66 JUNE 2022 • WORLD AQUACULTURE • WWW.WAS.ORG Biochar Application Charcoal and biochar have been used in agriculture to improve the water holding capacity and nutrient exchange capacity of soil to increase crop production. Then it was used to treat soil contamination with heavy metals/metals, toxins and pesticides. Charcoal and biochar are used for water filtration to detoxify water contaminants. In the 19th century, farmers in Germany, Austria and Switzerland began its use in animal husbandry, mainly in cattle and poultry farming. In animal husbandry, it is used to enhance animal health, increase nutrient intake, aid in the detoxification of contaminants, pollutants and other toxins in feed, reduction of antibiotic residues and reduction of enteric methane emissions. Feeding biochar promotes growth, enhances digestion and absorption of nutrients, improves gut health and indigenous flora, reduces pathogenic microbes and boosts the immune system of animals (Man et al. 2021). Recently researchers have begun to use biochar in in aquaculture to improve water and soil quality of the culture environment. In fish, biochar was first used as a feed additive in olive flounder feed (Yoo et al. 2005). Incorporation of biochar into fish feed enhances growth performance and nutrient utilization, improves blood profile, improves intestinal morphology, enhances beneficial gastrointestinal microbiota development, removes toxins from the body and reduces water quality degradation by reducing nitrogen emissions. Biochar Use in Fish Feed Biochar has excellent properties and can be used as an additive in fish feed. The addition of biochar to feed had several benefits to fish growth and immunity (Table 1). Wood biochar in feed increases growth rate and feed efficiency and decreases nitrogen excretion in olive flounder Paralichthys olivaceus (Yoo et al. 2005) and Nile tilapia Oreochromis niloticus (Samson 2019). Dietary bamboo biochar addition can improve growth performance, nutrient utilization, gut morphology, blood profile and improve water quality by reducing nitrogen excretion in tiger puffer fish Takifugu rubripes (Moe et al. 2009), olive flounder (Thu et al. 2010), striped catfish Pangasianodon hypophthalmus (Quaiyum et al. 2014) and common carp Cyprinus carpio (Mabe et al. 2018). Feeding 1 percent rice husk biochar in a feed for striped catfish increased growth performance, beneficial microbiota proliferation and water quality (Lan et al. (2016). Feeding 2 percent rice husk biochar in a feed increases the average length and width of gastric mucosa and intestinal villi of giant trevally Caranx ignobilis (Firdus et al. 2021). Mechanisms Adsorption of Toxic Substances Organic and inorganic pollutants, heavy metals (e.g., arsenic, cadmium, chromium, lead and mercury) and residues of drugs, pesticides, insecticides and herbicides pose a high health risk to animals. The mechanism of elimination of contaminants by biochar is its high absorption properties; toxic substances electrostatically interact with biochar and are trapped. Another possible mechanism is the high ion exchange capacity and high surface area of the biochar. Negatively charged ions of biochar attract the positively charged ions of toxic substances. When feed containing biochar reaches the intestine, it traps the toxic substance and reduces its harmful effects on animals. Another major benefit of supplementing biochar in feed is its enteral dialysis properties; it can remove lipophilic a nd hydrophilic toxins from blood (Schmidt et al. 2019). Adsorption of Mycotoxins Consumption of mycotoxin-contaminated feed by fish causes health problems (e.g., immunosuppressive disorders), carcinogenic and mutagenic effects, decreased activity of gastrointestinal microbes and overall reduced fish production. The high surface area and low pores size of biochar make it an excellent absorbent that binds to mycotoxin, reducing its assimilation and bioavailability in the gastrointestinal tract and minimizing its negative effects on fish (Schmidt et al. 2019). Detoxification of Plant Toxins The use of plant-based ingredients has detrimental effects on the fish growth and health because of intrinsic natural toxins and Biochar preparation unit: (A) raw material holding chamber (B) electrical heating kiln. (A) Paddy straw biochar, (B) sieving biochar to make a fine powder and (C) fish feed mixed and with and without rice straw and paddy straw biochar.
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