Description of the study area and experimental design: The experiment was conducted in a farmer’s pond under semiintensive rearing system in Shamgonj under Netrokona district for a period of 90 days from 1st June 2003 to 31st August 2003. A total of nine perennial ponds were divided under three treatments i.e. T1, T2 and T3 each having three replicates. In the experiment, four species were used. In every treatment the total stocking density was same (100/decimal) with different ratio of carp 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 area of each pond was 0.8 ha with an average water depth of 1.2 meters. All the ponds were more or less similar shape, size, basin confirmation 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 1160kg/acre. After 5 days of liming, urea and TSP were used in all of those ponds at the rate of 9.00 kg/acre and 6 kg/acre respectively.
Fingerling stocking: The experimental carp fry of same size group having average length and weight of 3.8 cm and 3.0 g respectively were collected from farmers having confirmed that the source of fry was BFRI (Bangladesh Fisheries Research Institute), freshwater station, Mymensingh. The carp fry were transferred within polythene bag supplied with O2. The fry of pangas was taken from the same farmer but the source was not the BFRI. The average length and weight of pangas fry were 6.3 cm 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 to 6% 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 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%.
Sampling: Sampling was done monthly in the morning at 9 Am to 10 AM. In each sampling, ten fish of each species from each pond were caught by cast net. Among these ten only three from each species were chosen for gut content analysis and the remaining fishes were released in the respective ponds after recording the weight and length. The weight was taken by ordinary balance and length was taken by measuring scale. Chosen fishes from each of the species were killed immediately by heating at the head and then bally were open carefully with the help of a scissor and knife. Only the anterior portion of the digestive tract of carps lying between the esophagus and the first major curve of the small intestine was taken instead of the entire gut. This was done because these fishes have not well-defined stomachs, and digestion is less advanced in this portion of digestive tract and representative food items mostly identifiable. McComish[4], Mckeehnime and Fenner[5] and Dewan)[6] have also adopted similar methods in their study. In case of pangs, the stomach was taken out. By this time sample was preserved in 10% buffered formalin for further study. Water samples were collected from a different depth of the pond for the qualitative and quantitative study of plankton. The collected water sample was 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. Water temperature was recorded on spot with the help of an ordinary thermometer graduated in a centigrade scale. An electronic pH meter was used to measure the pH of water. pH recorded by pH meter (Model-445, UK) on the spot at the same time.
Plankton counting and identification 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 .