REEVALUATING THE ESSENTIAL FATTY ACID REQUIRMENTS OF RAINBOW TROUT Oncorhynchus mykiss

There is a pressing need in aquaculture for alternative lipid-based, reduced fish oil-content feeds that provide adequate levels of essential fatty acids (EFA). Nutrient requirements must be well-defined to effectively formulate such feeds, and recent research has suggested that reported EFA requirements may not be fully accurate. Accordingly, we evaluated the growth performance of rainbow trout Oncorhynchus mykiss fed diets containing different combinations of n-3 and n-6 C₁₈ polyunsaturated fatty acids (C₁₈ PUFA) or long-chain polyunsaturated fatty acids (LC-PUFA).  Diets varied only in lipid source and composition, containing menhaden fish oil (positive control, "FO Control"), hydrogenated soybean oil (negative control, "EFA-free Control"), or soybean oil supplemented with ethyl esters of 18:3n-3 ("ALA"); 18:2n-6 and 18:3n-3 ("C18 PUFA"); 22:6n-3 ("DHA"); 20:4n-6 and 22:6n-3 ("ARA + DHA"); or 20:4n-6, 20:5n-3, and 22:6n-3 ("LC-PUFA").  Diets were randomly assigned to quadruplicate tanks (N = 4) of rainbow trout (10 fish/tank; ~25 g/fish), and fish were fed to satiation once daily for 8 weeks. At harvest, individual fish weights were collected to determine production performance. Data were analyzed by one way analysis of variance, followed by Tukey's HSD tests when omnibus tests indicated significant treatment effects (P < 0.05).

Expectedly, weight gain was significantly greater among fish fed the FO Control diet (433%) relative to the EFA-free Control (216%; Figure 1). Trout fed a diet supplemented with LC-PUFA gained more weight (276%) than fish fed EFA-free Control (Figure 1).  Results to-date appear to affirm that rainbow trout do not require preformed LC-PUFA in the diet, but there may be some bioenergetic efficiency in providing these nutrients directly.