2.1 Study area and duration The experiment was conducted in farmer’s ponds of Baraigoan under Trishal Upazila, Mymensingh for a period of 5 months from March 2018 to July 2018. A total of six perennial ponds were divided under two treatments i.e. T1 and T2 each having three replicates.
2.2 Pond preparation, Fingerlings stocking and feeding The ponds were rectangular in shape and the surface area of each pond was 0.041 ha (10 decimal). Each pond had inlet and outlet to enter and out of water when needed. Before starting the experiment the ponds were dried and freed from aquatic vegetation. After drying, liming (CaO) was done in all the ponds at the rate of 1 kg/decimal. Ponds were then filled with ground water at a depth of about 1.37 meter. Seven days after liming, all the ponds were manured with cow dung at the rate of 2470 kg/ha, Urea and TSP were used in all of those ponds at the rate of 37 kg/ha and 18.5 kg/ha, respectively. Soon after the appearance of light-plankton bloom, all the ponds were stocked with experimental fishes. Two treatments were tried in this experiment. In treatment-1 (T1), fingerlings of rohu (Labeo rohita), catla (Catla catla) and mirror carp (Cyprinus carpio var. specularis) were stocked. For treatment-2 (T2), equal number and ratio of rohu, catla and mirror carp fingerlings and as additional species of silver barb (Barbonymus gonionotus) fingerlings were stocked as per experimental design.
For the proper growth of fishes, commercially available mega fish feed was applied throughout the experimental period @ of 7 to 3% of their body weight. The feeding was adjusted on the basis of monthly sampling of fishes. In each sampling, ten fish of each species from each pond were caught by cast net. The weight was taken by ordinary balance.
2.3 Physico-chemical and Biological parameters The water quality parameters of pond water such as water temperature, transparency, dissolved oxygen (DO) and pH were monitored fortnightly throughout the experimental period between 09.00 and 10.00 hr. Temperature (0C) and dissolved oxygen (mg/l) were determined directly by a digital water quality analyzer Hanna DO meter (Model-HI 9146, Romania), pH by a digital pH-meter (Milwaukee pH meter, Model-PH55/PH56, USA), and transparency (cm) by a secchi disc. Plankton samples were collected fortnightly from experimental ponds to study the qualitative and quantitative estimates of plankton. For this purpose, 10 litre samples of pond water were collected from 1 meter depth of surface water of different areas and filtered through a fine mesh (25 mm) phytoplankton net. Filtered sample was taken into a measuring cylinder and carefully made up to standard volume with distilled water. Using a plastic tubing, water was siphoned off from the measuring cylinder and plankton were concentrated into 50 ml and preserved using 5% buffered formalin in small plastic vials for subsequent studies. From each 10 ml preserved sample, 1 ml subsample was examined using Sedgwick-Rafter cell (S-R cell) under binocular microscope. All the plankton present in 10 squares of the cell chosen randomly were counted and used for quantitative estimation by the following standard method of Rahman.
2.4 Harvesting of fish At the end of the experiment the water of the ponds was pumped out and all the fishes were harvested. After harvesting all fish were counted and weighed individually for each pond to assess the growth in weight (g) by using a balance.
2.5.1 Fish Production At the end of the experiment, all fish were harvested and the following parameters were used to determine the production of fishes at the experimental ponds. Individual stocking weight (g) Individual fish weight was taken using electric balance as gram (g) during stocking. Individual final weight (g) It was taken at the time of harvest and was expressed as gram (g). Weight gain (g) = Mean final weight (g) - Mean initial weight (g) Statistical analysis The collected data of the experiment were analyzed by individual sample t-test. Statistical significance was assessed using a probability level of P= 0.05