Dietary Regulation of Highly Polyunsaturated Fatty Acids Content in Farmed Nile Tilapia (Oriochromis niloticus) and African Catfish (Clarius gariepinus)

Abstract

Vertebrates including fish, cannot produce PUFA de novo as they lack the ∆12 and ∆15 desaturases required to desaturate oleic acid (C18:1 n-9) to linoleic acid (C18:2 n-6) and then to α- linolenic acid (C18:3n-3). Therefore, biosynthesis of long chain fatty acid occurs from dietary precursors, α-linolenic acid (C18:3n-3) and linoleic acid, C18:2 n-6 through a series of microsomal fatty acid elongase (ELOVL) and desaturase (FADS2) mediated reactions. As a result, dietary composition of fish feeds has direct influence on fish fillet fatty acids composition including the n-3 long chain highly polyunsaturated fatty acids such as eicosapentaenoic acid, (EPA, C20:5, n-3) and docosahexaenoic acid (DHA, C22:6, n-3). Traditionally, fish feeds have been formulated using fish oil (FO) as lipid supplement to provide the essential fatty acids needed for optimum fish growth. However, the ever growing aquaculture sector can not be supported by the current world production of fish oil, prompting research on the possible use of vegetable oil as a replacement to fish oil in fish feed. The current study investigated the use of linseed oil as a dietary lipid source of the precursor, α-linolenic acid (C18:3n-3) for EPA and DHA synthesis in Nile tilapia (Oriochromis niloticus) and African catfish (Clarius gariepinus) feeds. In addition, the effect of dietary linseed oil on the fatty acid profile of Nile tilapia and African catfish was also investigated. Six iso-nitrogenious diets were formulated with linseed: sunflower oil ratios as follows; diet 1(100: 0), diet 2 (75:25), diet 3(50:50), diet 4 (25:75), diet 5 (0:100) and washout diet (100% olive oil). Thirty tilapia and thirty catfish fingerlings with an average body weight of 2.6 g and 9.1g respectively, were set in triplicate tanks (1000-liter capacity) for each diet treatment and fed twice a day to apparent satiation, with commercial diet used as a control diet. Growth parameters were measured in terms of changes in body weight and length. Tissue fatty acid composition was determined using gas chromatography. Real time PCR with CYBR green as fluorescent dye was used to determine the expression of ELOVL and FADS2 genes in each feeding experiment. Survival rate and specific growth rate were significantly (P<0.05) lower with >50% dietary linseed oil composition. Influence of dietary fatty acid composition on tissue fatty acid composition was observed as tissue fatty acid content varied with dietary inclusion of vegetable oil. Significantly higher total n-3 fatty acids were observed with diet 1 (100% linseed oil) in tissues of both O. niloticus and C. gariepinus. Tissue composition of n-3 fatty acids decreased with decreased dietary composition of linseed oil in both fish species suggesting a correlation between dietary lipid and tissue fatty acids.Tissue arachidonic acid, ARA C20: 4 n-6, content was low in both the fish tissues relative to the dietary precursor, linoleic acid, (LA, C18: 2 n-6) suggesting a possible utilization of ARA especially in intensive fish culture. Polyunsaturated fatty acids, especially DHA and EPA, deposition in the tissues increased with the feeding period with the third feeding month recording significantly (P<0.05) higher DHA, total n-3 and n-3/n-6 ratio in both fish species compared to first feeding month suggesting a greater accumulation of n-3 HUFA with age. There was significantly (p<0.05) higher gene expression in fish groups fed >50% of dietary linseed oil in both species compared to fish fed diet containing 0% linseed oil. The esression of both ELOLV and FADS2 genes followed same pattern.Data on the fatty acid profiles for fish sampled from selected fish farms showed that differences in feeds used by different fish farmers contributed to differences in the fatty acid.Some of the experimental diets in the present study resulted into higher DHA and EPA than both wild fish and fish sampled from different fish farms thus its possible to have farmed fish with even better fatty acid profiles than wild fish. For wild fish sampled from different beaches along Lake Victoria, heavier fish had significantly higher n-3 HUFAs compared to lighter suggesting the effect of weight or age on tissue fatty acids profiles. In conclusion, dietary linseed oil >50% reduced growth and survival rate in tilapia and catfish, however, it increased tissue accumulation of long chain n-3 fatty acids which also increased with the length of feeding period. Further research will be necessary to determine other dietary components whose co-supplimentation with linseed will result into composition optimal survival rate, growth and at the same time enhance good levels of long chain fatty acids such as DHA and EPA.