Se is a naturally occurring element that interacts with fish’s environment through anthropogenic and natural means (see Figure 1). Thus, Se concentration in soil and aquatic environments depends on anthropogenic activities (industrial, mining, agriculture inputs, sewage sludge, oils, power plants, and mines) and natural occurrences (weathering rocks, volcanic eruptions, and atmospheric processes). There are ways in which Se can directly and indirectly get into the soil and aquatic environment, including erosion, runoff, and leaching that release coal ash into water bodies, particularly in Se-rich regions [4,30]. However, it also gets into the aquatic environment directly from mine discharges, agricultural fields, waste water, sewage disposals, and industrial discharges [6]. Khoshnood [31] highlighted the most frequent contaminants, such as heavy metals and pesticides, causing harm to fish in aquatic environments. According to Tan et al. [32], 40% of Se in the atmosphere and aquatic environment comes from industrial and mining activities. This percentage of Se was primarily influenced by human activities like burning coal and crude oil, which serves as an influencing factor [32]. In addition, these activities by humans have introduced high Se levels as micronutrient elements to the environment, causing imbalances in the food chain. The level of Se concentration in the aquatic environment in a particular area depends on a number of factors, such as the increase in demand for agriculture, industrial processes, mining power plants, chemical wastes deposited in the environment, from which Se comes from different sources, and the release of pollutants into the aquatic system [33,34]. Fish in the aquatic environment perform all metabolism processes, including Se intake, which may be carried out orally or assimilated into muscle and liver tissues.

Fish species make excellent models for detecting genotoxic chemicals in aquatic ecosystems [36,37] due to their abundance and specific habitats [38]. Most toxicity levels are based on uptake in the aquatic environment, which is substantially lower in natural populations than uptake through the food chain. Se concentrations in soils vary depending on the location, activities, type, and nature of the soils. Tan et al. [39] reported that Se content in various soil types varies from 1.07 to 6.69%. Thus, natural processes (physical and mechanical) occurring in the soil could lead to an uneven distribution of Se in the environment. In addition, the Se content in the soils of many countries varies. Studies have found Se levels of 0.4 mg/kg in most parts of the world’s soil, including North America, Ireland, Australia, and Israel, as well as in some Chinese provinces (72% of the area of China), New Zealand, and a considerable part of Europe [40,41] (Table 1).

Fish species absorb dietary Se in various forms, such as organic and inorganic [45,46,47]. These forms of dietary Se impact fish’s biological functions. The forms of dietary Se aid fish species in several ways depending on their health, size, species, feed composition, and experiment conditions [48]. The biological functions of fish in their aquatic environment may be supported by dietary Se use. However, the majority of dietary Se absorption by fish species is efficient due to their bulkiness or minute size [49].

As for Se in the diet of fish, there are a few different forms commonly used as inorganic, such as Na₂SeO₃ or sodium selenate (Na₂SeO₄) (Table 2). They differ in toxicity, with Na₂SeO₃ being slightly less toxic than Na₂SeO₄ [50]. Na₂SeO₃ is commonly used in fish feed and is also found in plant foods, meats, seafood, and dietary supplements that provide numerous health benefits, while Na₂SeO₄ is an insecticide used in agriculture to control insects and can be used as a fungicide [51]. Both forms are readily absorbed by fish and can meet their Se requirements. Na₂SeO₃ plays an important role in fish nutrition and tissues, as a study by Rathore et al. [19] showed that supplementing fish feed with inorganic Se positively affects Se content in Nile tilapia tissues.

Organic forms of dietary Se also include selenomethionine (SeMet) and selenocysteine (SeCyst). The two forms often come from natural sources such as yeast or plants. According to a study by Penglase et al. [52], fish feed enriched with Se provides fish with a higher proportion of essential nutrients and thus promotes their accumulation in their tissues. Additionally, dietary organic Se in fish feed has resulted in strengthened retention and increased Se levels in the fish. Since fish tissues accumulate both essential and non-essential trace elements [53], it is important to have an understanding of the various Se forms and their effects on fish.

SeMet is one of the essential amino acids. Fish can obtain SeMet from their diet, and it is known to play a crucial role in their overall health and biological functions such as growth and the immune system. A study by Liu et al. [54] demonstrated that SeMet supplementation in grass carp diets contributed to fostering growth performance, immune response, and antioxidant capacity.

SeCyst is an amino acid that incorporates Se into selenoproteins, thereby affecting reproduction, egg quality, and spawning performance in fish [26]. It is important for protecting fish from oxidative stress and maintaining cellular health, as shown by Kumar et al. [55]. In addition, SeCyst is an important component of antioxidant enzymes such as GPxs.

