| Nowadays, with the development of aquaculture industry, the demand of fish oil (FO) in producing aquatic feed is becoming more and more big. At the same time, people are consuming more and more FO products for their health care. What's worse, the wild fishery resources, which are the main source of FO, are trending to decrease. These would make it much harder to balance the supply and demand of FO. As a result, the price of FO is getting higher and higher, which at last have made a great influence on the sustainable development of aquaculture industry. This has lead the researchers on fish nutrition and feed try to find the alternative lipid sources of FO in the recent years. The vegetable oils (VO) are the best choice for their wider source and lower price, which are rich in linolenic acid (18:3n-3, LNA) and linoleic acid (18:2n-6, LN), but lack of HUFA (EPA, DHA and ARA) which are necessary for normal growth and development of fish. For freshwater fish, which have the capacity to convert LNA and LN into HUFA, the FO can be patially or totally replaced in their feed. On the contrary, FO is more necessary in the feed of seawater fish, since they can not convert LNA and LN into HUFA or such ability is very low. Fortunately, in recent years, we demonstrated for the first in marine fish that the herbivorous rabbitfish fish Siganus canaliculatus has the ability to convert LNA and LN into HUFA. In order to develop the cost-efficient feed using VO as the dietay lipid source, two growth experiments were carried out to obtain optimum dietary ratio of LNA to LN and PUFA level in rabbitfish feed by evaluating the effects of different diets on growth performance and tissue fatty acid composition of S. canaliculatus. The results were as follows:1. Study on the optimum LNA/LN ratio in diets of S. canaliculatusWith casein as a protein source, glycerol trioleate (HUFA–free) as a basic lipid source, and fish oil (rich in HUFA) or perilla oil (rich in LAN and HUFA–free) as EFAs source, seven formulated diets (D1–D7) were prepared with approximately equal contents of total protein (32%) and lipid (8%), but different composition of EFAs. D1 contained fish oil with 18.34% HUFA as control diet, while D2–D7 used the mixture of perilla oil and glycerol trioleate as the fat sources and the ratio of LNA to LA is 0.05:1, 0.47:1, 0.93:1, 1.35:1, 1.93:1 and 2.45:1, respectively. Twelve-week growth experiment was conducted to investigate the optimal dietary LNA/LN ratio for S. canaliculatus. The results showed that the weight gain, specific growth rate in D3, D5 and D6 groups were significantly better than those in D2 and D4 (P < 0.05), but showed no obvious difference with D1 (control) group (P > 0.05). Besides, the feed utilization, survival rates and protein efficiency ratio showed no significant difference among all trial groups.The difference of dietary LNA/LA ratio didn't affect whole fish's contents of biochemical composition including moisture, ash, crude protein and lipid, but markedly affected the fatty acid composition in liver and muscle. The contents of 18:1n-9 and 18:2n-6 in both liver and fillet were decreased with the increased of dietary LNA/LA ratio (P<0.05). Reverse result was observed for 18:3n-3 in muscle. Among the VO feed groups, the contents of eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in tissues increased with the increasing dietary LNA/LA ratios, and those in D6 and D7 groups were significantly higher than other groups. While opposite trend was obsevered for ARA, and the content of arachidonic acid (ARA) in muscle of D3 group was the highest and showed no difference with D1 (control) group. The above results demonstrate that the dietary LNA/LA ratios play an important role for the growth performance and biosynthetic ability of HUFA in S. canaliculatus. When the LNA/LA ratios were 0.47, 1.35 and 1.93, better growth performance and HUFA biosynthesis can be obtained. Taking the source and price of VOs, as well as the effects of dietary LNA/LA ratios on fish's growth and quality together, the optimum dietary LNA/LA ratio is suggested to be 0.47:1.2. The optimum PUFA requirement of S. canaliculatusWith fishmeal and soybean meal as protein sources, and fish oil, rapeseed oil, linseed oil, palm oil and soybean oil as lipid sources, six formulated diets (F1~F6) were prepared with approximately equal content of total protein (32%) and lipid (8%), but different fat sources and PUFA level. F1 contained fish oil as control diet, and F2~F6 used the mixture of rapeseed oil, linseed oil, palm oil and soybean oil as lipid sources, which comprised PUFA contents of 41.95%, 38.18%, 33.83%, 29.94% and 27.12% of total dietary lipid, respectively. A 9-week feeding trial was conducted to obtain the optimum dietary PUFA level for S. canaliculatus. The results were as follows:Dietary PUFA levels didn't affect the growth of S. canaliculatus. And the growth performance including weight gain, specific growth rate, feed utilization and protein efficiency ratio showed no difference between the VO dietary groups and control group. However, the survival rate decreased with decreasing dietay PUFA., and that in 27.21% PUFA group was significantly lower than that in 41.6% PUFA dietary group (P<0.05). Hepatosomatic and viscerosomatic indexes were negative correlated with decreasing dietay PUFA content, and that in F5 group was significantly higher than that in contorl and F2 groups. The fatty acid composition of tissue was markedly affected by different dietary fat sources and PUFA levels, and the former mirrored the later. The contents of 18:3n-3, 18:2n-6 and 18:1n?9 in fillet of VO groups were remarkedly higher than that of FO (fish oil) group. And opposite trend was obsevered for EPA and DHA. However, the fillet ARA level showed no significant difference among all trial groups. The total PUFA content didn't show significant difference in liver among all tial groups. The fillet total PUFA content in D2 group was highest and significantly higer than that in control group (P<0.05).Taking the source and price of VOs, as well as the effects of dietary PUFA contents on fish's growth, survival rate and fillet quality together, the optimum dietary PUFA level is suggested to be high than 29% of totally dietay lipid, and the optimal diet fat sources are the mixture of rapeseed oil, soybean oil, palm oil and linseed oil with the ratio of 4:1:6:1. |
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