Optimal dietary lipid requirement of advanced juvenile Nile tilapia Oreochromis niloticus

Abstract

Dietary lipids play an important role in fish nutrition as a source of energy,essential fatty acids,and as a vector for fat-soluble vitamins that are needed to maintain the biological structure and normal function of cell membranes.A number of studies have shown that increasing the proportion of lipid in fish feed reduces the need for protein and results in improved growth,feed conversion efficiency,and palatability,and reduces environ-mental pollution from the feed and its metabolites.However,little is known about the dietary lipid requirements of juvenile Oreochromis niloticus.Our objective was to determine the optimal dietary lipid requirement of ad-vanced juvenile O.niloticus.We randomly assigned fish(n= 360) with average initial body weight of(46.14± 4.67) g into 6 groups consisting of 4 replicates(n=15 fish/replicate).Each group was fed a diet containing either 0(control group),3%,6,9%,12%,or 15% fish oil for 8 weeks(actual lipid level was 0.20%,2.70%,6.11%,8.04%,11.13%,and 14.85%,respectively).We measured growth,lipid deposition,serum biochemical indexes,and lipid metabolism enzyme activities at the end of the trial.Weight gain rate(WGR),specific growth rate(SGR),and protein efficiency ratio(PER) initially increased with increasing lipid concentrations but then decreased.Conversely,the feed conversation ratio(FCR) first decreased then increased.Using second order regression analysis,we estimated that the optimal level of dietary lipid to promote WGR,FCR,and PER was 8.86%,9.75% and 9.40%,respectively.An increase in dietary lipid levels was associated with a significant increase in the hepatosomatic index(HSI) and crude fat content in both the whole body and muscle(P0.05).Serum total cholesterol(TCHO) and triglyceride(TG) levels first increased then decreased as the level of die-tary lipid increased.High density lipoprotein cholesterol(HDL-C) content first increased then plateaued as lev-els of dietary lipid increased.Broken-line regression analysis of the HDL-C content against dietary lipid level indicated that the dietary lipid requirement for optimal HDL-C content was 8.30%.Alanine aminotransferase(ALT) and aspartate aminotransferase(AST) activity were highest in the group fed 11.13% dietary lipid and lowest in the group fed 6.11% dietary lipid.Intestine lipase,lipoprotein lipase(LPL),hepatic lipase(HL) and total lipase(TL) activity tended to first increase and then decrease as the level of dietary lipid increased.Conversely,fatty acid synthase(FAS) activity decreased significantly(P0.05).Our comprehensive analysis of growth performance,body composition,serum biochemical indexes,and lipid metabolism enzyme activity suggests that the optimal level of dietary lipid for advanced juvenile O.niloticus is between 8.30%?9.75%.