The Effects of a Transition Diet on the Smoltification of Chinook Salmon

Abstract

AbstractTransition diets, often supplemented with salt and other additives, have been used to prepare Pacific salmon Oncorhynchus spp. being moved directly from freshwater to saltwater net pens for aquaculture production. However, the use of transition diets to prepare hatchery‐reared salmon released into freshwater prior to out‐migration to the ocean has shown mixed results. We compared the physiological (size, condition factor, Na+,K+‐ATPase [NKA] enzyme activity, survival during a saltwater challenge) and nutritional (proximate composition, fatty acid profile) condition of juvenile Chinook Salmon O. tshawytscha that were fed either a standard hatchery feed or a commercially available transition diet for the final 6 weeks of hatchery rearing to determine whether the transition diet altered smolt condition. At release, NKA activity of fish fed the transition diet was higher than that of fish fed the control diet, though survival during a saltwater challenge was similar between groups. Differences were seen in the proximate composition of the fish, with increased total body moisture and decreased lipid in the fish fed the transition diet relative to fish fed the control diet. Whole‐body fatty acid composition of fish was similar to the fatty acid composition of the feeds in both groups, though some key whole‐body fatty acids did not follow this trend. Palmitic acid (16:0), linoleic acid (18:2[n‐6]), eicosapentaenoic acid (20:5[n‐3]), and docosahexaenoic acid (22:6[n‐3]) varied between prefeeding trial samples and fish fed the two diets. Further research is required to determine whether the physiological changes that occurred are related to smoltification or the maintenance of osmotic balance while fish are fed additional salt.