Colin B. Cowey

Protein and Amino Acid Requirements

Over the past 25 years, considerable progress has been made in the study of the dietary nutrient requirements of fishes (for reviews see Cowey & Sargent, 1972, 1979; Halver, 1972; National Research Council, 1981, 1983; and Millikin, 1982). Despite some obvious similarities between fishes and other vertebrates in basic qualitative nutrient needs, the two groups have markedly different quantitative nutrient requirements. For example, the optimal dietary protein level required for maximal growth in farmed fishes is reported to be 50–300% higher than that of terrestrial farm animals (Cowey, 1975). In the main, these quantitative differences have been attributed to the carnivorous/omnivorous feeding habit of fishes and their apparent preferential use of protein over carbohydrate as a dietary energy source. However, the common expression by nutritionists (including major review authors) of nutrient requirements solely in terms of a ‘dietary percentage’ has itself limited value unless it is related to the feed intake and subsequent growth of the animal. This chapter attempts to relate the protein and amino acid requirements of fishes to the ‘growing animal’ with respect to its dietary feeding regime, developmental status, position in the aquatic food chain, and its physical environment. In addition, this chapter critically assesses the methodology employed by researchers for the measurement of nutrient requirements.

Aspects of intermediary metabolism in salmonid fish

The basic pathways of intermediary metabolism that have been investigated so far in salmonid fish are very similar to those occurring in mammals and other animals, though important differences do exist with regards to the presence/absence and physiological importance of some pathways. Salmonids like many other fish, have relatively high dietary requirements for both protein and essential amino acids which in some cases are more than twice those of rat, chicken and pig (see reviews by Mertz, 1972; Cowey, 1975, 1979; Ogino, 1980). Carbohydrates are relatively poorly utilised by fish and the evidence suggests that proteins together with lipids are the major sources of energy (Atherton & Aitken, 1970; Pieper & Pfeffer, 1978; Cr6ach & Serfaty, 1974; Mommsen et al., 1980). This contrasts to the situation in omnivorous mammals where, under normal nutritional conditions, protein catabolism is of little significance in supplying energy whereas carbohydrates and lipids are important energy sources. Phillips (1969) has suggested that 70% of dietary calories in trout feed is from protein, thus a greater percentage of dietary protein is metabolised for energy, rather than utilized for body protein synthesis. Since the end produce of N-metabolism in teleosts is ammonia rather than the more energy costly urea or uric acid (as in mammals, birds and reptiles), fish do not derive the same amounts of energy from dietary constituents as do mammals and hence food protein may have a higher metabolisable energy than carbohydrates. The energy requirements of fish (being poikilothermic) are lower than mammals which expend much energy to maintain their body temperature. Since fish body temperature varies with the water temperature, then reaction rates and occurrence of some pathways within the fish are affected by the environmental temperature. This article is a somewhat brief attempt to describe aspects of the intermediary metabolism of amino acids, carbohydrates and lipids that have been reported for salmonids (predominantly).

Nutrition : estimating requirements of rainbow trout

Abstract The formulation and manufacture of fish feed, the principal cost factor in fish production, must be based on sound information regarding nutritional requirements if the process is to be economical. Assessing nutrient requirements from growth experiments depends on the availability of a suitable model of growth, the analysis used to identify required levels of nutrients, choice of response variables, and the feeding scheme used to deliver the target nutrient to the fish. The review examines specific elements critical in experiments designed to estimate nutrient requirements for vitamins, proteins, amino acids, essential fatty acids, and minerals. Also provided are formulations for semi-purified diets and examples of data interpretation.

Amino acid requirements of fish: a critical appraisal of present values☆

Abstract There are large variations in the measured essential amino acid requirements of different species of fish when expressed as a proportion of the diet. The question of whether or not these are real differences is considered. Dietary amino acids are needed for growth and for maintenance, and the former is quantitatively much the more important in young, rapidly growing fish. It is noted that the amino acids laid down during growth are sensibly the same in different species. Maintenance is considered to consist of losses from the integument and intestine, from oxidation of amino acids, from conversion of amino acids to other N-molecules and from protein turnover. Losses from these causes are considered and are not thought likely to differ appreciably between species. When amino acid requirements are expressed as a proportion of the dietary protein, differences, while reduced somewhat, are still wide. The dilemma is illustrated by reference to the differences in amino acid requirement values for rainbow trout from different laboratories. Factors likely to affect the overall performance of fish in requirement studies (water quality, different sources of amino acids and so on) are enumerated, but are not thought likely to explain the observed discrepancies. Dietary energy density is an important factor affecting amino acid requirement, but there are uncertainties surrounding metabolisable energy contents of major dietary components and this tends to preclude expression of amino acid requirement in terms of metabolisable energy. Other methods of assessing amino acid requirement are regarded as subsidiary to, and confirmatory of, growth data. Where amino acid deficiencies lead to tissue pathologies it is important that the stated requirement level is such as to prevent such pathologies. A table of requirement values for channel catfish and trout is provided; it is based on all published values but with greater weight being given to studies characterised by high rates of growth. The relative proportions of essential amino acids in the requirement pattern of the two species bear a strong similarity.

