Effects of Different Ratios of DHA/EPA on the Growth and Fatty Acids of Cobia (Rachycentron Canadum) Juvenile
|Keywords||cobia (Rachycentron canadum) DHA EPA ratio growth fatty acids|
An experiment was conducted in laboratory to study the effects of different ratios ofDHA/EPA on the growth, survival and fatty acid composition in the muscle, liver and serum ofcobia (Rachycentron canadum) juvenile from August to October 2005. Three hundred andtwelve (312) cobia larvae (age 56-day, body weight 6.86+0.03g, body length 9.23_+0.06cm) wereselected and randomly assigned into 8 groups with triplicates each, total 24 tanks with 13 fisheach. Sea-water temperature was from 26 to 30.5℃and salinity from 25.35 to 33. Sevenformulated diets (D-1 to D-7) with the same composition and varying ratios of DHA/EPA (2.10,1.90, 1.70, 1.50, 1.30, 1.10, 0.90, respectively, justed by adding fish oils) and a control diet(DHA/EPA ratio, 1.33, without additional fish oil) were used to feed the cobia juvenile for 8weeks. Samplling were at the end of 0, 4th and 8th week, respectively.The experimental results were as follows: (1) higher livability(92.97％-100％) of cobiajuvenile was observed in all the experimental diets; (2) the fastest growth of cobia juvenile wasobserved in control diet and no significant difference in the growth of cobia juvenile fed dietswith varying ratios of DHA/EPA; (3) no significant difference (P＞0.05) was found in theweight gain rate (WGR), specific growth rate (SGR), feed conversation rate (FCR) and proteinefficiency (PE) in all experimental groups of juveniles; (4) FCR (1.12-1.34) in the first fourweeks was better than that in the last four weeks (1.57-2.27) for all fish.The results showed that fatty acid composition of cobia juvenile was significantly affectedby those of dietary fatty acids. The fatty acid composition in the muscle, liver and serum of thejuvenile varied with that of diets, particularly in the first 4 weeks. Fatty acid composition in theserum of cobia juvenile was strongly affected by dietary fatty acids as well as their metabolism,while weaker in the liver and muscle of the juvenile.Different degrees of fat-liver were observed in all experimental groups of fish and thelonger cultured time, the heavier fat-liver. The problem could be attributed to the higher fatcontent in diets.In conclusion, the growth of cobia juvenile was affected by the ratio of DHA/EPA as wellas their content in diets. The optimal ratio of DHA/EPA for cobia juvenile in the diet is 1.3,while their optimal amount is about 0.8％of dry diet in our experiment. Excessive n-3 HUFAsupplement in diet may be not available for the growth of cobia juvenile.