Performance of mono-sex tilapia (Oreochromis niloticus) in rice field with different ditch size

The effects of ditch size on growth and production of mono-sex tilapia, Oreochromis niloticus in rain fed concurrent rice–fish system were technically and economically evaluated for a period of 4 months. Three different ditch sizes were tried: 5%, 10% and 15% of the total cultivable rice field. The fish fingerlings stocking density (5000 ha−1) and management practices were similar in all plots. The water temperature, dissolve oxygen, pH, Secchi disc visibility, ortho-phosphate, ammonia nitrogen and alkalinity were found to be in the range of 31.5–31.9°C, 5.10–5.27 mg L−1, 7.56–7.62, 34.81–35.41 cm, 0.88–0.96 mg L−1, 0.014–0.021 mg L−1 and 66.23–69.76 mg L−1, respectively, and all the values were found suitable for tilapia fish farming. Sediment organic matter, available nitrogen and phosphorous were increased significantly (P < 0.05) in all plots after cultivating the fish. Larger ditch size (15% of the total land area) resulted in significantly (P < 0.05) higher specific growth rate, survival and fish production compared with the smaller (5%) and medium (10%) ditch sizes. However, significantly (P < 0.05) higher net income and benefit–cost ratio were obtained in medium ditch size with the values of 2.6% and 3.6% higher than the smaller and larger ditch sizes respectively. It is concluded that rice–fish farming in a rain fed ecosystem of Bangladesh with medium ditch size and stocking density of 5000 ha−1 mono-sex tilapia can achieve better economic return.