The experiment was conducted in the experimental pond of Sher-e-Bangla Agricultural University, Dhaka, Bangladesh has an area of 3 acres with an average depth of 2m. The pond was rectangular in shape, exposed to available sunlight and free from aquatic vegetation. The study period of the trial was 144 days from 26 June to 22 November 2014.
2.1. Design of the experiment The study was performed in completely randomized design with a total of three treatments with triplicate where three different densities were used in the experiments which were 100 fish m-3 (T1), 200 fish m-3 (T2) and 300 fish m-3 (T3). Twelve square shaped cages having an area of 1m³ were constructed to conduct this experiment. The frame of cages was made of a bamboo pole which was covered by black nylon net with the help of nylon twine. The mesh size of the net was 1 cm which did not allow the experimental fish fry to escape but allowed water to pass easily. An opening was kept on the upper side of the cage to supply feed and handling of the fishes whenever necessary. A floating feeding frame and a plastic sheet were set inside cages that prevented the passing out of feed outside cages (i.e. misuse of feeds). The cages were tied using nylon rope to the bamboo platform so that they remain fixed near to the platform. This platform was used to supply feed easily and monitoring the fish of the cages. Plastic bottle floats were used for suspension of the cages.
2.2. Fry collection and stocking Fries of stinging catfish were collected from a nursery farmer in Shomvuganj upazilla under Mymensingh district, Bangladesh. The fish fries were carried by oxygenated polyethelene bag to the experimental site. Then the fries were kept into a hapa for one week for conditioning. On 26th June 2014, the fries were released into the cages randomly according to the experimental design without considering male and female ratio. 2.3. Feeding of fish In order to meet the dietary requirement artificial commercial floating feed (30% protein) were provided to the fries twice daily. Initially the feeding rate was 20% of their body weight (Up to 3-10g body weight) then decreased to 10% (Up to 3- 10g body weight). Feed ration was adjusted forth-nightly by sampling the weight and considering the mortality. To confirm good water exchange and aeration into the cages were lifted and checked during sampling to clean the nets. In the experimental pond, no extra feed and fertilizer were applied.
2.4. Monitoring water quality Water quality parameters were measured monthly in the morning at 8 am. A portable Celsius thermometer (Digithermo WT-2), DO meter (Lutron, DO-5509) and pH meter (Eco Test r pH2) were used to determine temperature, dissolved Oxygen, and hydrogen ion concentrations, respectively. The ammonia-nitrogen concentrations of the caged water were determined by using HANNA instrument Test Kit.
2.5. Growth, Yield, and economic analysis At the end of the experiment, on 22nd November 2014 the fish were harvested by taking out cages from the pond. The harvested fish were counted and weighed to determine the mortality and production, respectively. The final average weight (g) of the individual fish in each cage was estimated by dividing the total final biomass in the cage by the number of survivors. Total weight increments (net yield) were estimated by deducting the biomass stocked from the biomass harvested and was expressed as kg m-3 . The specific growth rate (SGR, % day-1 ) was calculated as (lnW2—lnW1)/ t × 100, where W1 is the initial live body weight (g), W2 is the final live body weight (g), and t is the time in days. Survival rate was calculated as: (no. of fish harvested/ no. of fish stocked) ×100. Weight gain was calculated based on mean weight increments during the rearing period of 144 days. The feed conversion ratio (FCR) was calculated as feed consumption/weight gain. To choose the best result, Benefit Cost Ratio (BCR) was conducted to determine the economic returns of different treatments based on the sale price of the fish after harvest and the costs of fingerlings and feed. The fish fries and pelleted feed were purchased at the rate of Tk 2.00/piece and Tk 40.00/Kg, respectively (1USD=72Tk). The market price of adult stinging cat fish was considered as Tk 600.00/Kg in this analysis. The formula used for determining Benefit-Cost Ratio (BCR) was as below: BCR: Gross income / Total cost.
2.6 Statistical analysis All the collected data were analyzed statistically by one-way ANOVA using statistical software Statistix 10. Means were given with ± standard error (SE). This analysis was then followed by a Tukey-HSD test where significant differences were observed. Differences were considered significant when P>0.05.