Isolation of intake mediated effects demonstrate that the phenomic benefits of dietary omega-3 are nominal to Atlantic salmon when reared in a challenging (hypoxic) environment

The present study was designed to examine whether challenging environmental conditions (such as hypoxia) influence a fish's response to diet by changing responses to nutrient utilisation or down-regulation of appetite. To examine this, post-smolt Atlantic salmon (138.8 +/- 0.65 g/fish; mean +/- SD) were fed sub- (2.6% TFA; Ln3) and optimal (6.0% TFA; Hn3) levels of long-chain polyunsaturated omega-3 fatty acids (n-3 LC-PUFA) when maintained under either normoxic (8.1 +/- 0.6 mg/L) or hypoxic (6.8 +/- 0.5 mg/L) environmental conditions. To control for the anticipated effects of hypoxia on appetite and food intake, two additional treatments included the two diets that were pair-fed to the same intake levels as in the hypoxic treatments. The reduction in dissolved oxygen was accompanied by an increase in dissolved CO2 and decline in seawater pH, with no change in total alkalinity. After 138 days of the study the fish increased their average weight to between 616 and 720 g/fish subject to treatment. Hypoxia had a significant negative impact on final weight and weight gain, and these were the result primarily of effects on feed intake. No effects of treatments on feed conversion were observed. Significant diet x oxygen interactions were observed on whole-body lipid levels, but no other proximate compositional parameters. Sub-clinical effects of hypoxia and ration on several plasma chemistry parameters were observed, but few responses were diet related. Hypoxia affected plasma total protein, cholesterol, and red blood cell counts. Diet digestibility was typically not affected by hypoxia or ration, though a range of diet related effects were evident. Nutrient retention data showed evidence of elongation and desaturation of dietary short-chain PUFA, with clear effects of diet affecting the deposition of both omega-6 and omega-3 fatty acids. This study builds on findings from earlier studies, but crucially now include controls for the feed intake effects to clarify the roles of hypoxia on the nutrient and intake mediated effects. As a result of these controls, it is now clearer that many of the phenomic responses to long-chain omega-3 by Atlantic salmon are nominal and are largely unaffected by challenging environmental conditions like hypoxia. The inclusion of a series of pair-fed treatments, matching the feed intake levels of corresponding hypoxic treatments, shows that the effects of n-3 LC-PUFA appear to be largely intake mediated effects for Atlantic salmon post-smolts.