THE EFFECT OF PROTEIN AND ENERGY LEVELS ON MINERAL RETENTION IN TILAPIA FISH (O.niloticus)

The effect of different protein and energy levels in the diets on the retention of both major (Ca,P,K,Na, Mg) and trace-elements (Fe, Cu, Zn, Mn) was investigated on a total of 200 tilapia fish (O.niloticus), weighing 10gm each during a growth period of 10 weeks. Fish were fed on nine experimental diets having three levels of protein (28 ,32 ,36%) and energy (10,12,15 Mj/Kg digestible energy) and a control one. Experimental diets were tested, each for two replicates of aquaria. The ash retained in the fish body increased as the level of protein increased, while decreased as the level of dietary energy increased. The dry matter retained in the body increased as the energy level in the diets increased. Increasing the crude protein content of the diets, increased the retention level of calcium, phosphorus and potassium in the body of tilapia, while decreased as the energy levels increased. High retention values of Ca was found at 36% protein and 12 Mj/Kg DE, while for P and K were recorded at 32% protein level and 10 Mj/Kg DE. There was no significant (P>0.05) differences between treatments in the retention of Ca, P and K due to the effect of the protein and energy levels of the diets. The amount of sodium retained in the body of fish was decreased with increasing energy level. High retention value of Na was found at 32% protein and 10 Mj/Kg DE with utilization efficiency of 8%. Magnesium retained in the body of tilapia increased as the levels of protein and energy increased in the diets. High retention value of Mg was found at 36% protein and 12 Mj/Kg DE with utilization efficiency of 12% Increasing protein level in the diets resulted in increased retention of all trace elements (iron, copper, zinc,and manganese), but decreased at the high level (36%). The rising energy supply reduced the retained amount of the trace elements. All trace elements gave good retention in the body of tilapia with 32% protein and 10Mj/Kg DE except Fe at 36% protein and 12Mj/Kg DE. High retention value was found for zinc and low retention value for Mn and the utilization efficiency of trace elements ranged from 1% to 39%. On average, the amount of retained
elements per Kg (on dry basis) of tilapia fish body were : 20.94g Ca, 16.34g P, 8.52g K, 0.95g Na, 1.51g Mg, 13.6mg Fe, 1.91mg Cu, 200.9mg Zn and 1.25mg Mn. It could be concluded that, the retention of minerals were not significantly (P>0.05) affected by the level of dietary energy, while significantly (P<0.05) affected by the level of the dietary protein especially with Na, Fe, Zn and Mn. Among all the major elements, calcium, phosphorus and potassium supply in the feed are of major importance, while for trace elements, iron and zinc of the diets should be taken into consideration in view of the very high retention rate.