Juveniles of Cobia Rachycentron canadum raised in reduced salinity: can the addition of NaCl in the diet affect growth performance and osmoregulation?
Marine organisms in reduced salinity encounter a physiological challenge different from that naturally found in ocean salinity. This is because they become hyper-osmotic in relation to the environment. The literature suggests that the addition of salt in the diet can supply passive ion loss and, consequently, improve growth. Thus, the effects of dietary salt supplementation (SD) on growth, survival, osmoregulation and branchial histological changes were evaluated in juvenile Cobia (12 g) raised in salinity 5. The Cobia, Rachycentron canadum, has received the attention of researchers and investors worldwide due to its positive characteristics that elect a species with potential in marine fish farming. For 40 days, the fish were fed, daily in two shifts,with diets containing different levels of NaCl: 0.0; 2.5; 5.0; 7.5 and 10.0% of the dry weight of the basal diet (all in triplicate). At the end of the experiment, the branchial arches were collected for histological evaluation and determination of Na+, K+ -ATPase activity. Survival was 100% in all groups and there was no difference in the final average weight between treatments. However, 7.5 and 10% NaCl resulted in worse feed conversion rates and higher food consumption compared to the other groups. Branchial Na+, K+ -ATPase activity was statistically reduced when fish were given diets with levels of 2.5; 5 and 7.5% NaCl compared to the 0.0% group. The number of chloride cells in the control group (16 cells mm-2) differed significantly from the SD groups.An increase in cell proliferation was observed according to the increase in salt in the diet, reaching 41 mm-2 cells in the gills of fish in the 10% NaCl group. These data suggest that NaCl supplementation is not necessary for growth in salinity 5, despite the fact that the bijupira has less Na+, K+ -ATPase activity in groups with low NaCl addition to the diet.