5 things that hatchery nutritionists should know about selenium
1) What does selenium do?
Selenium is a critical micronutrient needed by animals in small quantities to make selenoproteins. Up to 50 different selenoproteins have been identified including three enzyme families and the amino acids selenomethionine, selenocysteine, and methylselenocysteine.
Selenoproteins play a critical role in the immune system and participate in hematopoiesis, the process of blood cell formation and blood cell differentiation (1).
For broodstock, selenium has a significant impact on fecundity. Our sister company, Sustainable Aquatics Inc. observes adding 3-8 ppm selenomethionine to broodstock feeds contributes to vitellogenin formation and the resulting egg yolk volume.
Selenoproteins regulate energy production by hormones, play a role in repairing damaged DNA and as an antioxidant preventing damage from reactive oxygen species (ROS) in the cells, gut and blood.
2) How do fish naturally get selenium in their diets?
Land based selenium deposits can be in a water soluble form (selenate) which over time migrated through rivers and getting concentrated in the ocean at concentrations of 60 ng/liter(2). This selenium is absorbed by phytoplankton and passed up the marine food chain. Freshwater streams and lakes, however, may or may not contain selenium depending on the selenium deposits in the underlying land mass. There are known zones where freshwater fish have selenium deficiencies.
3) How much selenium does my fish need?
For humans, National Institute of Health publishes the recommended daily allowance of selenium which varies depending on age, pregnancy and lactation. We have not located any reference source for how much selenium is required for each fish species much less for broodstock, first feeds and grow-out. Selenium ingredient suppliers typically suggest feeds contain .5-8 ppm of selenium as a start.
Also note that the bioavailability of selenium varies considerably. For example, selenomethionine is easily used by the animal to form the selenoproteins required for optimal health.
4) Can the selenium in my fish get depleted by other means?
Yes. Mercury and selenium have a very strong affinity for each other. Rising marine levels of mercury "tie up" selenium in compounds called mercury selenides. Selenium that is bound to mercury in this fashion is no longer available to the fish for selenoprotein formation. Conversely, the mercury that is tied up as mercury selenide, is no longer toxic to the animal nor to the humans consuming that product.
Certain selenoproteins also function as antioxidants helping to remove oxygen radicals (ROS) from the cells, gut and blood. However, selenium, like many other micronutrients and enzymes were not powerful enough to mitigate ROS on their own. Plants, algae, bacteria and ultimately arthropods devoted precious resources to make more powerful antioxidants called carotenoids. Astaxanthin, specifically the esterified 3S, 3'S isomer, is nature's most powerful antioxidant. Vertebrates evolved after astaxanthin was available in the food chain so did not need to acquire the mechanisms to synthesize astaxanthin de novo. Vertebrates ate arthropods and used the astaxanthin inside for ROS mitigation. Commercial hatchery diets that fail to include this isomer, can cause the animal to deplete their selenium reserves using selenium as an antioxidant instead to form other selenoproteins required for its other functions.
5) Should we use feeds that contain selenium?
We highly recommended the inclusion of selenium in hatchery diets. Our micronutrient supplement, Amplifeed Topcoat, contains selenomethionine, a highly bioavailable form of selenium produced by yeast. Amplifeed Topcoat also contains the 3S, 3'S form of astaxanthin that has been esterified and is 47X more bioavailable than astaxanthin extracted using the supercritical carbon dioxide method. Our astaxanthin protects selenium from being used as an antioxidant freeing it up to form other selenoproteins. We recommend higher inclusion rates in broodstock feeds, moderate inclusion rates during the allometric development phase and trace inclusion rates during grow out.
- Pacitti et al. 2016 Impact of selenium supplementation on fish antiviral responses: a whole transcriptomic analysis in rainbow trout (Oncorhynchus mykiss) fed supra nutritional levels of Sel-Plex® BMC Genomics (2016) 17:116
- Nakayama et. al. Chemical Analysis of Seawater for Recent Progress in Japan on Clean Speciation of Trace Elements, Analytical Sciences April, 1989, Vol 5 129