Overfeeding-induced obesity in rainbow trout (Oncorhynchus mykiss) linked to higher susceptibility to infection

Although overfeeding in aquaculture can decrease feed efficiency and increase wastage as well as bacterial loads, there are very few investigations on its effects on fish health. This study investigates the physiological changes in overfeeding-induced fish obesity and whether the feeding regimen increases vulnerability to infection. In total, 105 rainbow trout (average weight = 49.26 ± 2.78 g) were divided into three groups (normal-, intermediate-, and over-fed) and fed about 1.7%, 2.55%, and 3.58% of their body weights per day, respectively, for 8 weeks. Serological parameters and metabolites were analyzed to understand the correlation between obesity and physiological state of the fish. Fish at 8 weeks post-feeding (wpf) were challenged with Aeromonas salmonicida subsp. salmonicida at three concentrations of approximately 10³, 10⁴, and 10⁵ CFU/fish by intraperitoneal injection. The results showed that the hepatosomatic index (at 2, 4, and 8 wpf), body mass index, and serum triglyceride values (at 4 and 8 wpf) of overfed fish were significantly higher than those of the other groups, suggesting that obesity was successfully induced. Moreover, overfed fish showed significantly higher concentrations of succinate, valine, and isoleucine in their serum compared to the intermediate and control groups, which are known to be associated with obesity. The levels of alanine aminotransferase (ALT) and aspartate transaminase (AST), which serve as indicators of liver function, were consistently higher between 2 and 8 wpf compared to both the normal and intermediate groups. This suggest that overfeeding has detrimental effects on the liver; thus, these enzymes can serve as early indicators of overfeeding. In the challenge test, the fish in the overfed group showed notably higher mortality than those in the other two groups. Our findings therefore suggest that overfeeding for even short periods of time could harm fish health and increase their vulnerability to infection.