Dietary lipids play important roles in the energy production processes of animal tissues and as the source of essential fatty acids (EFA). Besides these functions they do have other important dietary roles as carriers of certain non-fat nutrients, notably the fat-soluble vitamins A, D and K. Recent studies on EFA in fish have demonstrated that EFA requirements of fish differ considerably from species to species. On the other hand, the results of studies on the energy requirements of fish during recent years have indicated that in the carnivorous fish such as rainbow trout, eel, yellowtail and plaice which have limited ability to utilize carbohydrates of high mol. wt as an energy source, dietary lipids play an important role in this respect and have a sparing action on dietary protein.

Lipid nutrition in fish

To date, aquatic feeds have depended heavily on fishmeal and fish oil as their source of protein and lipid. However, the feed industry is encountering shortfalls in the availability of these ingredients because of a decline in the number of fish captured in the wild and the increased human demand for some of the species currently being used for fishmeal and oil production. Therefore, efforts are now being directed in different parts of the world to finding alternative quality ingredients, which ideally are less expensive and readily available for use in practical diets. The data accrued have shown that a large proportion of both fishmeal and fish oil can be replaced by other protein and lipid sources. However, it is emphasized that an optimal essential amino acid balance be maintained and that the n-3 highly unsaturated fatty acid requirement be satisfied when combining economical protein and lipid ingredients. Newly developed feeds should aim at being nutrient-dense in order to reduce the output of solid, P and N waste. This can be done through improving nutrient availability, optimizing the digestible protein to energy balance of the diet, and replacing dietary fishmeal with alternate ingredients. These diets should also be effective for maintaining good health and improving disease resistance in fish through enhancement of immune responses. A wholesome approach to culturing fish would be to use appropriate feeding standards that are aimed not only at improving economic returns but also at developing a lasting cohabitation of sustainable aquaculture and a cleaner environment. Furthermore, in the 2lst century aquaculture would still retain its place as a prominent source of food protein, signifying that fish feed research remains a forceful area in discovering better feeds for the industry.

https://www.jstage.jst.go.jp/article/fishsci1994/68/2/68_2_242/_pdf

Strategies for further development of aquatic feeds.

Tentative qualitative and quantitative amino acid requirements have been reported for the major species of fish reared for market or as replacement stocks for natural waters. Most work has concentrated upon juvenile fish or upon rapidly growing young market fish; these have high protein dietary requirements (30-50%) that are in direct contrast to the homothermic terrestial animals. Net protein utilization from the diet is similar or slightly better than that found in avian species, but energy needs are much lower in fish and as a result the body protein deposition in fish is larger (about 5 g protein/MJ for the chick versus about 10 g protein/MJ for young fish). Qualitative amino acid requirements appear identical for all fish species examined; arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine are all required for normal growth and metabolism. Quantitative requirements differ only slightly among species that have been tested. Salmon have higher arginine requirements than other fish examined. Catfish appear to have a lower requirement for histidine and threonine, and the Japanese eel seems to need more tryptophan in the diet. However, when the quantitative requirements for indispensable amino acids are expressed as a percentage of the protein fed, then a remarkable harmony appears between values needed for maximal growth for most species examined. A review of Tables 3 and 4 will disclose the paucity of information available considering the large number of fish species reared commercially over the world. Most commercial diet formulations have relied upon the work done on salmon, catfish, and carp, and their amino acid and protein requirement values have been used. Remarkably, these diets have produced other species of fish economically. Sparing effects of one amino acid on another have only been studied with cystine-methionine and tyrosine-phenylalanine. Arginine and analogues of methionine have been used as good nitrogen sources for salmon. Isoleucine-leucine ratios have been measured and experiments indicate some growth inhibition when the isoleucine-leucine ratio was greater than 2/1. Valine at abnormally high levels also inhibited growth. Much more work needs to be done on the effects of subtle differences in amino acid ratios in the diet, and major emphasis should be placed on the important role of the dispensable amino acids in fish nutrition.(ABSTRACT TRUNCATED AT 400 WORDS)

Protein and Amino Acid Requirements of Fishes

Dietary protein requirement of the red drum (Sciaenops ocellatus) and relative use of dietary carbohydrate and lipid

Protein requirements of grass carp fry (Ctenopharyngodon idella Val.).

The relationships between dietary energy sources and the utilization of dietary proteins have been investigated with rainbow trout and carp. In rainbow trout, lipid was utilized more effectively than carbohydrate (α-starch and dextrin) as an energy source for utilization of dietary protein. The values for PER and NPU of casein at low protein levels were markedly different according to the kind of energy sources used. Accordingly, the protein level producing the maximum growth rate also differed according to the main energy sources used. On the other hand, carp utilized carbohydrate effectively as an energy source.

Jing Yum Ghiou

Papers

Protein nutrition in fish. VI. Effects of dietary energy sources on the utilization of proteins by rainbow trout and carp.