Essential fatty acids in the diet of silver perch (Bidyanus bidyanus): effect of linolenic and linoleic acid on growth and survival

The development of more cost-effective feeds for silver perch, depends on a better understanding of the nutrient requirements of this species. The dietary requirements of silver perch for linoleic acid (LA) and α-linolenic acid (ALA) were examined in a 4 x4 factorial experiment, with diets containing incrementally increasing proportions of LA (10%, 20%, 30%, 40% of total fatty acids (TFA)) and ALA (0%, 10%, 20%, 30% of TFA) and only trace amounts of the highly unsaturated fatty acids (HUFA). Three additional treatments were included in the study: a reference diet (FF) that contained significant amounts of HUFA, a defatted reference diet (DF), and a defatted reference diet with its lipid composition reconstituted to mimic fatty acid composition of the FF diet (FF-DF). The lipid content was kept constant across all diets (100 g kg-1), except for the DF diet (22 g kg-1). After feeding the experimental diets to silver perch fingerlings (mean initial weight.1.8+/-0.12 g) for 57 days, weight gain was highest in fish fed the diets containing HUFA: FF diet (12.0+/-1.13 g) and FF-DF diet (10.7+/-0.67 g). Fish fed the DF diet had the lowest weight gain (7.0+/-0.96 g). The growth rate of silver perch fed the LA/ALA series of diets increased with increasing dietary LA content to a maximum when the LA content was about 27% of TFA, after which the growth rate declined with further inclusion of LA. Silver perch did not appear to respond to dietary ALA. Feed conversion ratio (FCR) did not vary significantly with changes to dietary LA or ALA content, but was significantly better with the diets containing HUFA. Survival was >80% in all treatments and did not vary significantly with treatment. These results demonstrate that silver perch require the dietary total lipid content to be greater than 22 g kg-1. In the absence of HUFA, and with a total lipid content of 100 g kg-1, juvenile silver perch require a dietary LA content of 27% of TFA (equivalent to 17 g kg-1 of diet) for optimum growth. There appears to be no nutritional benefit in formulating silver perch diet to contain a specific level of ALA when the diet contains some LA (>8% of TFA or >5.3 g kg-1 of diet). However, the enhanced weight gain of the fish fed the FF and FF-DF diets demonstrates that silver perch have a requirement for some or all of the HUFA present in fish oils.