Proteins and Amino Acids (GROSS PROTEIN REQUIREMENTS)

Gross protein requirements have been determined for a few species of fish (see Table 1). Simulated whole egg protein component of test diets contains an excess of indispensable amino acids. These diets were kept approximately isocaloric by adjusting total protein plus digestible carbohydrate components to a fixed amount as the protein diet treatments were varied over the ranges tested. Tests in feeding fry, fingerling, and yearling fish have shown that gross protein requirements are highest in initial feeding fry and that they decrease as fish size increases. To grow at the maximum rate, fry must have a diet in which nearly half of the digestible ingredients consist of balanced protein; at 6-8 weeks this requirement is decreased to about 40 percent of the diet for salmon and trout and to about 35 percent of the diet for yearling salmonids raised at standard environmental temperature (SET). See Figures 1 and 2. Gross protein requirements for young Catfish appear to be less than those for salmonids. Initially feeding fry require that about 50 percent of the digestible components of the ration be protein, and the requirement decreases with size. Some feeding trials with salmon have indicated direct relationships between changes in the protein requirements of young fish and changes in water temperature. Chinook salmon in 7 C water require about 40 percent whole egg protein for maximum growth; the same fish in 15 C water require about 50 percent protein. Salmon, trout and catfish can use more protein than required for maximum growth because of efficiency in eliminating nitrogenous wastes in the form of soluble ammonia compounds through the gill tissue directly into the water environment. This system for eliminating nitrogen is more efficient than that available to fowl and mammals. Fowl and mammals consume energy to synthesize urea, uric acid, or other nitrogen compounds which are excreted through the kidney tissue and expelled in urine. Digestible carbohydrate and fat will spare excess protein in the diet as long as the protein requirement for maximum growth is met.
Basically the fish must be given a diet containing graded levels of high quality protein and energy and adequate balances of essential fatty acids, vitamins and minerals over a prolonged period. From the resulting dose/response curve the protein requirement is usually obtained by an Almquist plot. These differences in apparent protein requirement are thought to be due to differences in culture techniques and diet composition.
The relatively high dietary protein levels required for maximal growth of certain fish such as grass carp, Ctenopharyngodon idella, and Brycon spp. are surprising as these fish are omnivorous. Brycon spp. are grown on unwanted fruit and other plant material of low protein content and under these conditions there is presumably a substantial contribution to their protein intake from a natural food chain.
Protein requirement of eurythaline fish such as the rainbow trout, Salmo gairdneri, and the coho salmon, Oncorhynchus kisutch, reared in water of salinity 20 ppt is about the same as the requirement in freshwater. No data are available for the protein requirement of these species in full strength sea water.(35 ppt).