Fatty acid composition, osmolality, Na+, K+-ATPase activity, cortisol content and antioxidant status of rainbow trout (Oncorhynchus mykiss) in response to various dietary levels of eicosapentaenoic acid and docosahexaenoic acid

This study was conducted to evaluate the effect of dietary eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) levels on the fatty acid composition, salinity tolerance and antioxidant status of rainbow trout (Oncorhynchus mykiss). Four diets were formulated with total EPA and DHA contents of 5.41, 9.55, 13.97 and 17.88 g/kg (abbreviated as ED-5.41, ED-9.55, ED-13.97 and ED-17.88 respectively). Rainbow trout (initial weight of 90.61 +/- 9.25 g) were fed the experimental diets for 8 weeks to accumulate significant differences in fatty acid composition and subsequently underwent salinity acclimation. Our results indicated that high dietary EPA and DHA significantly improved the EPA and DHA content in fish tissues. The serum osmolality of fish returned to their freshwater values in the ED-9.55, ED-13.97 and ED-17.88 groups. The Na+, K+-ATPase (NKA) activity of fish in the ED-13.97 group changed dramatically to adapt the fish to the hypertonic environment. Moreover, there was no significant difference in the serum cortisol concentration and liver catalase (CAT) activity of fish in the ED-13.97 group during salinity acclimation. The liver superoxide dismutase (SOD) activity in the ED-13.97 group was significantly higher than that in ED-5.41 and ED-9.55 groups at the end of salinity acclimation. The muscle malondialdehyde (MDA) content in the ED-13.97 group was significantly lower than that in the ED-17.88 group before salinity acclimation and significantly lower than the ED-5.41 and ED-17.88 groups on day 7 of acclimation. The results of this study indicate that the rainbow trout in the ED-13.97 group exhibited optimal salinity acclimation performance.