Survival, growth and digestive enzyme activity of juveniles of the fat snook (Centropomus parallelus) reared at different salinities

The effect of salinity on survival, growth and activity of digestive enzymes was evaluated in the fat snook (Centropomus parallelus). Juveniles of 76 days after hatching (0.35 g) were reared at 5, 15 and 35 ppt, in triplicate, for 50 days, at 0.6 fish/l. Snook presented excellent survival (> 93.3%) at 5, 15 and 35 ppt, demonstrating the euryhalinity of the species. At the end of the experiment, no differences in weight and specific-growth rates (mean 1.8%/day) were observed, however, total and standard length values were higher at 15 ppt when compared to those at 5 ppt (P > 0.05). The best results in food conversion ratio (1.3) and digestive enzymes activity were obtained at 15 ppt. The activity of total alkaline proteinase was significantly affected at this salinity (0.124 ± 0.006 Δ absorbance366 nm/min/ml/mg protein), being two-fold and six-fold higher, compared to 35 and 5 ppt, respectively. The activity of total amylase was higher at 15 and 35 ppt (mean 0.016 ± 0.001 μmol reducing sugar/min/ml/mg protein), compared to 5 ppt (P < 0.05). Results indicate that fat snook reared at 15 ppt presented a higher potential for a more efficient digestibility and nutrient absorption, especially proteins. Additionally, at this salinity, the energetic demand for osmoregulation is probably reduced by the isosmotic medium, leading to growth enhancement. In terms of production costs, feeding expenses can be lowered at this salinity due to a better food conversion ratio.