Effects of dietary lipid and air replenisher on the growth and lipid deposition of steelhead trout (Oncorhynchus mykiss) confined underwater

In this study, we investigated the effects of dietary lipids (DL) on the growth and lipid deposition of steelhead trout (Oncorhynchus mykiss) confined underwater using various equipment. Three diets formulated with 6%, 13%, and 20% DL (F6, F13, and F20) were fed to the fish. Fish (initial weight: 75.01 ± 1.36 g) in each dietary group were randomly cultured in three equipment groups: the control group; the submergence group, in which mesh and airstones were used to keep the experimental fish from the water surface; and the replenisher group, in which an air replenisher and airstones were utilized under the mesh. The results of this 70-day study showed that the growth and crude lipid content of the steelhead trout fish body increased significantly with increasing DL levels. No significant differences were observed in the liver among the different equipment groups, whereas the lipid contents in the muscle and the viscerosomatic index significantly decreased in the submerged group fed F20. The fitted curve analysis showed that the energy expenditure of continuous swimming and the success of replenishing the swim bladder may determine the strategy adopted by trout to cope with negative buoyancy. If the swim bladder was easily replenished, steelhead trout tended to convert excess energy into lipid stored in muscles, thereby reducing body density and maintaining neutral buoyancy. Otherwise, trout must burn lipid in their muscles to ensure energy supply to keep swimming for lift. We concluded that a high-lipid diet compensated for the loss of the energy used in resistance to negative buoyancy, and growth performance of steelhead trout improved