Effects of urea fertilization on nutritional value of natural food and digestibility for tilapia Oreochromis niloticus

The effects of urea fertilization on nutritional value of the food sources and the digestibility of nutrients were evaluated for the tilapia Oreochromis niloticus. Three treatments of urea (16, 33 and 50 g/are/days, corresponding respectively to T16, T33 and T50), compared to a control without urea (T0) have been tested during one month in plastic tanks (1 m3), in witch were stocked fingerlings of O. niloticus. The food sources in tanks include suspended solids and the sediment. Estimation of the total weight shows a positive effect of the urea on the increasing of the suspended solids biomass, with an organic enrichment (82 and 71%) and proteins/Energy ratio (12 and 13 mg/kJ) enhancement for the resource from treatments T33 and T50. The effect of the urea on the quality of the sedimenting resource seems on the other hand not very perceptible; the organic fraction representing no less than 25% of the total biomass. The chemical analysis indicates that the food ingested by the fishes contains less 10% of fibers (HROM) and so on the percentages of total organic matter (AFDW) in the stomach increase linearly with the level of treatment. This qualitative enhancement also expresses in terms of proteins contribution (20 and 23%), with high Proteins/Energy (15 and 17 mg/kJ) ratios and better digestibility of organic matter (45 and 57%) and proteins (66 and 71%) for food ingested by the fishes of treatments T33 and T50. However, the high concentrations of ammonia measured in these treatments seem to be damaging to the survival of fishes.