FISH NUTRITION
407
The NPU of a further series of protein mixtures determined by Nose
(1963) on rainbow trout is shown in Table VI ; the effect of supplementation with limiting amino acids is demonstrated.
H . The assimilation of ingested protein
The percentage of ingested food protein actually assimilated by
the animal is approximated by the expression:
Food nitrogen-faecal nitrogen x 100
Food nitrogen
In general two methods have been used to evaluate assimilation of
protein namely (i) direct measurement of food nitrogen and faecal
nitrogen (e.g. Gerking, 1955a; Edwards et al., 1969; Birkett, 1969) and
(ii) an indirect procedure in which an inert (not absorbed) indicator
material is included at a known concentration in the food. The ratio of
indicator to that of nitrogen in the faeces is determined, and by
comparison with the ratio of indicator: nitrogen in the food the
percentage assimilation can readily be obtained. The inert indicator
usually employed in experiments with fish is chromic oxide.
The f i s t method suffers from the fact that quantitative collection
of faecal material is necessary and this is by no means easy in experiments with fish. Should any of the faecal nitrogen be even slightly water
soluble the method becomes very difficult. In experiments on the influence of the feeding rate on protein absorption by sunfish Gerking
(1955a) appears to have simply measured the dry weight of faecal
material, apparently assuming it to be of constant composition
irrespective of feeding rate.
The indirect procedure does not necessitate quantitative collection
of faeces and as faecal material can be obtained by applying finger
pressure to the ventral sides of the fish the possibility of chemical leakage from faeces to water is averted. However experiments with
mammals have shown that in certain species excretion of chromic oxide
is not complete and that diurnal variation in chromic oxide excretion
may occur (Knapka et al., 1967). As the chromic oxide technique has
frequently been applied to fish some demonstration that temporal
variations in its excretion pattern do not occur would be useful.
Equally interesting would be comparative trials with other inert
indicators (cerium-144 and polyethylene have been used with
mammals).
Hastings (1969) has tabulated values for the digestibility of a large
number of proteins or materials containing protein. The fish used were
mainly either trout or channel catfish and the method employed was
407
The NPU of a further series of protein mixtures determined by Nose
(1963) on rainbow trout is shown in Table VI ; the effect of supplementation with limiting amino acids is demonstrated.
H . The assimilation of ingested protein
The percentage of ingested food protein actually assimilated by
the animal is approximated by the expression:
Food nitrogen-faecal nitrogen x 100
Food nitrogen
In general two methods have been used to evaluate assimilation of
protein namely (i) direct measurement of food nitrogen and faecal
nitrogen (e.g. Gerking, 1955a; Edwards et al., 1969; Birkett, 1969) and
(ii) an indirect procedure in which an inert (not absorbed) indicator
material is included at a known concentration in the food. The ratio of
indicator to that of nitrogen in the faeces is determined, and by
comparison with the ratio of indicator: nitrogen in the food the
percentage assimilation can readily be obtained. The inert indicator
usually employed in experiments with fish is chromic oxide.
The f i s t method suffers from the fact that quantitative collection
of faecal material is necessary and this is by no means easy in experiments with fish. Should any of the faecal nitrogen be even slightly water
soluble the method becomes very difficult. In experiments on the influence of the feeding rate on protein absorption by sunfish Gerking
(1955a) appears to have simply measured the dry weight of faecal
material, apparently assuming it to be of constant composition
irrespective of feeding rate.
The indirect procedure does not necessitate quantitative collection
of faeces and as faecal material can be obtained by applying finger
pressure to the ventral sides of the fish the possibility of chemical leakage from faeces to water is averted. However experiments with
mammals have shown that in certain species excretion of chromic oxide
is not complete and that diurnal variation in chromic oxide excretion
may occur (Knapka et al., 1967). As the chromic oxide technique has
frequently been applied to fish some demonstration that temporal
variations in its excretion pattern do not occur would be useful.
Equally interesting would be comparative trials with other inert
indicators (cerium-144 and polyethylene have been used with
mammals).
Hastings (1969) has tabulated values for the digestibility of a large
number of proteins or materials containing protein. The fish used were
mainly either trout or channel catfish and the method employed was