Influences of water salinity and dietary fat content on growth, nutrient composition and fillet quality of Nile tilapia (Oreochromis niloticus)

In order to study the combined effects of water salinity and dietary fat content on the growth, fillet nutrient composition and quality of Nile tilapia (Oreochromis niloticus), we designed the following experiment. Three different salinities (0, 8, 16) were set to raise O. niloticus initial weight (5.0±0.2) g. And in each salinity-group, O. niloticus was fed with two lipid-content diets, medium and high lipid diet respectively (MFD, 6% and HFD, 12%). After 8 weeks of breeding, the fish were sampled, and the growth, serum biochemical parameters, muscle nutritional composition and fillet quality related parameters were assayed. The results showed that, among MFD groups, the fish in medium salinity group (salinity 8) showed the highest final weight (FW), carcass ratio (CR), mesenteric fat index (MFI) and contents of total protein, amino acids and lactate in the muscle, but decreased feed conversation ratio (FCR), viscerosomatic index (VSI), hepatosomatic index (HSI) and pH of muscle, compared with the control group (salinity 0) and high salinity group (salinity 16). And compared with the control group and middle salinity group, the fish in MFD-high salinity group showed increased FCR, condition factor (CF), VSI, HSI, whole fish lipid, hepatic triglyceride (TG) and serum glucose, lactate, TG and glutamic oxaloacetic transaminase (GOT), but reduced whole fish moisture, muscle moisture, whole fish ash, meat yield (MY) and centrifugal water loss. In HFD group, with the increase of salinity, FW, survival rate and MFI were gradually decreased, and FCR, VSI, HIS and CF were increased. In HFD group, the fish in medium salinity group showed reduced muscle total protein and amino acid content. HFD-high salinity cultured fish showed increased whole fish ash, hepatic TG, serum glucose, GOT, muscle total protein, muscle TG and PL content, and decreased whole body lipid, hepatic glycogen, MY, centrifugal water loss, pH. However, either in freshwater or saltwater, the fish fed with HFD showed higher fat accumulation and muscle lactic acid content, but lower survival rate and MY, compared with MFD. Particularly in saltwater, the HFD fed fish showed significantly higher fat accumulation and muscle lactic acid content, but lower survival rate, FCR and MY. These results showed that suitable salinity (e.g. 8) of aquaculture water environment can improve the growth and flesh quality of O. niloticus, but the combination of HFD and high water salinity had negative effect on growth and flesh quality of O. niloticus.