Effects of Protein, Lipid, Feeding Levels and Their Interaction ... 77adversely affect body composition (Erfanullah and Jafri, 1998; Hanley, 1991;
El-Sayed and Garling, 1988; Garling and Wilson, 1977), as well as create
problems associated with pelleting and development of rancidity in stored feed
(Jauncey, 1982). Fattiness is often undesirable in fish cultured for food and
increasing the dietary protein level may be a strategy for producing a leaner
product. There were no interactive effects of protein and lipid on WG and
SGR. This could indicate that the effect of different levels of protein is
independent of the level of lipid in the diet of O. niloticus. The protein sparing
effect of high lipid, the main factor of the research has been reported in many
fish species such as in hybrid tilapia (O. niloticus x O. aureus) (Gaylord and
Gatlin, 2000; Jauncey, 2000; Teshima et al., 1985; El-Sayed and Garling,
1988) but high dietary lipid level (12%) did not show any further improvement
on growth parameters of low or high dietary protein diet compared to low lipid
diet (8%) at the same feeding level in the present study. Similar results have
been reported in seabass (Catacutan and Coloso, 1995). Thus the protein
sparing action of lipid reported by many researchers was not observed in this
study. This could indicate that O. niloticus is unable to secrete sufficient
bile/lipase to meet digestive demands at the highest lipid level. As all the diets
have similar energy level, it is probably that O. niloticus can efficiently utilize
carbohydrate better than lipid because FI decreased at high lipid level with the
same dietary protein level.
From the results, it was quite obvious that low dietary protein enhanced
higher triglyceride and cholesterol compared to high dietary protein level. This
is because percentage feed intake per body weight was generally high at low
dietary protein and the extra energy generated by high feed intake which
cannot be utilized by fish was stored as fat which in turn after metabolism
increased the plasma TG and CHOL levels. Similar observations have been
reported for carp (Shimeno et al., 1995) and tilapia (Shimeno et al., 1993).
When fish are fed on low dietary protein diets, their blood plasma TG and
CHOL levels go up. High dietary protein was reported to cause low plasma
TG in African catfish (Clarias gariepinus) (Matter et al., 2004) which is in
agreement with our findings. Also, similar to our results were the findings of
Cheng et al. (2006) in grouper and also (Chen et al., 2009) in juvenile tilapia
(O. niloticus) where low TG and CHOL were observed from fish fed on high
dietary protein.
Crude lipid level has no significant influences on plasma TG and CHOL at
the end of the experiment which is conflicting with the results of Cheng et al.
(2006) in grouper. The reason could be the difference in species. May be
O.niloticus can utilize dietary lipid more efficiently than grouper. Meanwhile,