Selenium is an essential nutrient for fish and diet is the supply route. In fishmeal-based diets, selenium supplementation is not considered since fishmeal is a good source of this mineral. As the industry moves to plant-based diets, its supplementation has become more important to increase its availability. The maximal permitted selenium concentrations in animal feeds, including fish feeds, in the EU is 0.5 mg Se kg−1 in complete feed.
Selenium influences the immune responses and helps mitigate stress effects often at nutritional levels higher than the minimal requirements. This increase in dietary selenium also increases the load to the environment.
Skretting, together with a team of Norwegian researchers, evaluated the supplementation of selenium, either as inorganic or organic, in Atlantic salmon post-smolt in vivo and in vitro. The basal diet was formulated to be low in fishmeal and contain 0.24 mg kg−1 of selenium. Six other diets with selenium inclusion of 0.15, 0.4, 0.7 or 1.1 mg kg−1 as sodium selenite (SS) and 0.15 or 0.4 mg kg−1 as L-selenomethionine (SM) were formulated from the basal diet. The diets were fed to Atlantic salmon post-smolt for nine weeks. At the end of the feeding trial, samples were taken for in vivo and in vitro assessment.
Researchers found that the minimal selenium requirement for Atlantic salmon fed plant-based feeds containing 0.24 ppm basal selenium level was met by 0.4 ppm SS supplementation or by 0.15 ppm SM supplementation (total selenium, 0.41 ppm). The SM supplementation complied with the existing EU regulations, but SS did not.
Increasing SM supplementation to 0.4 ppm (total, 0.65 ppm) indicated health benefits, however, the feeds were non-compliant. Further, the use of SM was predicted to reduce the selenium load to the environment from Norwegian salmon farming by up to 70%. From the fish health perspective, dietary selenium level required in plant-based feeds to maintain body selenium homeostasis and improved health status of Atlantic salmon post-smolt exceeds the existing EU maximum limit of 0.5 mg Se kg−1 complete feed.
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