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A ten months investigation was conducted to know the status of bacterial flora in the farmed hybrid catfish (Clarias hybrid) and respective pond water from the fish pond of a commercial fish farm of Bangladesh. The average total load in pond water, slime and kidney of fish varied from 2.1×10 3 to 4.1×10 6 CFU/ml of water, 5.6×10 3 to 4.4×10 9 CFU/g of slime and 1.8×10 1 to 2.7×10 4 CFU/g of kidney. Coryneforms and Flavobacterium were the dominant bacteria in the observed fish-organs and pond-water. Higher bacterial load was recorded in the months of September and October. Sensitivity of the isolated Aeromonads and Pseudomonads to various antibacterial agents was determined. Higher percentage of resistant Aeromonads and Pseudomonads were observed against oxytetracycline followed by erythromycin. On the contrary, the percentage of resistant Aeromonads and Pseudomonads was lower against oxolinic acid.

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 À1 ) 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 À1 , 7.56-7.62, 34.81-35.41 cm, 0.88-0.96 mg L À1 , 0.014-0.021 mg L À1 and 66.23-69.76 mg L À1 , 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 À1 mono-sex tilapia can achieve better economic return.

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