Abstract
The potential of inclusion of freshwater prawn (Macrobrachium rosenbergii) and mola (Amblypharyngodon mola) in rice-fish culture systems was evaluated through four on farm trials in Mymensingh area of the central Bangladesh from 2005 to 2006 with a view to develop a sustainable rice-fish culture system to ensure nutritional security and economic benefit of the rural people. Effects of different densities of freshwater prawn on mola in concurrent method of rice-fish culture systems were investigated in the first experiment. The stocking density of mola was 20,000 ha1 in all treatments and four densities of freshwater prawn were maintained at 10,000, 15,000, 20,000 and 25,000 ha', respectively in treatment 1 (D-10), treatment 2 (D-15), treatment 3 (D-20) and treatment 4 (D-25) in triplicates. Water quality parameters and plankton abundance were monitored fortnightly. The results of four months culture period showed that the average production of prawn ranged from 222-388 kg ha, mola 51-68 kg ha1 and rice 2,880-3,710 kg ha1 in four treatments. Both prawn and mola had better production in treatment D-15, where the stocking density of prawn was 15,000 ha'. In the second experiment the effects of different stocking densities of freshwater prawn on its growth and production in relation to the presence of small indigenous species mola were evaluated in modified rice fields after rice harvest. There were four treatments with three replications in each. Keeping the stocking density of mola fixed at 20,000 ha' in each treatment, four densities of freshwater prawn (treatments) were maintained at 10,000, 15,000, 20,000 and 25,000 ha ̈1, respectively. All water quality parameters were found within the suitable ranges for freshwater prawn culture except high temperature in August and September. There was a homogenous abundance of plankton communities in all treatments throughout the experimental period. Total production of mola ranged between 124 and 152 kg ha' during the four months culture period. The average prawn survival ranged from 49-57% without any significant difference among treatments. Freshwater prawn production ranged from 294 to 596 kg ha' with significantly higher production (P<0.05) in 3rd treatment where 20,000 ha1 freshwater prawn was stocked. This treatment also resulted in higher net profit margin (74%) indicating that stocking at a combination of 20,000 ha' freshwater prawn and 20,000 ha mola may be the optimum densities for prawn - mola culture in the modified rain-fed rice fields after rice harvested. The third trial was carried out to evaluate the growth and production performance of tilapia (Oreochromis niloticus) and catla (Catla cata) with freshwater prawn and mola in farmers' rice fields by alternate method of rice-fish culture systems. Mola and freshwater prawn were stocked at 20,000 ha in all the treatments. Catla was stocked in treatment-I (Cpm) at the rate of 2,500 ha, tilapia was stocked in treatment-II (Tpm) at the rate of 2,500 ha1 and both catla and tilapia were stocked in treatment-III (CTpm) with same stocking density of 1,250 ha' each. The highest total average production of 2,142 kg ha' was observed in treatment Cpm and the highest average production of prawn 730 kg ha1 was also found in the same treatment. Treatment CTpm showed comparatively better production of 1,526 kg ha1 than treatment Tpm with 1,295 kg ha'. Mola showed better production when both tilapia and catla were stocked with mola and prawn. Higher benefit-cost ratio (2.30:1) was found in treatment Cpm followed by treatment CTpm (1.99:1). Gender effects of freshwater prawn in polyculture with major carps, catla and rohu (Labeo rohita) and small fish mola was evaluated in the fallow rice fields in the fourth experiment. All male prawn with carps and mola were stocked in treatment-I (MP), mixed sex of prawn with the said species stocked in treatment-II (MFP) and all female prawn with the said species stocked in treatment-III (FP). Freshwater prawn, mola, catla and rohu were stocked, respectively at 20,000 ha', 20,000 ha', 1,750 ha1 and 750 ha in all treatments. All water quality parameters were found within the suitable ranges for fish-prawn polyculture. The highest production of prawn 697 kg ha' was obtained in treatment MP, which was 34.7% and 56.2% higher than treatments MFP and FP, respectively. Catla and mola showed better production in treatment MP than other two treatments. Production of rohu was almost similar in all treatments. Higher total production of all fish and prawn (1,619 kg ha'') and higher benefit-cost ratio (2.10:1) were observed in treatment MP, where all-male prawns were stocked.