Effect of dietary protein to energy ratio on performance of nile tilapia and food web enhancement in semi-intensive pond aquaculture

When fish have only access to formulated feed, the optimal dietary protein to energy ratio (P:E) for tilapia ranges between 18 and 23 g.MJ(-1). In pond culture, where natural foods complement administrated feed, increasing the carbon: nitrogen (C:N) ratio stimulates the natural food productivity. This study assessed if lowering the dietary P:E ratio (and thus increasing the C:N ratio of the feed input in the pond) below the optimal P:E ratio affects fish productivity, food web dynamics and nitrogen balances in semi-intensively managed tilapia ponds. Twelve ponds, each divided into three equally-sized compartments, were assigned to test the effect of two diets, which differed in P:E ratio (19 vs. 14 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 (+/- 16) g.compartment(-1) and the experiment lasted 60 days. Decreasing P:E ratio enhanced tilapia production and specific growth rate (P < .05; 1195 vs. 985 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 5% of body weight and unaffected by treatments. Averaged over both diets, survival and feed conversion ratio increased with increasing feeding level (P < .001). Diet composition did not alter measured water quality, and abundance and diversity of the related parameters of the food web. The total amount of N accumulated in the pond(-) was lower with the low P:E ratio diet (i.e., low protein diet). The data on N gain and N balance at the pond level suggest that the food web productivity was stimulated by reducing the dietary P:E ratio below the reported optimal levels in the literature. It is hypothesized that the optimal dietary P:E ratio is dependent on the culture intensity (extensive, semi-extensive or intensive pond culture).