396
C. B. COWEY AND J. R. SAROENT
is assimilated (Singh and Nose, 1967). The question of caloric content in
a series of diets containing very different amounts of protein is in fact
quite complex because food energy losses during assimilation may vary
with the nutritional balance of the diet. Again a large reduction in
calories may deplete protein stores rapidly, but, as the stores decrease,
the caloric requirements seem to decrease.
After a ten week feeding period, DeLong et al. found that weight
gains of chinook salmon fingerlings were optimal when protein comprised 40% (water temperature 8.3"C) and 55% (water temperature
146°C) of the dry diet.
A dietary scheme similar to that of DeLong et al. was employed in
studies on the dietary protein level of plaice at a water temperature of
16OC (Cowey et al., 1970a) ; casein (supplemented with arginine, cystine,
methionine and tryptophan to simulate whole egg protein) was used as
the protein source. We found no levelling off of the curve of weight
gain up to the very high protein levels (70%) in the dry diet ; whether
or not nitrogen retention continued to increase at these levels was not
determined. A possible explanation for the failure of the response
curve to reach a plateau in these experiments may be that the diets
differed in their calorific value to the fish. In fact, if a calorific value of
6.7 kcalfg is assumed for protein, and dietary protein levels up t o 70%
are employed, it becomes extremely difficult to make all the diets
isocaloric.
In a subsequent experiment with o-group plaice we have used
freeze dried cod muscle (for amino acid composition see Porter et aE.,
1968) as the protein source in a series of diets (Table 11) of increasing
protein content. The use of freeze dried cod muscle helps to circumvent
problems of acceptability related to taste and texture. In these
experiments optimal weight gain occurred on diets containing 50yo
protein (Fig. 2).
Ogino and Saito (1970) used casein as the protein source in
experiments on young carp. The weight gain curve did not reach a
plateau up t o the highest level of dietary protein used (55%). On the
other hand, gain in total body protein increased linearly with protein
intake up to a dietary protein level of 38%, and thereafter increases in
protein intake did not result in any further gain in total body protein.
A dietary level of 38% for casein was regarded as optimal for young
carp.
Several studies have been made on channel catfish fed mixtures of
proteins at different levels (Nail, 1962; Simco and Cross, 1966; Deyoe
and Tiemeier, 1968) and these have indicated dietary protein requirements for optimal growth in the region of 25% or more. In these
C. B. COWEY AND J. R. SAROENT
is assimilated (Singh and Nose, 1967). The question of caloric content in
a series of diets containing very different amounts of protein is in fact
quite complex because food energy losses during assimilation may vary
with the nutritional balance of the diet. Again a large reduction in
calories may deplete protein stores rapidly, but, as the stores decrease,
the caloric requirements seem to decrease.
After a ten week feeding period, DeLong et al. found that weight
gains of chinook salmon fingerlings were optimal when protein comprised 40% (water temperature 8.3"C) and 55% (water temperature
146°C) of the dry diet.
A dietary scheme similar to that of DeLong et al. was employed in
studies on the dietary protein level of plaice at a water temperature of
16OC (Cowey et al., 1970a) ; casein (supplemented with arginine, cystine,
methionine and tryptophan to simulate whole egg protein) was used as
the protein source. We found no levelling off of the curve of weight
gain up to the very high protein levels (70%) in the dry diet ; whether
or not nitrogen retention continued to increase at these levels was not
determined. A possible explanation for the failure of the response
curve to reach a plateau in these experiments may be that the diets
differed in their calorific value to the fish. In fact, if a calorific value of
6.7 kcalfg is assumed for protein, and dietary protein levels up t o 70%
are employed, it becomes extremely difficult to make all the diets
isocaloric.
In a subsequent experiment with o-group plaice we have used
freeze dried cod muscle (for amino acid composition see Porter et aE.,
1968) as the protein source in a series of diets (Table 11) of increasing
protein content. The use of freeze dried cod muscle helps to circumvent
problems of acceptability related to taste and texture. In these
experiments optimal weight gain occurred on diets containing 50yo
protein (Fig. 2).
Ogino and Saito (1970) used casein as the protein source in
experiments on young carp. The weight gain curve did not reach a
plateau up t o the highest level of dietary protein used (55%). On the
other hand, gain in total body protein increased linearly with protein
intake up to a dietary protein level of 38%, and thereafter increases in
protein intake did not result in any further gain in total body protein.
A dietary level of 38% for casein was regarded as optimal for young
carp.
Several studies have been made on channel catfish fed mixtures of
proteins at different levels (Nail, 1962; Simco and Cross, 1966; Deyoe
and Tiemeier, 1968) and these have indicated dietary protein requirements for optimal growth in the region of 25% or more. In these