Effects Of Dietary Lipid Levels On Growth, Fat Deposition, Fatty Acid Profiles And Fatty Acid Synthase Of Gift Strain Nile Tilapia,Oreochromis Niloticus

Lipids are the best form of nutrients as energy provider and energy reserve for fish, they also can provide the essential fatty acids (EFA). The deficiency of dietary lipid can lead to many side effects on fish, such as lower protein bioavailability, deficiency of lipid-soluble EFA. But the excessive lipid can lead to over-deposition of fat and low resistance to disease. So the dietary lipid should be optimal. It was mainly through the growth of fish to determination the suitable requirements of dietary lipid levels, the changed of critical enzyme activities in fat metabolism reflects a situation of lipid requirement of fish from molecular level.The juvenile fish of GIFT (Genetically Improved Farmed Tilapia) strain tilapia were used in this study. Triplicate groups of fish were fed for90days with the experimental diets formulated with increasing lipid levels (lipid levels group of1.73L,3.71L,5.69L,7.67L,9.64L and16.55L) using fish oil as the lipid source. The study was mainly focused on the effects of deitary lipid levels on the growth, fat deposition, fatty acid synthetase (FAS) activity and its gene expression, in order to provide related theoretical data for the study of fish lipid nutrient, lipid metabolism and enzymology. The main results were as follows:1. Effects of dietary lipid levels on the growth of GIFT strain tilapiaWhen the dietary lipid levels percentage ranged from5.69%to9.64%, the fullness of GIFT were significantly higher than other groups (p<0.05), the lipid content beyonded this range could decrease the fullness. The ratio of the intestine length to body length in group7.67L was significantly higher than in the group3.71L (p<0.05), which indicated that the feed of group7.67L may promote the growth of the intestine of GIFT. The hepatosomatic index of the group1.73L and16.55L were significantly higher than other groups (p<0.05), while the feed with the percentage of lipid ranged from3.71%to9.64%had no significant effect (p>0.05). With the exception of the group1.73L, viscerosomatic index increased with the increase of the lipid level of the feed. The viscerosomatic index of the group1.73L was significantly higher than that of the group3.71L. Results indicated that the excess and less of the lipid content in feed both significantly affected the shape and the volume of the liver. The feed with right lipid level could promote the growth of intestine. The feed with suitable fish oil added could decrease the viscera weight of the body.The dietary lipid levels had significant effect on the growth of the GIFT. The higher dietary lipid level, the less feed intaken by fish. The more or less lipid could reduce the weight gain and specific growth ratio. High dietary lipid level significantly reduce the feed conversion ratio (p<0.05) and enhanced the protein utilization. At the end of the experiment in7.67L group, the fish had large average weight and body length, weight gain and specific growth ratio were also higher than other groups. It indicated that when the dietary protein was30%and the dietary lipid level was7.67%, the GIFT had best growth performance.2. Effects of dietary lipid levels on fat deposition and fatty acid profilesThe fat contents in muscles and liver of GIFT had great relationship with the dietary lipid levels. When the dietary lipid levels increased from9.96%to17.27%, the fat contents in the muscles and liver also ascended from25.92%to32.04%while mesenteric fat content sustained around90%, which was not affected by the dietary lipid levels.The main types of fatty acids in the excrement were C14:0, C16:0, C18:0, C18:2n-6, C16:n-9, C18:n-9, C20:n-9, C22:n-9, in which both C16:0and C18:n-9accounted for70-80%of total fatty acids. The proportion of C16:0in excrement increased steadily and the proportion of C18:n-9decreased gradually as the dietary lipid level increased. The n-3PUFA were not detected in the excrement, but about1-8.71%of C18:2n-6in excrement was detected. It indicated that GIFT could absorb and utilize the n-3PUFA easily.The profiles of fatty acids in muscle and liver reflected the profiles of fatty acids in feed. When the dietary lipid levels increased, saturated fatty acids (SFA) and multi-unsaturated fatty acids (MUFA) had obvious descending tendency and the proportion of n-3PUFA in muscle were increasing and then was up to the proportion of it in the fish oil steadily, while the SFA in liver were decreasing and n-3PUFA were increasing. The MUFA had no significantly changes in liver. The Σ-3/Σn-6index were ascending from0.90to4.02in muscle as it were increasing from2.61to4.62in liver, which indicated that the index was more stable in liver than in muscle. Compared with muscle and liver, mesenteric profiles of fatty acids was very stable, which could not be affected by the dietary lipid levels. The profiles of fatty acids in fish head and fish skin was similar to that in muscle.3. Effects of dietary lipid levels on fatty acid synthetase (FAS) activity and its gene expression When the dietary lipid levels increased, the FAS activity in the liver had descending tendency, but the differences were not significant (p>0.05). The differences of the FAS activity in liver were very small among the groups when the dietary lipid levels ranged from1.73%to3.71%. When the dietary lipid levels were more than or equal to9.64%, the FAS activity in liver increased in a large extent. When the dietary lipid was sufficient, the expression abundances of FAS gene in liver and muscle decreased so that the biosynthesis of the fatty acids was reduced.