2.1 Duration and location of the study The experiment was carried out for a period of four months (March 2009 to June 2009) in irrigated rice fields at Bijoynagar of Godagari Upazila under Rajshahi district, Bangladesh. The study site belonged to a typical drought prone Barind area.
2.2 Experimental Design The experiment was conducted in a randomized complete block design (RCBD) with three treatments and three replications for each treatment. The treatments were: (T1) rice combined with 4940 individuals/ha of tilapia (Oreochromis niloticus), (T2) rice combined with 4940 individuals/ha of Thai sarpunti (Barbodes gonionotus) and (T3) rice combined with 4940 individuals/ha of carpio (Cyprinus carpio).
2.3 Field Management Experimental rice plots (mean area of 0.14 ha) were more or less similar in size. The fields were ploughed with the help of a power tiller and then leveled properly by laddering to keep equal water depth throughout the field. Dyke around the land was constructed at the height of 25 cm. A small ditch (1.5 m x 1.5 m x 1 m) was constructed at the lower part of the field to provide refuge during high water temperature and low water depth. All the experimental plots were prepared well and fertilized with urea, triple super phosphate (TSP) and murate of potash (MP) at the rate of 200 kg/ha, 150 kg/ha and 75 kg/ha, respectively as recommended by BRRI (1999). The present experiment was conducted during Boro season. High yielding variety (HYV) rice BR-28 was selected for this experiment. Alternate row spacing of 35 cm and 15 cm was followed for transplanting rice seedling following Hossain et al. The plant to plant distance was 20 cm. The alternate row spacing would provide enough space for easy movement of fish and allow adequate sunlight to ensure enough biological production.
2.4 Stocking and management of fish Fifteen days after transplantation of rice seedlings, fields under different treatments were stocked with fish fingerlings. All the treatments of rice-fish system belonged to a similar stocking density of 4940 fish ha-1. The mean initial weight of tilapia, sharpunti and carpio were 20.30, 20.35 and 20.80 g, respectively. Fishes were grown for a period of 120 days in the plots and rice was harvested just after harvesting the fishes.
2.5 Chemical analysis of soil To determine the nutrient concentration of soil, the soil samples were collected at a depth of 0-15 cm from the surface of each treatment. The samples were collected in two installments: first before rice transplantation and finally, after the harvest of rice and fish then the samples were air-dried and ground to pass through 2 mm sieve. Then the parameters of soil viz., pH, organic matter and total nitrogen were analyzed by following standards methods outlined by Jackson, Black and Page.
2.6 Water quality parameters During the study period, the status of physicochemical parameters of water like temperature, transparency, dissolved oxygen, pH, alkalinity, ammonia, nitrate-nitrogen, phosphatephosphorus were recorded fortnightly. Water temperature, transparency, dissolved oxygen and water pH were recorded directly in the field with the help of a Celsius thermometer, secchi disc, a digital electronic DO meter (YSI, Model 58) and an electrical pH meter (JENWAY, Model 3020). The concentration of ammonia-nitrogen (mg L-1), total alkalinity (mg L-1) and free carbon dioxide (mg L-1) of water samples were determined by using a HACH kit (FF-2, USA). The plankton were identified up to genus level and enumerated by the following.
2.9 Statistical Analysis Data related on water and soil quality parameters, rice and fish production, and economics under different treatments were subjected to one way ANOVA (Analysis of Variance) using computer software SPSS (Statistical Package for Social Science, version-20). The mean values were also compared to see the significant difference from the Duncan Multiple Range Test.