Description of the study area and experimental design The experiment was conducted in farmer’s pond under semi-intensive rearing system in Shamgonj under Netrokona district for a period of 90 days 1st June 2003 to 1st September 2003. A total of twelve perennial ponds were divided under three treatments i.e. T1, T2 and T3 each having four replicates. Four species were selected for the experiment and stocked the density of 100/decimal with different ratio of carps and pangas. In case of carp only three Indian major carps were used viz rohu (Labeo rohita), catla (Catla catla), and mrigal (Cirrhinus mrigala). The ratios of rohu, catla, mrigal and pangas in the three treatments were at 35:17.5:17.5:30 (T1), 30:15:15:40 (T2) and 25:12.5:12.5:50 (T3), respectively.
Pond preparation The average size of the ponds varied from 0.8 acre to 1.0 acre with an average depth of water 1.2 m to 1.5 m. All the ponds were more or less similar shape, size, basin conformation and bottom type. The ponds were flood free rain fed, free from aquatic vegetation and well exposed to sunlight. Each ponds have inlet and outlet to provide water and when needed. After selection, at first broken dikes and holes of all ponds were repaired. After that, all kinds of aquatic vegetation (floating, emergent, submerged and spreading) were removed manually and the branches of all trees on the ponds were trimmed off. The predatory and undesirable fishes were eradicated by netting repeatedly and cleaned by poisoning with rotenone at the rate of 5 ppm. Liming was done immediately after poisoning at the rate of 100.0 kg/acre. Three days after liming, all the ponds were manured with cow dung at the rate of 1000kg/acre. After 5 days of liming, urea and TSP were used in all of those ponds at the rate of 15.0 kg/acre and 7.5 kg/acre, respectively.
Fingerling stocking Fingerlings were stocked 7 days after application of fertilizer when light plankton bloom was appeared. The experimental carp fry of same size group having average length and weight of 3.14, catla 4.20 and mrigal 3.60 g, respectively were collected from farmers having confirmed that the source of fry was BFRI (Bangladesh Fisheries Research Institute), freshwater station, Mymensingh. Transportation of fry was accomplished in polyethylene bags with oxygen to avoid stress and injury. The fry of pangas was taken from the same farmer but the source was not the BFRI as they don’t produce them. The average length and weight of pangas fry were 2.5 inch and 6.69 g respectively. All the fry were acclimatized with experimental pond water in polythene bag and then stocked at 4.00 PM.
Supplementary feeding Throughout the experiment for the proper growth of fishes supplementary feed was given to pangas at the rate of 8% of their body weight. The feeding was adjusted on the basis of monthly take fish weight. The feed was supplied in the dough form and feeding was done directly without any feeding trays. Half of the ration was supplied at 9.00 AM and the remaining half was supplied at 4.00 PM. The composition of experimental feed was wheat bran 20%, rice bran 30%, mustard oil cake 20%, meat and bone meal 20%, wheat 9%, vitamin premix and mineral 1%.
The water quality parameters of pond water such as temperature, pH and dissolved oxygen was measured by a portable DO meter (Lurton, Model 5510) and recorded at the monthly interval in the morning at 9 Am to 10 AM. The temperature was measured by a Celsius thermometer and pH was recorded with the help of a pH meter (Model-445, UK). Plankton abundance in the experimental ponds was also estimated monthly intervals. Ten litters of water samples were collected from different depth of the pond and preserved with 10% buffered formalin for 7 days to settle down the plankton at the bottom and then concentrated by draining out the upper portion of the sample through siphoning to 15 ml in plastic vials for subsequent studies. For the quantitative study of plankton, a drop of the concentrated plankton sample was taken by a dropper and then on the Lund chamber, a simple counting chamber for nanoplankton. After pouring the sample, the counting chamber was covered with a coverslip so as to eliminate the air bubbles and left to stand for a few minutes to allow the plankton to settle down. Then counting chamber was placed under an electric binocular microscope and the plankton was counted. The mean number of plankton was recorded and expressed numerically per liter of water of each pond. The qualitative analysis of plankton was done according to Ward and Whipple, Prescott and Bellinger.
Economic analysis: Economic analysis was done to estimate the net profit from the carp-pangas polyculture system. The approximate cost of each diet was calculated on the basis of Mymensingh local market price (2003) of the entire ingredient used. The cost of leasing ponds was not included in the total cost. The cost of different dietary ingredients, cost per kg diet and cost of different fingerlings used in the experiment has been presented.
Statistical analysis The data obtained from different treatments were tabulated and analyzed by using a statistical package program by one way ANOVA and then significant variations of mean values of growth, survival and yield among the treatments were confirmed by Tukey test following the key given by Zar.