The Effects of Fertilization on Growth and Production of Nile Tilapia in Rain-Fed Ponds

Semi-intensive production of Nile tilapia Oreochromis niloticus commonly utilizes organic and inorganic fertilizers to increase primary production and fish yield. Most experiments on such systems are done at fish culture stations where water supplies are readily available, and water loss through evaporation or by seepage is replaced regularly, often weekly. Rainfall during such experiments may also flush the ponds, reducing nutrient concentrations and improving water quality. In such systems, regular fertilization at high input levels can result in yields approaching 3,500 kg/ha over 5 months, or extrapolated yields of up to 10,000 kg · ha-1 · yr -1 (Knud-Hansen et al. 1993, Diana 1996). Optimal fertilization rates in Thailand determined to be 28 kg N · ha-1 · wk-1 and an N:P ratio of 4:1 (KnudHansen et al., 1993). The fertilization schedule was weekly in these ponds, where combinations of chicken manure and urea or triple super phosphate were used for inputs. Knud-Hansen et al. (1993) believed that inorganic fertilizer produced higher yields than manures at the same loading rates. However, Diana et al. (1994) found reductions in alkalinity in some ponds fertilized with inorganic fertilizers alone which they believed were due to carbon extraction for photosynthesis. No declines in alkalinity were noted in ponds fertilized with manure and inorganic fertilizer or ponds receiving inorganic fertilizer and feed.