Abstract
The effects of biotechnical management of all-male freshwater prawn on pond ecology and production in polyculture systems were studied at the Fisheries Field Laboratory, Bangladesh Agricultural University, Mymensingh during 2007 and 2008. In the first experiment, the effects of selective harvesting (SH) and claw ablation (CA) of blue-clawed prawns on water quality and production in all-male freshwater prawn-fish polyculture ponds were compared with control (Co) in quadruplicate. Male prawn Macrobrachium rosenbergii (mean individual weight 5.58 ± 0.86 g), silver carp Hypophthalmichthys molitrix (17.93 ± 3.53 g), catla Catla catla (23.07 ± 4.00 g) and small fish mola Amblypharyngodon mola (0.52 ± 0.08 g), respectively were stocked at the densities of 12,000, 2,000, 500 and 20,000 ha⁻¹ in all ponds. Prawns were fed daily with pelleted feed and ponds were fertilized regularly with urea, TSP and cow-dung. Selective harvesting and claw ablation of blue-clawed prawns in respective treatments started on day 60 and continued at 15-day intervals during 137 days culture period until final harvest. Water quality parameters and plankton abundance did not differ significantly (P>0.05) among the treatments. The treatment SH resulted in significantly higher (P<0.05) net production of freshwater prawn (437 kg ha⁻¹) with higher survival and mean weight followed by CA (354 kg ha⁻¹) and Co (322 kg ha⁻¹). The combined net production of prawn and fish was also higher in SH (1,244 kg ha⁻¹) compared to CA (1,161 kg ha⁻¹) and Co (1,137 kg ha⁻¹), although the fish production did not differ significantly among the treatments. The periodic selective harvesting (SH) of blue-clawed prawns showed better economic return with a BCR of 1.71. In the second experiment, the effects of three different combinations of stocking density of catla and silver carp on pond ecology and production in all-male freshwater prawn-fish polyculture system were evaluated in triplicate. Stocking densities of silver carp (25.03 ± 3.83 g) and catla (22.26 ± 4.10 g), respectively were maintained at 2,000 and 500 ha⁻¹ in treatment SC₂₀₀₀C₅₀₀, 1,500 and 1,000 ha⁻¹ in treatment SC₁₅₀₀C₁₀₀₀ and 1,000 and 1,500 ha⁻¹ in treatment SC₁₀₀₀C₁₅₀₀, while the densities for all-male prawn (5.16 ± 0.59 g) and mola (1.34 ± 1.03 g) were fixed in all the treatments at 12,000 and 20,000 ha⁻¹, respectively. Management practices were same for all the treatments. During 122 days culture period, blue-clawed prawns were harvested twice at 15-day intervals before the final harvest. Plankton and macro-benthos abundance and water quality parameters (except transparency and chlorophyll a) did not vary significantly among the treatments. Mean weight of silver carp at harvest was significantly lower in treatment SC₂₀₀₀C₅₀₀ (174.1 g) and that of catla in SC₁₀₀₀C₁₅₀₀ (129.2 g). Treatment SC₁₅₀₀C₁₀₀₀ resulted in significantly higher net production of silver carp and catla (461 kg ha⁻¹). This treatment also yielded significantly higher combined net production of all the species (874 kg ha⁻¹), even though the differences in production of prawn and mola among the treatments were not significant. In the third experiment, the effects of prawn density on pond ecology, production and economics in all-male freshwater prawn polyculture system were studied. Three treatments named according to the prawn densities as MP₁₅₀₀₀, MP₂₀₀₀₀ and MP₂₅₀₀₀ were stocked in triplicate with male prawn juveniles (5.33 ± 0.72 g) at 15,000, 20,000 and 25,000 ha⁻¹, respectively. Silver carp (18.74 ± 1.64 g), catla (15.85 ± 1.74 g) and mola (1.15 ± 0.67 g) densities were fixed in all treatments at 1,500, 1,000 and 20,000 ha⁻¹, respectively. Management practices were same for all the treatments. Blue-clawed prawns were harvested four times at 15-day intervals before the final harvest during 152 days culture period. Water quality parameters, plankton and macro-benthos abundance did not differ significantly among the treatments. The mean weight of prawn decreased significantly with increasing density. However, the net production of prawn was higher in treatments MP₂₅₀₀₀ (661 kg ha⁻¹) and MP₂₀₀₀₀ (640 kg ha⁻¹) than in MP₁₅₀₀₀ (535 kg ha⁻¹). Silver carp and catla production did not differ significantly among the treatments, while mola performed better with higher net production in MP₂₀₀₀₀ (125 kg ha⁻¹). Treatments MP₂₀₀₀₀ and MP₂₅₀₀₀ resulted in significantly higher combined net productions of all the species (1,688 kg and 1,608 kg ha⁻¹, respectively) than that of MP₁₅₀₀₀ (1,501 kg ha⁻¹). The treatment MP₂₀₀₀₀ fetched higher profit (US$ 2,597 ha⁻¹) with a BCR of 1.79. All in all, the study revealed that the periodic selective harvesting of blue-clawed prawns is a suitable biotechnical management strategy for minimizing the size heterogeneity of male prawns in favour of enhancing production and profit. Applying this management strategy, a polyculture of all-male prawn with silver carp, catla and mola at the stocking densities of 20,000, 1,500, 1,000 and 20,000 ha⁻¹, respectively was found to provide an optimum and sustainable production with higher profit.