54 - Influence of Selenium on the Fate of Mercury in Fish: An Aquaculture Study

This study explores the potential valorization of tuna byproducts as an aquafeed by the assessment of dietary selenium (Se) and its effect on the fate of mercury (Hg) in fish. Tuna byproducts represent more than 65% of the original product mass and are usually discarded. These non-commercialized parts of tuna constitute a significant source of nutrients and are currently considered as an alternative for developing a sustainable fishmeal product. The high Hg content however raises a concern. Selenium concentration and Hg:Se dietary molar ratios have been recognized as a more suitable criteria than methylmercury (MeHg) content for risk assessment in seafood. Rainbow trout juveniles were exposed to a 6-month feeding trial with single or multiple Hg and Se species supplementation at realistic concentration levels, totaling 12 different conditions. Two different types of aquafeeds based on plant (containing 0.25 µg(Se)/g) or tuna byproduct (containing 0.25 µg(Hg)/g and 8.0 µg(Se)/g) were enriched with 2.5 µg(MeHg)/g, and/or 1.5 µg(Se)/g supplied as selenite (Se(IV)) or selenomethionine (SeMet). The determination of Hg and Se in muscle, liver, kidney, brain, and blood tissues reveals that concentrations increased linearly during the first 12 weeks, with major bioaccumulation in liver (up to 12 µg(Hg)/g and 380 µg(Se)/g). Despite dietary exposure to equal MeHg concentration, tuna byproducts lead to lower Hg content than plant-based diets. In muscles and blood, a decrease of Hg levels of 38 and 65% respectively, was observed. These results suggest that Se content in tuna byproducts could play a crucial role in Hg regulation. The study raises the question of the selenoneine extent, reported in tuna tissues, which would explain such singular trend. Tuna byproducts-based diets resulted in muscle Hg content below the threshold established for human consumption by the European Commission (5 µg(Hg)/g), which was exceeded when using plant-based aquafeeds.

authors: Claudia Marchán-Moreno