Temporal Dynamics of Oxylipins and Antioxidant Metabolites in Rainbow Trout Muscle Are Driven by Diet Type Rather Than Methylmercury or Selenomethionine Supplementation

Tuna by-products in aquafeed pose challenges due to heavy metals like mercury (Hg), which has pro-oxidant effects. However, they also contain selenium (Se), an antioxidant oligoelement. This study assessed the oxidative impact of these elements in juvenile rainbow trout over a 6-month feeding trial. Fish were fed plant-based or tuna-based diets, with or without supplementation of methylmercury (MeHg) and selenomethionine (SeMet). Muscle samples were collected at 0, 21, 84, and 168 days to analyze fatty acid profiles, non-enzymatic oxylipins, and antioxidant metabolites (glutathione, vitamin E). Tuna-fed fish exhibited higher oxylipin levels and lower antioxidant metabolites compared to plant-fed fish. Notably, oxylipin levels remained elevated at 84 and 168 days in tuna-fed fish, correlating with a decrease in the GSH:GSSG ratio. In contrast, plant-fed fish showed a transient increase in oxylipins at 84 days, which normalized by day 168, aligning with a temporary drop in the GSH:GSSG ratio. MeHg and SeMet supplementation did not alter these trends. Surprisingly, these findings, along with a previous study, suggest that prolonged oxidative stress in tuna-fed fish is more closely linked to Se bioaccumulation than Hg. Future work should investigate dietary Se concentration and form to better understand these results.