Abstract
The impact of introduction of two small fish mola (Amblypharyngodon mola) and punti (Puntius sophore) in carp polyculture system was studied at both on-station in the Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University (BAU) and on-farm in the farmers pond from June 2000 to November 2002. An indoor experiment was also carried out to evaluate the plankton cleaning rate as well as nature and extent of food competition between the two small fishes.Though there was no significant difference in water quality parameters, plankton population and production of fish, the overall highest total fish production (2862 kg/ha/142 days) was obtained from the treatment where punti and mola (25,000 punti+25,000 mola=50,000 small fish/ha) were stocked together at higher densities, followed by the treatment where punti and mola (12,500 punti+12,500mola=25,000 small fish/ha) were stocked together at lower densities. The production was minimum when only mola (25,000/ha) was stocked with carps. When only punti (25,000/ha) was stocked with carps, production was lower than the treatments where punti and mola were stocked together. Production was higher than the treatment where only mola was stocked with carps and the control (2405 kg/ha/142 days), where only carps were stocked.A parallel polyculture experiment was carried out in 25 rural ponds; stocking characteristics of small fish was same but major carp combination was slightly different than on station. Here catla was partially replaced by silver carp and grass carp as per farmers' choice. The harvested biomass of punti and total small fish obtained from the ponds in which twice the number of small fish had been stocked was double that of other treatments. In contrast the harvested biomass of mola was not significantly different in ponds stocked with 12500 and 25000 mola/ha. Individual growth rate, weight, survival, biomass and yield as well as total harvesting biomass and yield of large carp were not affected by the addition of small fish.Next experiment, the polyculture of carps with different proportion of punti and mola was carried out in the field laboratory; where stocking density of major carps i.e. catla, rohu and common carp were kept at 9,900/ha with a ratio of 1:1:1 in each of the four treatments. Stocking density of punti and mola were 30,000/ha at 1:2, 1:1 and 2:1 ratio in three treatments PMM, PM and PPM, respectively; no small fish was stocked in control. No significant difference in water quality parameters, plankton population and production of fish was observed, but the best production was found in treatment PPM where punti and mola were stocked in 2:1 ratio.A parallel experiment was again carried out in 28 rural ponds, with the objectives of comparing the outputs of on-station and on-farm trials and increasing the farmers' understanding through their participation. Formal and informal training of farmers were also carried out to improve their understanding. The production of carps in control was higher than its production in any treatment where small fish were added at any ratio. The addition of 300 small fish/100 m² pond marginally significantly.