FISH NUTRITION
395
radioactivity. Those amino acids which contained radioactivity could
clearly be synthesized by the fish itself from ordinarily available
materials and were not essential dietary constituents ; on the other hand
those amino acids which did not contain radioactivity could not be
synthesized by the fish and it was inferred that they were dietary
essential amino acids. The same ten amino acids were found indispensable for these marine fish.
An identity of essential amino acid requirements in those fish which
have been studied is apparent, and this pattern is similar to that of
warm blooded animals. In fact there is a marked uniformity of essential
amino acid requirement in the vertebrate kingdom and indeed this
uniformity extends also to those arthropods which have been
examined (Kasting and McGinnis, 1958 ; Cowey and Forster, 1971).
D. Dietary protein requirement
The minimum amount of nutrient needed to produce maximal
growth is of particular concern to nutritionists. The growth-response
plot for given nutrients and given organisms frequently assumes a
hyperbolic shape, i.e. the response to a continuing constant increase in
the nutrient is a steadily diminishing increment in growth. Pinally
growth increments cease altogether indicating that the limit of response
has occurred and the curve reaches a plateau. The requirement for
the particular nutrient is then fixed by the intersection between the
plateau line and the initial linear portion of the plot. This approach
has been used in delineating quantitative requirements for proteins and
amino acids in fish.
The level of protein intake necessary for maximal growth has been
investigated in several fish species. One of the first of these was by
Halver and his colleagues (De Long et al., 1958) on chinook salmon.
Fish were fed a partially defined diet in which the protein was supplied
by a mixture of gelatin, casein and crystalline amino acids of overall
amino acid composition simulating that of whole egg protein. The latter
protein is the most nearIy perfectly utilized protein by mammals, and
growth on it is something of a yardstick in mammalian nutrition. Diets
containing different levels of protein were formulated by substituting
greater or lesser amounts of dextrin for protein, on a weight basis, and
a series of diets containing between 5% and 65% protein (in the dry
matter) were used. The diets so formed were said to be " isocaloric
on a total energy basis '' but it must be questionable whether they were
isocaloric to the fish. I n the first place it is doubtful if dextrin and
protein have the same calorific value for fish, and secondly when
increasing amounts of dextrin are fed to fish proportionately less of it
395
radioactivity. Those amino acids which contained radioactivity could
clearly be synthesized by the fish itself from ordinarily available
materials and were not essential dietary constituents ; on the other hand
those amino acids which did not contain radioactivity could not be
synthesized by the fish and it was inferred that they were dietary
essential amino acids. The same ten amino acids were found indispensable for these marine fish.
An identity of essential amino acid requirements in those fish which
have been studied is apparent, and this pattern is similar to that of
warm blooded animals. In fact there is a marked uniformity of essential
amino acid requirement in the vertebrate kingdom and indeed this
uniformity extends also to those arthropods which have been
examined (Kasting and McGinnis, 1958 ; Cowey and Forster, 1971).
D. Dietary protein requirement
The minimum amount of nutrient needed to produce maximal
growth is of particular concern to nutritionists. The growth-response
plot for given nutrients and given organisms frequently assumes a
hyperbolic shape, i.e. the response to a continuing constant increase in
the nutrient is a steadily diminishing increment in growth. Pinally
growth increments cease altogether indicating that the limit of response
has occurred and the curve reaches a plateau. The requirement for
the particular nutrient is then fixed by the intersection between the
plateau line and the initial linear portion of the plot. This approach
has been used in delineating quantitative requirements for proteins and
amino acids in fish.
The level of protein intake necessary for maximal growth has been
investigated in several fish species. One of the first of these was by
Halver and his colleagues (De Long et al., 1958) on chinook salmon.
Fish were fed a partially defined diet in which the protein was supplied
by a mixture of gelatin, casein and crystalline amino acids of overall
amino acid composition simulating that of whole egg protein. The latter
protein is the most nearIy perfectly utilized protein by mammals, and
growth on it is something of a yardstick in mammalian nutrition. Diets
containing different levels of protein were formulated by substituting
greater or lesser amounts of dextrin for protein, on a weight basis, and
a series of diets containing between 5% and 65% protein (in the dry
matter) were used. The diets so formed were said to be " isocaloric
on a total energy basis '' but it must be questionable whether they were
isocaloric to the fish. I n the first place it is doubtful if dextrin and
protein have the same calorific value for fish, and secondly when
increasing amounts of dextrin are fed to fish proportionately less of it