Nutritional requirements of fish

The impetus for accurate information on the nutrient requirements of fish derives very largely from the development, in many parts of the world, of an aquaculture industry that is dependent on artificial feeds. At the same time such information can provide the basis for comparative nutrition, whereby features of the nutrition of cold-blooded, water-breathing and mainly, carnivorous vertebrates which differ from the pattern largely common to omnivorous mammals are identified. Since this topic was last addressed (Cowey, 1988) the aquaculture industry has continued to grow, fuelled both by the continuing overexploitation of the marine environment, resulting in declining yields, and by the high quality of aquaculture products. The first serious studies on nutritional requirements of fish were made in the 1950s. Since then much has been learned concerning fish husbandry, water quality, pellet quality (water stability) of fish diets and so on. Consequently, general methodological standards in nutritional experiments have improved greatly and some of the early values for nutrient requirements need to be revised, usually downward.

Lipid nutrition in fish

Abstract 1. 1. Two main forms of neutral lipid are available to fish in the natural environment, namely triacylglycerols and wax esters. 2. 2. There is evidence that triacylglycerols can be hydrolysed completely to free fatty acids and glycerol in the gastro-intestinal tract and absorbed as such. Fatty alcohols resulting from wax ester hydrolysis are oxidized to the corresponding acid and thereafter follow pathways of fatty acid metabolism. 3. 3. Polyunsaturated fatty acids in fish tissues are predominantly of the ω3 series. 4. 4. Fatty acids of the ω3 series have essential fatty acid activity for fish. Some species have the ability to convert linolenic acid (18:3ω3) rapidly to longer chain polyunsaturated acids (20:5ω3, 22:6ω3) that have full essential fatty acid activity. Other species lack this ability and the polyunsaturated ω3 acid must be supplied preformed in the diet for maximal growth and freedom from pathology.

Some effects of selenium deficiency on enzyme activities and indices of tissue peroxidation in Atlantic salmon parr (Salmo salar)

Abstract Two groups of Atlantic salmon ( Salmo salar ), mean weight 6 g, were given diets of differing selenium content (deficient 0.017 mg Se/kg; supplemented 0.944 mg Se/kg) and adequate vitamin E content (40 mg α -tocopheryl acetate/kg) for 28 weeks. The final weight and packed cell volume of salmon given the Se-supplemented diet were significantly greater than those of the Se-deprived fish. Blood vitamin E concentrations, and liver, blood and brain Se concentrations were all significantly lowered in Se-deficient salmon. No gross pathologies were observed but pathology was evident in pancreatic tissue (loss of integrity in endoplasmic reticulum and increased vacuolation) from Se-deficient salmon. Glutathione peroxidase (EC.1.11.1.9) activity was greatly reduced in liver of the Se-deficient salmon although there was no indication of any compensatory non Se-dependent glutathione peroxidase activity. Hepatic glutathione S-transferase (EC.2.5.1.18) activity, plasma pyruvate kinase (EC.2.7.1.40) activity, erythrocyte fragility and kidney reduced glutathione were all increased in Se deficiency.

Digestibility and bioavailability of dietary selenium from fishmeal, selenite, selenomethionine and selenocystine in Atlantic salmon (Salmo salar)

Abstract Four groups of Atlantic salmon (Salmo salar), mean weight 68 g, were given diets for 4 weeks, in which the selenium (Se) was supplied from either fishmeal, sodium selenite, DL-selenomethionine or DL-selenocystine. Selenomethionine was the most digestible (92%) and fishmeal (47%) the least digestible source of Se. This was reflected in plasma Se concentration which was highest in fish given selenomethionine, although the activity of the enzyme glutathione peroxidase (GSH-Px) which contains Se was not significantly increased. Measurement of the GSH-Px:Se ratio indicated that Se supplied as selenite or selenocystine was a better source of Se for plasma GSH-Px than was Se from selenomethionine or fishmeal. Se concentration and GSH-Px activity in liver and erythrocytes were not significantly different in fish given the experimental diets for 4 weeks.

Gluconeogenesis by isolated hepatocytes from rainbow trout Salmo gairdneri

Abstract 1. 1. A procedure for the preparation of intact, metabolically viable trout liver parenchymal cells by the use of collagenase is described. 2. 2. Cell integrity was judged to be satisfactory by microscopic appearance, exclusion of vital dyes and succinate, and retention of intracellular enzymes. 3. 3. Gluconeogenesis from [U-14C]labelled alanine, pyruvate and lactate was shown to proceed in these cells. 4. 4. The effects of glucagon, quinolinate, aminoxyacetate and α-cyanohydroxycinnamate on gluconeogenesis were also investigated.

Aspects of ammoniogenesis in rainbow trout, Salmo gairdneri

1. 1. Factors affecting the formation and excretion of ammonia in trout have been examined. 2. 2. During passage through the gills, the plasma concentrations of ammonia and glutamate fell significantly, while that of glutamine remained unchanged. 3. 3. The tissue distribution of several enzymes was investigated. Glutamate dehydrogenase and glutaminase were mainly found in liver and kidney; AMP deaminase in muscle; and glutamine synthetase in brain. 4. 4. Glutamate is oxidised by trout liver mitochondria predominantly by the GIDH pathway (60%) while 40% is converted to aspartate. 5. 5. The concentrations of NAD+, NADP+, NADH, NADPH and several metabolic components affecting GIDH activity were measured in freeze-clamped livers. 6. 6. Kinetic parameters of liver G1DH were determined. 7. 7. The results indicate that transdeamination is the major ammonia forming process in trout liver and kidney.