Essential Fatty Acids in the Diet of Rainbow Trout (Salmo gairdneri): Lipid Metabolism and Fatty Acid Composition

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Abstract
The fatty acid composition of the liver, heart, kidney, brain and body lipids of fish fed either a fat-free diet or those containing known amounts of oleic, linoleic, or linolenic was determined. A fat-free diet or one containing oleic acid as a sole lipid source resulted in elevated levels of eicosatrienoic acid (20:3ω9). Dietary linoleate and linolenate both depressed the triene levels. The former acid elevated 20:4ω6 and 22:5ω6 concentrations and the latter increased 22:6ω3 tissue levels. Dietary lipids affected the fatty acid composition of phospholipids to a greater degree than those in the neutral lipids. The demonstrated requirement for the ω3 fatty acids by fish suggest that the 20:3ω9/22:6ω3 ratio in the phospholipid fraction be used as an index of essential fatty acid nutrition. Fish receiving 0.7% or more of linolenate in the diet had 20:3ω9/22:6ω3 ratios of less than 0.4. Diets which produce this ratio value or less appear to be adequate in the ω3 fatty acids (linolenate series) and fulfill the nutritional requirements of young fish as judged by growth and other physiological parameters.