2. Materials and methods 2.1 Experimental site and pond facilities The experiment was carried out for a period of 120 days from June to September 2011, in nine experimental ponds have been located in nine villages in Kotalipara Upazilla under Gopalgonj distract. The ponds were same in size (30±0.15 dec.) and similar in shape and depth.
2.2 Pond preparation The ponds were drained out completely and aquatic weeds were removed manually. Liming was done in all ponds at the rate of 1 kg/decimal. Four days after liming the ponds were manured with cow dung at the rate of 2 kg/decimal. One week after liming the ponds were filled with water and fertilized with urea and TSP at the rate of 100 gm/decimal and 50 gm/decimal respectively. TSP was soaked overnight, then urea and TSP were dissolved together and spread manually on pond water surface at sunny day (10-11 am).
2.3 Collection of experimental fish and stocking All the tilapia fingerlings with mean initial weight of 10 ± 0.25 gm were collected from Genetic hatchery, Avaynagar Jessore and were stocked at a rate of 200/dec (49400/ha).
2.4 Experimental design The experiment was carried out with three treatments (T1, T2 and T3) each with three replications.
2.5 Feeding Fertilization was done weekly in the ponds of all treatments at the same rate (cow dung, 2 kg/dec; urea, 100g /dec and TSP, 50 g/dec). Supplemental feeds were given in treatment-2 (T2) (rice bran: mustard oilcake; 3:1) and in treatment-3 (T3) (commercial pellet floating feed Nourish containing 30% protein). The feed was applied at the rate of 10% of the body weight of fishes at the beginning of the experiment, then it was reduced to 7% after one month and 3% after two months. Feed was applied twice a day, half in the morning (9.00 am) and the rest in the afternoon (4.00 pm). The supplementary feeds, such as rice bran and mustard oilcake were given to the fish in dough form. For this purpose, mustard oilcake was soaked overnight, mixed with rice bran and small balls were made. The feeds were gently thrown over the ponds water on a particular site of the ponds regularly.
2.6 Sampling Five percent of the total fish were sampled fortnightly by a cast net to monitor the fish growth and to adjust feeding rates. The weight of fish during sampling was measured by using a portable digital balance.
2.7 Water quality parameters The water quality parameters such as air temperature, water temperature, dissolved oxygen (DO), water pH, soil pH, ammonia, transparency and total alkalinity were recorded fortnightly. The temperature and dissolved oxygen of the ponds were determined by a DO meter. Secchi disc visibility was measured using a Secchi disc at the time of water sampling. The water pH was recorded by a pH meter.
2.8 Statistical analysis Computer analysis of the data was done by using MS excel, SPSS (Statistical Package for Social Science) version-20. Significance was assigned at 0.05% level.
2.9 Economic analysis An economic analysis was carried to estimate the net profit from different treatments. The analysis was based on local market prices for harvested fish and all other items. The cost of leasing ponds was not included in the total cost. An additional 7.5% on total cost was included as operational cost. The net return was measured by deducting the gross cost from the gross return per decimal. The benefit-cost ratio was also measured as a ratio of net benefit to gross cost.