Due to a rapid growth in aquaculture and limited access to marine resources, marine fish oil and fish meal in feeds to carnivorous marine fish species such as Atlantic salmon (Salmo salar) have been replaced with plant ingredients in the last decade. The change from marine to plant feed ingredients has altered the nutritional composition of salmon feeds, reducing the levels of essential micro-nutrients that are naturally high in fish meal such as selenium (Se). Several studies have indicated the need for Se supplementation in plant-based feed to marine carnivorous fish to maintain the natural high levels of Se in marine food products and to cover Se requirements in farmed fish. Selenium has a narrow range between its toxic and its beneficial effects. Supplementation of aquafeeds with organic Se-methionine (SeMet) forms or inorganic selenite hence requires toxicological assessment to set safe upper limits that protect fish health and consumer safety (Berntssen et al. 2017, Berntssen et al. 2018, Berntssen et al. 2019).