Experimental design The experiment was conducted using a completely randomized block design into two treatments with three replications for each. Stocking density of prawn juvenile was same in both treatments. The differences between both treatments were in the management practices. The treatments were traditional and organic farming of prawn. The treatment of traditional farming is abbreviated as TT and the organic farming as TO. In treatment TO, formulated feed with the locally available ingredients was used, and maize flour and bamboo kanchi were provided for maintaining C:N ratio 20 and developing periphyton, respectively. But in treatment TT, commercially available prawn feed was applied, and maize flour and bamboo branches (kanchi) were not supplied. Rotenone, lime and fertilizer were used only during the preparation of ponds but no fertilizers or other types of chemicals or antibiotics were used in both treatments after stocking. Experimental site and pond preparation The experiment was carried out at the pond facilities of Fisheries Field Laboratory of the Faculty of Fisheries, Bangladesh Agricultural University (BAU), Mymensingh for a period of 90 days from 31 August to 28 November 2009. Six rectangular earthen ponds with an area of 130 m2 and an average depth of 1.2 m each were used for this research. The ponds were rain-fed, well exposed to prevailing sunlight and being used for research over last 20 years. All unwanted fishes were eradicated by rotenone application at the rate of 100 g pond-1. Lime (CaCO3) was applied to all ponds at the rate of 250 kg ha-1 on Day 1. On day 4, ponds were filled with groundwater from a deep tube-well. On day 6, 12 side shoots of bamboo (locally known as kanchi ) per m2 water surface area with a mean diameter of 2.8 cm were posted vertically into the bottom mud in TO ponds, excluding a 0.5 meter wide free perimeter. This resulted in an additional substrates surface area of 40 m2 for periphyton development equaling 60% of the pond surface area. On day 9, all ponds were fertilized with urea and triple super phosphate (TSP) at the rates of 100 and 100 kg ha-1, respectively. Ponds were left for 10 days postfertilization to allow plankton development in the water column and periphyton growth on substrates (treatment TO), and subsequently stocked with prawns. Stocking and pond management In all ponds, post larvae (PL) of freshwater prawn (individual weight 5.47 ± 0.02 g) procured from a nearby commercial hatchery was stocked according to the experimental design (2 prawns m -2). No fertilizers were used after stocking in both treatments. In case of treatment TO, a locally formulated and prepared pellet feed (2 mm) containing about 24% protein (% dry matter basis) with a C/N ratio close to 15:1 was used. The feed was applied considering the body weight of prawn at a daily feeding rate of 10% body weight at the start of experiment, and gradually reduced to 5% body weight at the end of the culture period. Feed was distributed evenly over the pond’s surface, twice daily at 7.00 to 18.00 hours. Individual weights of minimum 10% of initially stocked prawn in numbers were taken at monthly interval to estimate the biomass and adjust the feeding rate. Prawns were sampled using a cast net in both treatments but in treatment TO, it was done after eliminating some bamboo kanchi, which were re-positioned after the sampling. In treatment TO, locally purchased maize flour was used as carbohydrate source for manipulating the C/N ratio to 20:1. The preweighed maize flour was mixed with pond water in a beaker and evenly distributed over the pond surface directly after the application of feed at 07:00 and 18:00 hours. Determination of water quality parameters Water samples were collected using a horizontal water sampler from three locations of each pond. Water quality parameters, temperature (degree Celsius, thermometer), dissolved oxygen (HACH Sension 8), pH (CORNING 445 pH meter) and transparency (Secchi disc) were monitored in situ at 0900 h on a weekly interval. Total alkalinity (titrimetric method) and nutrients, NO2-N, NO3-N, NH3-N and PO4-P concentrations (HACH kit model DR 2010) were determined in the late morning (between 0900 to 1000 h) at monthly basis (APHA, 1992). Before nutrient analysis, water samples were filtered through a glass microfibre filter paper (Whatman GF/C, Whatman International, Maidstone, England) using a vacuum pressure air pump. The filter papers containing particulate matter were preserved in 10 ml of 90% acetone in a test tube and kept in refrigerator for 24 hours for subsequent chlorophyll a analysis. The papers in the test tube were ground by using a glass rod, centrifuged (Denlay centrifuge, model BS-400) for 10 minutes at 3,000 rpm and made ready for chlorophyll a analysis. Afterward, supernatant water was poured to a cuvette and chlorophyll a was determined using spectrophotometer (Spectronic Genesys 5, model 336001) at 664 and 750 nm wave lengths, following Boyd (1979). Assessment of plankton population Plankton samples were collected monthly by pooling 10 Liter of water from five locations in each pond and passing it through a plankton net (mesh size 45 µm). The concentrated samples were preserved in small plastic bottles with 5% buffered formalin. Plankton numbers were estimated using a SedgewickRafter (S-R) cell and was left to stand for 15 minutes to allow plankton to settle. Then, the planktons on 10 randomly selected fields of the chamber were counted under a binocular microscope (Swift, M-4000). Taxa were identified to genus level using keys from Bellinger (1992). Plankton abundance was calculated using the following formula (Azim et al., 2001): N = (P×C×100)/L
Where, N is the number of plankton cells or units per liter of original water; P, the number of plankton counted in 10 fields; C, the volume of final concentrate of the sample (ml); L, the volume (L) of the pond water sample. Economic analysis An economical analysis was performed to estimate the net return and benefit-cost ratio in both treatments. The following equation was used: R = I – (FC + VC + Ii) Where, R = net return, I = income from prawn and tilapia sale, FC = fixed/common costs, VC = variable costs and Ii = interest on inputs. The benefit cost ratio was determined by following equation: Benefit cost ratio (BCR) = Total net return / Total input cost. The wholesale prices per kg of large, medium and small prawn were TK. 450, 300 and 200, respectively. The prices of inputs and prawn correspond to the Mymensingh wholesale market prices in July to December 2009 and are expressed in Bangladeshi taka (1US$ = 70 BDT). Statistical analysis Independent samples T-Test was performed for comparing water quality parameters, plankton, periphyton and benthic macorinvertibrates data, and growth and production of prawn and tilapias as well as economics between both treatments. Survival and percent data were analyzed using arcsine-transformed data, but percent values were reported. The assumptions of normal distribution and homogeneity of variances were checked before analysis. All statistical tests were carried out at a 5% level of significance using SPSS (Statistical Package for Social Science) version 16.0