Age, ration level, and exercise affect the fatty acid profile of chinook salmon (Oncorhynchus tshawytscha) muscle differently

This study was undertaken to investigate the effect of sustained physical exercise (SS, swimming speed) on the fatty acid profile of muscle in PIT-tagged all-female chinook salmon (Oncorhynchus tshawytscha) in relation to their age and ration level (RL; i.e., maximum ration, RL100 or 75% of maximum, RL75). Accordingly, body weight (BW), specific growth rate (SGR), and total muscle lipid content (TLC) were considered as covariates in data analyses. In addition, plasma levels of thyroid hormones (thyroxine, T4 and 3,5,3′-triiodo-l-thyronine, T3) were compared to the muscle fatty acid (FA) compositions of individual fish to determine if there were any associations between thyroidal status and FA percentages. During the 300-day study, body weight increased from 75 to 440 g. Fish age explained most of the changes found in muscle FA composition [that is, R2 was 0.23 for saturated fatty acids (SAFAs), 0.65 for monounsaturated fatty acids (MUFAs), and 0.71 for polyunsaturated fatty acids (PUFAs) with p<0.0007 in all cases]. Reduction in RL had less influence on FA composition (p>0.15 for SAFA, and R2 was 0.27 for MUFA and 0.34 for PUFA, with p<0.0001 in each of the latter cases). SS only affected MUFA significantly (p<0.07 with R2=0.09). The observed effects of fish age and SS on FA composition were to a large degree uninfluenced by BW, SGR, or TLC, while the effects of RL were markedly diminished when these covariates were included in the statistical model. Thus, the changes in body weight and/or lipid content were viewed as being the direct cause for the alteration in muscle FA profile seen with the main factor RL. The effect of SS was pronounced only if fish with a reduced RL were compared. In this situation, the level of PUFA, especially docosahexaenoic acid (DHA; 22:6 n-3), decreased and MUFA increased. Plasma T4 was unrelated to muscle FA levels, but plasma T3 was correlated positively with muscle MUFA and negatively with PUFA. We conclude that in order for exercise to affect chinook salmon fillet FA composition, it has to be combined with restricted feeding. The data also imply that in accordance with higher vertebrates, T3 is also involved in the regulation of FA metabolism in fish.