FISH NUTRITION
405
dietary protein levels near the maintenance level and as the proportion
of dietary protein increases NPU falls. At low dietary protein levels the
amino acid composition of the protein is the limiting feature and
maximal NPU values are obtained ; at higher dietary protein levels a
greater proportion of the protein is burned as energy. High dietary
protein levels are however necessary to obtain reasonably rapid growth
6 C
5!
5c
41
v PU
40
31
3c
30
4 0
50
60
7 0
80
90
100
Dietary protein calories as 2 total CalOrleS
containing freeze dried cod muscle.
FIG. 4. Relationship between dietary protein level and NPU in plaice fed diets
in fish. A similar relationship between dietary casein level and NPU
was found by Ogino and Saito (1970) in their experiments with carp. At
10% dietary casein the NPU exceeded 60, but at 66% dietary casein
NPU was about 26. However weight gain on the latter diet was three
times that on the 10% casein diet.
The regression line for NPU against dietary protein is substantially
linear (Fig. 4) and it can be expressed by the equation NPU,,,, =
75.3 - 0.48P, where NpU(,p, is the operative net protein utilization