The need for essential fatty acids (EFA)of chum salmon in a freshwater environment was examined by feeding diets containing different levels of methyl esters: 18:2ω6, 18:3ω3, 20:5ω3, and a mixture of 20:5ω3 and 22:6ω3 (ω3HUFA), The EFA-deficient diet resulted in poor growth, low feed efficiency, high mortality, and swollen pale livers from the 2nd week of the feeding. The addition of either 18:2ω6 or 18:3ω3 to the diet improved growth conditions, but the mortality was not effectively reduced by the addition of 0.5% 18:3ω3. On the other hand, the addition of ω3 fatty acids, not only 20:5ω3but also ω3HUFA, to the EFA-deficient diet vastly improved growth and feed efficiency, and the supplemental effect of 0.5% 20:5ω3 and 0.5% ω3HUFA on the growth of chum salmon slightly exceeded that of 1% 18:3ω3. The best weight gain and feed efficiency were obtained in the fish receiving the diet supplemented with both 1% 18:2ω6 and 1% 18:3ω3 or ω3HUFA.
The EFA-deficient diet raised the levels of the “abnormal” polyunsaturated 20:3ω9and monoethylenic fatty acid, 18:1, and these levels were lowered by bothω6 andω3 fatty acids. ω3HUFA and 20:5ω3 appeared to be more efficient than 18:2ω6 and 18:3ω3 in reducing the levels of these fatty acids. The requirement of chum salmon in freshwater environment for linoleic and linolenic acids was found to be around 1%, or 0.5-1% for ω3HUFA.