Nile tilapia (Oreohromis niloticus) with different initial body sizes were
divided into three size groups (mean initial size): small (1.5 g), medium: (9.0g) and
large : (16.0 g) and reared for 112 days in glass aquaria. Under each size, the fish
were divided into five groups and fed on five experimental diets containing 15
,20,25,30 and 35% dietary crude protein levels. Ten fish were placed randomly in
each of 30 glass aquaria (100 I water capacity/each) in order to study the effects of
the initial body size and dietary protein levels on the growth performance and feed
and nutrients utilization of tilapia. Fish were hand fed twice daily, six days a week at
rate of 4% of its body weight. The results indicated that the absolute values of fish weight (g/fish) , fish gain
and average daily gains were significantly (P<0.05) increased with increasing the
initial body size from 1.5 to 9.0 and 16 g /fish, receptively. However, values of specific
growth rate (SGR%) were significantly (P<0.05) decreased with increasing the initial
body size. On the other hand, increasing dietary protein levels from 15 to 35% CP
significantly (P<0.05) increased growth performance criteria. Fish body composition at the end of the experiment showed a slight increase
in carcass dry matter and crude protein contents with increasing the initial body size
and dietary protein levels. However, values of carcass ether extract and ash were
decreased. Energy contents (Kca1/100g) of fish carcass at the end of the experiment
were similar in all treatments. The results showed that values of feed conversion ratio (FCR) , protein utilization
{expressed as protein efficiency ratio (PER) and protein productive value(PPV%)}and
energy utilization (EU)were significantly (P<0.05) improved with increasing the initial
fish body size . However, values of FCR and EU% were significantly (P<0.05)
improved but PER and PPV% were decreased with increasing the dietary protein
levels. Finally, it could be concluded that small size fish was more efficient and achieved
higher growth performance than large size fish. Increasing the dietary protein level
from 15 to 35% CP increased the growth performance, however, feed utilization
criteria were decreased.