In intensive aquaculture production, high growth rates and high feed efficiencies are essential. However, the required high growth performance is more often associated with increased levels of stress. Stress can be from various sources such as high stocking densities, pathogenic pressure, osmotic imbalance, other environmental factors and animal handling. When fish and shrimp experience stress, enhanced levels of reactive oxygen species (ROS) and a suboptimal antioxidant status and immunity are observed. Selenium (Se), an essential trace element, will support the animal during these stressful conditions. For example, it functions as a vital component of selenoenzymes, which play an important role in reducing ROS and maintaining a healthy antioxidant status.
Selenium contents in aquafeeds under pressure and in need for supplementation
Beneficial effects of Se in aquaculture diets are well-recognised. Nevertheless, due to increased usage of plant-based meals, to replace fishmeal and fish oil, the amount of Se in aquafeeds has been decreasing over the past decades. One of the consequences of these decreased Se levels in the feed is the decreased content of Se in fish, throughout the body and in the fillet (Betancor et al., 2016).
Decreased Se contents in the body are known to be unfavourable for animal health and performance. To counteract this trend, aquafeed producers and fish farmers have the possibility to include organic and inorganic Se sources in their diets. These chemical forms affect the Se bioavailability differently. In general, animals are unable to incorporate dietary inorganic Se sources in body protein, but this is possible for organic Se, in the form of L-selenomethionine (Figure 1). Next to that, it is commonly accepted that organic Se shows greater bioavailability compared to inorganic Se. This combination of high storage capacity and bioavailability is the reason that organic Se sources, in the form of L-selenomethionine (Excential Selenium 4000, Orffa Additives BV) are more functional compared to inorganic Se sources, for example sodium selenite.
Organic Se is involved in numerous biological processes which benefit the antioxidant and immune systems. These systems are negatively affected when Se levels are inadequate, showing the importance of maintaining sufficient Se levels in the diet. Additionally, Se is beneficial for health but decreasing Se concentrations in the fish body is problematic for humans, as fish fillet is one of the major sources of Se in the human diet. Supplementation of Se in the diet is thus a necessity – in order to maintain fish health and to maintain the health benefits of consuming fish and shrimp.