EFFECT OF DIETARY PROTEIN TO ENERGY RATIO ON PERFORMANCE OF NILE TILAPIA AND FOOD WEB ENHANCEMENT IN SEMI-INTENSIVE POND AQUACULTURE

 
K. A. Kabir,* M.C.J. Verdegem, J.W. Schrama,  J.A.J. Verreth, M. J. Phillips
 
Aquaculture and Fisheries Group, Wageningen Institute of Animal Sciences, Wageningen University, PO Box 338, 6700 AH, and Sustainable Aquaculture Program, WorldFish Bangladesh, House # 22B, Road #7, Block # F, Banani, Dhaka-1213.
k.kabir@cgiar.org;
 

In (experimental) conditions where there is no natural food web, the optimal dietary digestible protein to digestible energy ratio (DP:DE) for tilapia ranges from 18 to 26 g.MJ−1. In pond culture, which valorizes the potential of the natural food web, increasing the C:N ratio stimulates the food web productivity. This study assessed if reducing the dietary DP:DE ratio (i.e., increasing C:N input in the pond) below the optimal DP:DE ratio affects the productivity of tilapia kept under semi-intensive conditions, the food web dynamics and nitrogen balances at the pond level. 12 ponds, each divided into three compartments, were assigned to test the effect of two diets, which differed in DP:DE ratio (17 vs. 13 g.MJ-1). Three feeding levels (no, low and high) were nested in each pond in a split plot design. Initial fish biomass was 1166 g.compartment-1 and the experiment lasted 60 days. Decreasing DP:DE ratio enhanced biomass gain and specific growth rate (P<0.05; 1195 vs. 986 g.compartment-1; and 1.76 vs 1.55 %.d-1). Body composition of tilapia was unaffected by diet and feeding level. Despite the difference in performance final fat content was equal at all treatments; being only 5%. Averaged over both diets, survival and feed conversion ratio increased with increasing feeding level (P<0.05). Diet composition did not alter any of water quality parameters and abundance and diversity related parameters of the food web. The total amount of N accumulated in tilapia N per pond was enhanced when dietary DP:DE ratio was decreased. The data on N gain and N balance at the pond level, suggested that the food web productivity was stimulated by the reduction in dietary DP:DE even below the assumed optimal levels reported in literature. It is hypothesized that the optimal dietary composition is dependent on the culture condition (i.e. extensive; semi-extensive versus intensive pond culture).