Effects of dietary lipid levels on sub-adult triploid rainbow trout (Oncorhynchus mykiss): 1. Growth performance, digestive ability, health status and expression of growth-related genes

A growth trial was conducted to evaluate the effects of dietary lipid levels on the growth performance, feed utilization, digestive ability, health status and expression of growth-related genes of female triploid rainbow trout (Oncorhynchus mykiss). Six isonitrogenous diets, containing 46% protein were formulated with graded lipid levels of 6.6 (Diet 1), 12.3 (Diet 2), 14.8 (Diet 3), 19.5 (Diet 4), 22.8 (Diet 5) and 29.4% (Diet 6). Each diet was fed to quadruplicated groups of fish with initial average weight of 233 g in freshwater cages for 80 days. Results showed that there were significant linear and quadratic responses in growth performance, feed utilization, digestive ability and health status of sub-adult fish (P < .05). Final weight, specific growth rate (SGR), feed intake (FI), stomach protease activity, activity of lipase in pyloric caeca, activity of amylase in pyloric caeca and intestine showed a general increasing trend with increasing dietary protein level. Feed conversion ratio (FCR) decreased with increasing dietary protein level. No significant difference of intestinal morphology was observed in any of the groups (P > .05) except that the highest value for muscular layer thickness and density of goblet cells were found in Diet 6 treatment (P < .05). When examining health status, no negative effects were found in the antioxidative capacity of the intestine and liver (P > .05), however, the lowest value of plasma total anti-oxidative capacity (T-AOC) content and highest plasma malonaldehyde (MDA) content were shown in Diet 6 treatment (P < .05). There was no significant difference in plasma enzymes in any of the groups (P > .05) except for activities of aspartate aminotransferase (AST) and creatine kinase (CK), which increased as dietary lipid levels increased (P < .05). The expression of growth-related genes was at first significantly improved (P < .05) and then kept consistent (P > .05). Broken-line regression analysis of FCR showed that the minimum dietary lipid level of triploid rainbow trout was 23.3% in the present study. In summary, we found that triploid rainbow trout could use or tolerate high dietary lipid level (up to 29.4%) with no negative effect on fish growth, feed utilization, liver or intestine health. The minimum dietary lipid level for sub-adult triploid rainbow trout was estimated to be 23.3% based on FCR under the present conditions.