Effect of Dietary Linoleic Acid (18:2n-6) Supplementation on the Growth Performance, Fatty Acid Profile, and Lipid Metabolism Enzyme Activities of Coho Salmon (Oncorhynchus kisutch) Alevins

SIMPLE SUMMARY: In China, coho salmon is becoming an increasing farming species in recent years. Thus, full understanding of the fatty acid requirements of coho salmon is particularly relevant, especially during the larval stage, since they need high-energy demands and rapid lipid metabolism. As a vital essential fatty acid, linoleic acid (18:2n-6) also plays an important role in the larval stage due to the production of longer-chained and more essential highly unsaturated fatty acids (HUFA). However, there is a lack of knowledge about the dietary linoleic acid (18:2n-6) requirement for this fish species. In this study, six isonitrogenous and isolipidic diets were formulated to contain different dietary linoleic acids (18:2n-6) by increasing corn oil at the expense of coconut oil to study the growth performance, fatty acid profile, and hepatic lipid metabolism enzymes of coho salmon alevins. The results showed that the dietary linoleic acid (18:2n-6) affected the growth performance, muscle fatty acid profile, and hepatic lipid metabolism enzyme activities of coho salmon, and determined the optimal demand of linoleic acid based on the specific growth rate (SGR) and feed conversion ratio (FCR). ABSTRACT: A 12-week feeding trial aimed to evaluate the effects of dietary linoleic acid (LA, 18:2n-6) on the growth performance, fatty acid profile, and lipid metabolism enzyme activities of coho salmon (Oncorhynchus kisutch) alevins. Six experimental diets (47% crude protein and 15% crude lipid) were formulated to contain graded LA levels of 0.11%, 0.74%, 1.37%, 2.00%, 2.63%, and 3.26%. Each diet was fed to triplicate groups of 50 alevins with an initial body weight of 0.364 ± 0.002 g, which were randomly assigned to 18 white plastic tanks (0.8 × 0.6 × 0.6 m, 240 L/tank). Fish were reared in a freshwater flow-through rearing system and fed to apparent satiation four times daily. The survival rate was not significantly different among the treatments (p > 0.05). However, the 1.37% LA group significantly improved the final body weight and specific growth rate (SGR) (p < 0.05) of alevins. The feed conversion ratio (FCR) in the 1.37% LA group was significantly lower than those in other groups (p < 0.05). The whole-body lipid content significantly decreased (p < 0.05) with dietary LA levels increasing from 0.74% to 2.00%. The fatty acid composition of the total lipid in muscle was closely correlated with those in the diets. The dietary LA level of 1.37% led to significantly higher activities of liver lipoprotein lipase (LPL) and hepatic lipase (HL) than those of other groups (p < 0.05). Hepatic malate dehydrogenase (MDH) and fatty acid synthase (FAS) decreased with the increase in the dietary LA levels from 0.11% to 1.37%. The lowest MDH and FAS activities were obtained in the 1.37% LA group (p < 0.05). This study indicated that an appropriate amount of dietary LA was beneficial for the growth and lipid metabolism of coho salmon alevins, and the results of the quadratic regression analysis of the SGR and FCR indicated that the optimal dietary LA requirements were 1.25% and 1.23% for coho salmon alevins, respectively.