This study aimed to investigate stocking density during larviculture of Colossoma macropomum in recirculating aquaculture system with slightly saline water. Densities of 10 (D10), 30 (D30) and 50 (D50) larvae/L were tested with five replicates each. Larvae were fed with Artemia nauplii during the first 15 days and with commercial diet from day 16 to day 30. Water was maintained at a salinity of 2.01 ± 0.41 g of salt/L, and performance and survival were evaluated throughout the experiment. At the end of the experiment, larvae of each tank were classified according to size as small—S (<18.0 mm and <0.08 g), medium—M (>18.0 mm and <22.2 mm, >0.08 g and <0.15 g) and large—L (>22.2 mm and >0.15 g), and the size distribution was calculated for each stocking densities evaluated. Larval weight, length and specific growth rate were not affected by the tested stocking densities throughout the experiment (p > .05). Heterogeneity was recorded at 30 days of larviculture for all tested densities with a predominance of class M animals. Survival during feeding with Artemia was not affected by the tested densities (p > .05). Survival during feeding with commercial diet was highest for D10 and lowest for D30 (p < .05). Global survival, considering the experiment as a whole, was similar among the tested stocking densities (p > .05). Larviculture of C. macropomum can be successfully carried out at stocking densities of up to 50 larvae/L during the first 30 days of rearing in slightly saline water in recirculating aquaculture system.

It is known that the level of dietary protein modulates the enzymatic activity of the digestive tract of fish; however, its effect at the molecular level on these enzymes and the hormones regulating appetite has not been well characterised. The objective of this study was to evaluate the effect of CP on the activity of proteases and the expression of genes related to the ingestion and protein digestion of juveniles of red tilapia (Oreochromis sp.), as well as the effects on performance, protein retention and body composition of tilapia. A total of 240 juveniles (29.32 ± 5.19 g) were used, distributed across 20 tanks of 100 l in a closed recirculation system. The fish were fed to apparent satiety for 42 days using four isoenergetic diets with different CP levels (24%, 30%, 36% and 42%). The results indicate that fish fed the 30% CP diet exhibited a higher growth performance compared to those on the 42% CP diet (P < 0.05). Feed intake in fish fed 24% and 30% CP diets was significantly higher than that in fish fed 36% and 42% CP diets (P < 0.05). A significant elevation of protein retention was observed in fish fed with 24% and 30% CP diets. Fish fed with 24% CP exhibited a significant increase in lipid deposition in the whole body. The diet with 42% CP was associated with the highest expression of pepsinogen and the lowest activity of acid protease (P < 0.05). The expression of hepatopancreatic trypsinogen increased as CP levels in the diet increased (P < 0.05) up to 36%, whereas trypsin activity showed a significant reduction with 42% CP (P < 0.05). The diet with 42% CP was associated with the lowest intestinal chymotrypsinogen expression and the lowest chymotrypsin activity (P < 0.05). α-amylase expression decreased with increasing (P < 0.05) CP levels up to 36%. No significant differences were observed in the expression of procarboxypeptidase, lipase or leptin among all the groups (P > 0.05). In addition, the diet with 42% CP resulted in a decrease (P < 0.05) in the expression of ghrelin and insulin and an increase (P < 0.05) in the expression of cholecystokinin and peptide yy. It is concluded that variation in dietary protein promoted changes in the metabolism of the red tilapia, which was reflected in proteolytic activity and expression of digestion and appetite-regulating genes.

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While some studies have investigated the effects of dietary protein on the activities of protease in Nile tilapia, little information exists about these enzymes and hormones regulating appetite at the molecular level. Therefore, we investigated the performance, protease activities and expression of genes related to protein intake and digestion in juvenile tilapia fed with diets containing different levels of crude protein (CP): 25.14, 31.12, 36.60 and 42.05%. The fish were fed to apparent satiation three times a day for 65 days. The animal performance and the retention rates of protein and energy linearly decreased with an increase in dietary CP content. A positive quadratic effect of CP on pepsinogen was observed, although the acid protease activity was not affected. Trypsinogen and trypsin activity in the intestine presented similar patterns, showing a positive quadratic response to dietary protein levels. A linear increase in intestinal chymotrypsinogen expression was observed, but the activity of chymotrypsin showed a positive quadratic response. In addition, the expression of cholecystokinin (cck) and peptide yy (pyy) increased linearly with increasing CP. Dietary protein modulates the activity of alkaline proteases as well as the expression of these protease genes and hormones involved in appetite regulation. These changes can help expand our understanding of feed intake and digestion control, thereby improving feed formulations for this species.

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