AMINO-ACID-REQUIREMENTS OF FISH - A CRITICAL-APPRAISAL OF PRESENT VALUES

There are large variations in the measured essential amino acid requir ements 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 mainten ance, and the former is quantitatively much the more important in youn g, rapidly growing fish. It is noted that the amino acids laid down du ring growth are sensibly the same in different species. Maintenance is considered to consist of losses from the integument and intestine, fr om oxidation of amino acids, from conversion of amino acids to other N -molecules and from protein turnover. Losses from these causes are con sidered and are not thought likely to differ appreciably between speci es. 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 ac id requirement values for rainbow trout from different laboratories. F actors likely to affect the overall performance of fish in requirement studies (water quality, different sources of amino acids and so on) a re enumerated, but are not thought likely to explain the observed disc repancies. Dietary energy density is an important factor affecting ami no acid requirement, but there are uncertainties surrounding metabolis able energy contents of major dietary components and this tends to pre clude expression of amino acid requirement in terms of metabolisable e nergy. Other methods of assessing amino acid requirement are regarded as subsidiary to, and confirmatory of. growth data. Where amino acid d eficiencies 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 ba sed on all published values but with greater weight being given to stu dies characterised by high rates of growth. The relative proportions o f essential amino acids in the requirement pattern of the two species bear a strong similarity.