Dietary available phosphorus restriction and skeletal integrity in stomachless fish Gnathopogon caerulescens and Carassius auratus the effects of ferric chloride supplementation

Phosphorus is abundant in most feed ingredients and is classified into two forms: available and unavailable phosphorus. Available phosphorus is absorbed in the intestines of fish and used for various physiological needs. Any excess available phosphorus is excreted in urine, which is water-soluble and directly stimulates the growth of algae and macrophytes in surrounding water bodies. Therefore, minimizing the available phosphorus content in feeds to match the exact requirement level of fish is necessary to reduce the environmental impacts of aquaculture and promote its sustainable development. This study aimed to develop a technology to minimize the available phosphorus content in aquaculture feeds using ferric chloride as a phosphorus-binder. In a soybean meal-based diet devoid of inorganic phosphorus, the dietary addition of ferric chloride had no measurable effect on bone ash content or bone formation in experimental fish: Gnathopogon caerulescens and Carassius auratus, both stomachless fish belonging to the cyprinid family. However, in a purified diet containing a normal concentration of inorganic phosphorus, the dietary addition of ferric chloride decreased the bone ash content of the fish (p<0.01). Similarly, in a commercial diet containing inorganic phosphorus, dietary ferric chloride decreased the bone ash content of the fish (p<0.01). In conclusion, in diets containing soluble inorganic phosphorus, supplementing with ferric chloride can reduce the available phosphorus content, thereby decreasing the phosphorus burden on surrounding water bodies. However, when reducing the dietary available phosphorus content, it is advisable to occasionally monitor the bone ash content or check for bone deformities in cultured fish to prevent clinical deficiencies.