FISH NUTRITION
391
From a knowledge of the nitrogen content of the food, the fish and the
faecal matter, various functions were derived, mainly by plotting the
nitrogen content of the food absorbed against the nitrogen content of
the fish. Fruitful discussion of Birkett’s data is not easy because various
inconsistencies occur, for example, summation of all Birkett’s data for
yearling plaice show that a live weight gain of 15 g (345 mg N) was
apparently achieved by the consumption of 14.9 g of food (118 mg N).
In addition, Artemia, which was used as food for yearling plaice,
contains a significant amount of nitrogen in the form of chitin as well
as protein so that deductions based on nitrogen content alone may not
be valid. It is also felt that some experimental measurements of nitrogen excretion are necessary for a proper description of nitrogen balance.
B. The calorijc value of protein
Growth rate, diet and metabolic rate are inter-related and in
evaluating their relationship all three terms are normally expressed in
energy units. It is common practice with experimental or practical
diets to state their calorific value and to quote the number of dietary
calories required to produce a unit weight of fish. One ought thus to be
able to compare directly the calorific values of any published diets.
Unfortunately this state of affairs does not reign, largely because
different workers have ascribed different calorific values to dietary
protein. Thus Phillips et al. (1966), Hastings and Dupree (1969) and
others use a value of 4.1 kcal/g dietary protein, other workers
(Brett et al., 1969) a value of 5.7 kcal/g protein.
The total energy from 1 g of protein is about 5.7 kcal and if the end
product of protein metabolism in the fish in question is entirely ammonia
then the biologically available energy in the food protein (or that
portion of it which is assimilated) equates to 5.7 kcal/g. When,
however, the excretory product contains a carbon fragment as in the
case of urea then the biologically available energy from 1 g of food
protein is considerably less than 5.7 kcal/g (when urea is excreted it is
4.3 kcal/g).
It is thus of some importance to establish both qualitatively and
quantitatively the nitrogenous end products of cultured fish if the
calorific value of their diets are to be accurately assessed. Unfortunately information concerning excretory products is meagre, and
moreover little of it refers to species which are either being farmed or
are likely to be farmed. The most reliable data is that of Wood (1958)
relating to the marine teleosts Leptocottus armatus Girard (Sculpin),
Platichthys stellatus (Pallas) (starry flounder) and Taeniotoca lateralis
(blue sea perch). Wood measured the total nitrogen excreted by these
391
From a knowledge of the nitrogen content of the food, the fish and the
faecal matter, various functions were derived, mainly by plotting the
nitrogen content of the food absorbed against the nitrogen content of
the fish. Fruitful discussion of Birkett’s data is not easy because various
inconsistencies occur, for example, summation of all Birkett’s data for
yearling plaice show that a live weight gain of 15 g (345 mg N) was
apparently achieved by the consumption of 14.9 g of food (118 mg N).
In addition, Artemia, which was used as food for yearling plaice,
contains a significant amount of nitrogen in the form of chitin as well
as protein so that deductions based on nitrogen content alone may not
be valid. It is also felt that some experimental measurements of nitrogen excretion are necessary for a proper description of nitrogen balance.
B. The calorijc value of protein
Growth rate, diet and metabolic rate are inter-related and in
evaluating their relationship all three terms are normally expressed in
energy units. It is common practice with experimental or practical
diets to state their calorific value and to quote the number of dietary
calories required to produce a unit weight of fish. One ought thus to be
able to compare directly the calorific values of any published diets.
Unfortunately this state of affairs does not reign, largely because
different workers have ascribed different calorific values to dietary
protein. Thus Phillips et al. (1966), Hastings and Dupree (1969) and
others use a value of 4.1 kcal/g dietary protein, other workers
(Brett et al., 1969) a value of 5.7 kcal/g protein.
The total energy from 1 g of protein is about 5.7 kcal and if the end
product of protein metabolism in the fish in question is entirely ammonia
then the biologically available energy in the food protein (or that
portion of it which is assimilated) equates to 5.7 kcal/g. When,
however, the excretory product contains a carbon fragment as in the
case of urea then the biologically available energy from 1 g of food
protein is considerably less than 5.7 kcal/g (when urea is excreted it is
4.3 kcal/g).
It is thus of some importance to establish both qualitatively and
quantitatively the nitrogenous end products of cultured fish if the
calorific value of their diets are to be accurately assessed. Unfortunately information concerning excretory products is meagre, and
moreover little of it refers to species which are either being farmed or
are likely to be farmed. The most reliable data is that of Wood (1958)
relating to the marine teleosts Leptocottus armatus Girard (Sculpin),
Platichthys stellatus (Pallas) (starry flounder) and Taeniotoca lateralis
(blue sea perch). Wood measured the total nitrogen excreted by these