ARTICLE FEED & ADDITIVE MAGAZINE August 2023 67 small amounts to fish diets to enhance feed consumption. They serve various purposes, including improving feed stability, enhancing color appearance, stimulating feed intake, and preserving feed quality. By incorporating specific ingredients or mixtures into the basic feed, the specific requirements of the cultured organism can be met. While plant or animal protein sources can be used as substitutes for fish meal from a nutritional standpoint, they may have limitations in terms of palatability. To address this, the inclusion of feed attractants and flavors in the diet can promote faster food intake and reduce the leaching of water-soluble nutrients by minimizing the food's duration in the aquatic system. As a result, improved feed intake leads to enhanced feed efficiency, minimizing losses and maximizing utilization. A. FREE AMINO ACIDS Different fish species have varying feeding behaviors and consume a combination of prey and aquatic plants. These feeds naturally contain free amino acids that are highly soluble in water. Due to the sensitivity of fish taste receptors, these free amino acids are attractive and elicit electrophysiological responses related to taste. Aromatic amino acids, specifically non-polar ones, are particularly effective in stimulating feed intake. The mixture of free amino acids is more attractive than the whole preferred food organism. Certain free amino acids, such as L-alanine, L-glutamic acid, glycine, and L-arginine, possess attractive properties. Palatability of free amino acids varies among species, with neutral amino acids being more palatable than mixtures of acidic and neutral amino acids in European eels. In sea bream, L-alanine and L-serine were found to be more palatable. Various amino acids, including L-glutamine, L-tyrosine, glycine, L-arginine, L-histidine, L-threonine, L-valine, L-tryptophan, and L-aspartic acid, have been reported as feed stimulants. However, certain amino acids like L-cysteine, L-alanine, L-serine, L-lysine, L-norvaline, L-leucine, and L-isoleucine are less attractive and rarely act as stimulants. In rainbow trout, L-isoleucine and L-leucine are recognized as feed stimulants. The essential amino acid requirements in fish species are similar to those in terrestrial animals. However, it has been observed that the essential amino acids themselves do not contribute to the palatability of the feed in common carp. In fact, some amino acids can act as deterrents, imparting an aversive taste to fish. Certain amino acids, including L-valine, L-phenylalanine, L-histidine, L-proline, L-cysteine, and L-methionine, have been found to decrease the palatability of the feed significantly. These deterrent amino acids stimulate an aversive taste response in fish, leading to complete rejection of the diet when formulated with these amino acids. The taste properties of amino acids are stereotypic, with D-isomers of amino acids being deterrents and lacking taste significance, while L-forms of amino acids are highly palatable in rainbow trout. B. BETAINE Betaine, a water-soluble quaternary ammonium compound, was first discovered in sugar beet juice. It is recognized by the olfactory bulb and gustatory system of fish, making it an attractive feed stimulant. Found naturally in various fish diets, betaine is commonly used as an additive in aquaculture to enhance feed consumption. When added to plantbased or non-fish meal diets, betaine increases diet attractiveness, although reduced feed intake has been observed in certain fish species. Betaine also acts as a flavor enhancer for other amino acids in the diet. Additionally, it plays a role in osmoregulation and methyl donation, protecting cells from osmotic pressure changes. Studies have shown positive effects of betaine on feed intake and growth in juvenile grouper, rainbow trout, cobia, and crustaceans like Macrobranchium rosenbergii, Peneus monodon, and P. indicus by improving food-searching behavior. C. NUCLEOTIDES AND NUCLEOSIDES Fish acquire nucleotides through diet or synthesis pathways. These nucleotides act as feed stimulants. Nucleotides such as IMP, inosine, and their breakdown products enhance feed intake in various aquatic animals. Krill synthetic extract and yeast RNA extract increase feeding rates. Marine organisms with higher IMP show improved performance
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