainable circadian oscillator. The present study is one of only a few investigating diel feeding rhythms, feeding-entrainable locomotor activity rhythms and effects of temporal distribution of feed on growth performance in cultured flatfish. The finding that magnitude and duration of FAA in greenback flounder were influenced by meal size and meal duration of a single daily meal is perhaps the first obtained for fish, and may have implications for the elucidation of the feeding entrainment pathways which remain unknown for all animals.