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C. B. COWEY AND J. R. SAROENT
largely the chromic oxide indicator. Values obtained with most animal
proteins were high, 80% or more being assimilated. Plant proteins
(soyabean meal, wheat middlings, rice bran, distillers’ solubles,
cottonseed meal and so on) tended to be less well assimilated, values
being in the range 70% or below. Hastings has rightly pointed out that
these values refer normally to a single set of conditions and that
factors such as feeding rate, nutrient level in the diet, fish size, water
temperature and so on may affect digestibility.
Preliminary results on the assimilation of protein by plaice have
given a similar picture ; animal proteins (white fish meal, fish protein
concentrate) and single cell protein are all assimilated to an extent
greater than 90%. Soyabean meal (defatted) on the other hand is only
68% assimilated.
In the experiments of Gerking referred to above, sunfish were fed
mealworm at a series of daily rates between 1.16 and 3.59% of their
body weight each day. Protein assimilation remained uniformly high
(about 97%) at all levels.
Inaba et al. (1963) examined the assimilation of protein (mainly
white fish meal) at dietary levels of between 24 and 45% by rainbow
trout. Protein absorption was uniformly high (85-94%) at all dietary
levels used. Kitimikado et al. (1964a, b) also using rainbow trout found
that digestibility of proteins was relatively low in small fish (5.6 g), the
effect being especially marked when white fish meal was the test protein
(assimilation only 40%). This value was improved to 73% when the
fish meal was finely ground as by passing it through a 60 mesh screen.
The same workers showed that high dietary starch levels (60430%)
had a detrimental effect on the assimilation of both casein and white fish
meal by larger fish (20 g live weight). On the other hand dietary lipid
values of up to 30% did not affect the assimilation of either casein or
white fish meal.
Within the limits of practical diets it would therefore presently
seem that protein assimilation is not greatly affected by other dietary
considerations.
I . Food energy and protein requirement
The protein requirements of animals may be spared by nonprotein foodstuffs by virtue of their calorific properties. Experience with
mammals indicates that low energy diets result in an increased catabolism of labile protein which is associated with the need to ameliorate the
energy deficiency in the metabolic pool. On the other hand, higher
dietary caloric levels lead to increased retention and utilization of
dietary protein. It might be considered that optimal dietary calorie
C. B. COWEY AND J. R. SAROENT
largely the chromic oxide indicator. Values obtained with most animal
proteins were high, 80% or more being assimilated. Plant proteins
(soyabean meal, wheat middlings, rice bran, distillers’ solubles,
cottonseed meal and so on) tended to be less well assimilated, values
being in the range 70% or below. Hastings has rightly pointed out that
these values refer normally to a single set of conditions and that
factors such as feeding rate, nutrient level in the diet, fish size, water
temperature and so on may affect digestibility.
Preliminary results on the assimilation of protein by plaice have
given a similar picture ; animal proteins (white fish meal, fish protein
concentrate) and single cell protein are all assimilated to an extent
greater than 90%. Soyabean meal (defatted) on the other hand is only
68% assimilated.
In the experiments of Gerking referred to above, sunfish were fed
mealworm at a series of daily rates between 1.16 and 3.59% of their
body weight each day. Protein assimilation remained uniformly high
(about 97%) at all levels.
Inaba et al. (1963) examined the assimilation of protein (mainly
white fish meal) at dietary levels of between 24 and 45% by rainbow
trout. Protein absorption was uniformly high (85-94%) at all dietary
levels used. Kitimikado et al. (1964a, b) also using rainbow trout found
that digestibility of proteins was relatively low in small fish (5.6 g), the
effect being especially marked when white fish meal was the test protein
(assimilation only 40%). This value was improved to 73% when the
fish meal was finely ground as by passing it through a 60 mesh screen.
The same workers showed that high dietary starch levels (60430%)
had a detrimental effect on the assimilation of both casein and white fish
meal by larger fish (20 g live weight). On the other hand dietary lipid
values of up to 30% did not affect the assimilation of either casein or
white fish meal.
Within the limits of practical diets it would therefore presently
seem that protein assimilation is not greatly affected by other dietary
considerations.
I . Food energy and protein requirement
The protein requirements of animals may be spared by nonprotein foodstuffs by virtue of their calorific properties. Experience with
mammals indicates that low energy diets result in an increased catabolism of labile protein which is associated with the need to ameliorate the
energy deficiency in the metabolic pool. On the other hand, higher
dietary caloric levels lead to increased retention and utilization of
dietary protein. It might be considered that optimal dietary calorie