Se is a chemical element that can naturally occur in soil. Soil serves as the primary source of Se, influencing its content in food, vegetables, and drinking water [39]. Hence, the amount of Se in the soil directly influences the amount of Se in fish. Soil parent material, physical and chemical properties, and other factors like soil type and land use could influence Se distribution in soil [64]. Se distribution in soil varies geographically and depends on factors such as parent material, climate, and land use practices. It can be found in varying concentrations worldwide (see Table 3), with higher levels typically found in regions with seleniferous deposits [64]. The bioavailability of Se in soil depends on its chemical form, as it can exist in different oxidation states (−2 to +6), with selenate (SeO₄⁻) and selenite (SeO₃⁻) being the most common forms. Na₂SeO₃ is more mobile and bioavailable, while SeO₄⁻ is prone to adsorption and immobilization in soil [65]. Soil contains various sources and forms of Se, including Se sulfides and Se ions. It is acidic, oily, and rich in organic matter. Se exists in various organic and inorganic forms, with selenopyrite or selenopyrite fixed to organic matter. According to Saha et al. [44], dietary Se exists in selenide (Se²⁻), SeO₃²⁻, and SeO₄²⁻ forms, found in acidic, low redox soils, predominant in acidic, medium redox soils (0 to 200 mV), and dominant in alkaline, high redox soils (500 mV). Moreover, soil pH, organic matter content, and redox conditions significantly influence Se concentrations in soil. Acidic soils with low organic matter content can increase Se availability, while alkaline or high organic soils may reduce its mobility [66]. The interaction of Se in soil can have impacts on the aquatic environment, including fish populations. When Se-rich soils are eroded or irrigation water containing Se is used, Se can enter nearby water bodies through runoff or leaching. This can result in elevated Se concentrations in aquatic environments [67].

In the soil environment, the ideal Se levels, which support a variety of living organisms, including some fish species, particularly the African catfish in terms of extreme conditions, some crustaceans like crab, lobster, and prawns, as well as mollusks like snails and slugs, should exist. The soil environment serves in the capacity of producing food, transferring energy within food webs, releasing water, and regulating carbon [26,68]. Living organisms, including fish in the soil and aquatic environment, require adequate Se based on their acceptability for their survival and welfare. However, high Se concentration depends on soil types and characteristics such as sedimentary and organic matter, whereas magmatic rocks are poor in Se concentration and have a low Se content [69]. In areas where aquaculture structures are unmodernized, they allow external sources of Se to enter aquatic environments through runoff.

Moreover, authors have reported the global average Se concentration in soils at 0.1 and 2 mg/kg [70], 0.01–0.4 mg/kg [71], and 0.03–2.0 mg kg⁻¹, with an average of 0.40 mg/kg [71,72]. The soil has multiple Se levels, and its characteristics are high, toxic, deficient, marginal, and moderate in levels of 0.40–1.00 mg/kg, 1.00–3.00 mg/kg, >3.00 mg/kg, <0.125 mg/kg, and 0.125–0.175 mg/kg [73], respectively, depending on type and characteristics. However, the average Se concentration level in soil globally falls within the range of 0.01–0.4 mg/kg [74,75]. The levels of Se in the soil vary based on the region or countries, as indicated in Table 3.

Many fish species’ biological processes are positively impacted by dietary Se, including thyroid regulation, growth promotion, improvement of fertility, immune system enhancement, and delay of aging [83,84,85], as well as antioxidant activity [86,87]. In contrast, Mechlaoui et al. [88] discovered that gilthead sea bream fed SeMet had better growth performance and higher antioxidant content than those fed sodium selenite (Na₂SeO₃). Ma et al. [89] demonstrated that dietary Se supplementation resulted in improved growth of grass carp. Abdel-Tawwab et al. [58] studied African catfish fed diets containing 0.2, 0.4, or 0.6 mg/kg Se and were also exposed to copper toxicity. They found that organic Se improved growth and reduced the negative effects of copper toxicity on physiology. Elia et al. [90] confirmed that fish fed Se-supplemented diets had higher growth rates and Se accumulation than the control group. Zhu et al. [91] investigated the effect of dietary Se on growth, body composition, and hepatic GPxs activity of largemouth bass; the researchers discovered that dietary Se supplementation improved growth and body composition and increased hepatic GPxs activity in largemouth bass, which may have a positive effect on their health. Han et al. [92] found that higher dietary Se content positively affected the overall growth performance of gibel carp compared to diets with lower Se content. Saffari et al. [93] confirmed that dietary Se is important to improve the growth and productivity of common carp and Ashouri et al. [94] reported that dietary Se concentrations of 0.1, 0.2, and 0.4 mg/kg improved the growth of common carp, muscle composition, and antioxidant status. Ibrahim et al. [8] confirmed that Nile tilapia fed nanoselenium (nanoSe) supplements had better growth rates and feed efficiency than those fed Se bulk supplements. Se is also important for biological functions like promoting fish growth, immune function, and health. It also boosts fish immunity, antioxidation, the stabilization of cell membranes [95,96], and the maintenance of normal metabolism in the body [30]. Fish feeding on regulated Se may support fish species performance by contributing to weight gain and specific growth rates in aquatic animals [64,93,94]. Studies on Wuchang bream have shown that Se increases growth hormone (GH) and insulin-like growth factor (IGF)-I levels in serum and messenger ribonucleic acid (mRNA) transcription levels of growth hormone receptor 2 (ghr2) and insulin-like growth factor 1 (igf1) in the liver [97]. It effectively promotes growth in various fish species, including grass carp, gibel carp, and rainbow trout [92,98].