World Aquaculture 5 Before fecal collection, all possible care was taken during feeding so that no uneaten feed settled to the tank bottom. The fecal collectors were removed from the tanks and the tanks were thoroughly cleaned 30 min after feeding. After collection, fecal samples were freeze-dried and analyzed to estimate the digestibility of protein, lipid and energy. Results and Discussion Red sea bream exposed to a 24L:0D photoperiod showed the highest total weight gain and specific growth rate [SGR (percent) = 100 × (lnW2-lnW1)/time (days), where, W1 and W2 indicate the initial and final weight (g)] compared with fish exposed to other photoperiods (Figure 3). Weight gain in fish exposed to 24L:0D was 44.4 percent higher than that of fish exposed to 12L:12D. Similarly, feed intake in fish reared under 24L:0D photoperiod was 41.0 percent higher than those reared under 12L:12D (Figure 4). Feed conversion efficiency [FCE ( percent) = 100 × {wet weight gain (g) / dry feed intake (g)], was higher in fish exposed to 24L:0D and 16L:8D (Figure 4). The higher food intake in continuous photoperiod is a result of diurnal fishes being more active under continuous photoperiods and having greater foraging activity when food is delivered. It is also related to a positive effect of growth hormone on appetite (Johnsson and Björnsson 1994). Feeding time is also one of the important factors causing variation in feed intake among the treatments. It is generally assumed that the fish take more feed when the feeding time coincides with the time of maximum appetite. Therefore, the remarkable higher food intake and FCE in 24L:0D suggested that the feeding strategy in fish exposed to that photoperiod reflected most closely the times of maximum appetite. Fig. 3. Variation in weight gain and specific growth rate (SGR) among photoperiods. [SGR ( percent) = 100 × (lnW2-lnW1)/time (days), where, W1 and W2 indicate the initial and final weight (g), respectively]. Fig. 4. Variation in feed intake and feed conversion efficiency (FCE) among photoperiods. [FCE ( percent) = 100 × {wet weight gain (g) / dry feed intake (g)}]. The digestibility of protein, lipid and energy was higher in fish exposed to 16L:8D and 24L:0D (Table 1). This was the result of a longer time interval between feeding times in fish exposed to 16L:8D and 24L:0D that have allowed the most efficient digestive process. This might have improved digestion and retention efficiency in both treatments. This resulted in a significantly higher FCE in fish exposed to 16L:8D and 24L:0D. These results suggest that growth was influenced by photoperiod through better food conversion efficiency and not just through stimulated food intake (Boeuf and LeBail 1999). The lower growth performance in 6L:6D, in spite of a longer time interval between feeding times, may be attributed to the dissipation of energy for other purposes. In the aquaculture industry, fish stress, which is simply defined as any threat to or disturbance of homeostasis, is a growing concern inasmuch as it has caused reduced growth rate, disease resistance and food intake and increased mortality. Therefore, although higher growth performance was observed in fish reared under 24L:0D photoperiod, it would be premature to propose that photoperiod as optimal for rearing fish without careful analysis of how it photoperiod affects stress level. To clarify whether or not 24L:0D caused Table 1. Apparent digestibility coefficient (ADC) of protein, lipid and energy in fish exposed to different photoperiods. ADC (%)1 6L:6D 12L:12D 16L:8D 24L:8D Protein 94.6 94.4 96.2 95.4 Lipid 91.5 91.5 93.6 93.5 Energy 87.2 86.3 87.1 88.2 1ADC of nutrients or energy (%) = 100 × [1 – {(dietary Cr 2O3 / fecal Cr2O3) × (fecal nutrient or energy / dietary nutrient or energy)}]
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