2.1 The study area The study was conducted in Mongla, Sadar Upazilla of Bagerhat district and Dakop Upazilla of Khulna district throughout a production cycle from July 2014 to June 2015.Three semi-intensive farms S & M Fish Farm, bajlu Fish Farm and Gazi Fish Farm were randomly selected in each upazilla, which were categorized in three (T1, T2, T3) treatment. In each category (treatment) of farms, three ghers were considered as replicates. The Ghers ranged in size between 0.26 and 0.53 ha (mean 0.39 ha) and a mean depth of 1.7 m throughout the growing cycle. All the Ghers were similar in configuration, basin and contour type, well-exposed to sunlight and natural air flow.
2.2 Shrimp farming techniques and farm management The Farm owners were interviewed for detailed information on husbandry and management practices using FGD tools. Farm records were used to quantify the manure and fertilizers, supplemental feeds, shrimp harvests and to have information on the management practices applied and inputs used. Per hectare shrimp yield was calculated from the final biomass obtained in each individual Gher. Gher preparation began from mid-February to mid March with ploughing the enclosed land and encircling it with fence, which was left for about a week for drying under the sunshine. after one week of drying water was introduced by allowing the high tide of new moon or full moon to enter into the Ghers. Then Bleaching Powder (CaOCl) Cl was applied at the rate of 250 kg/ ha. After lime application the ghers were aerated by electric aerator and pulled horra up to 7 days. Then Molases, wheat bran or rice bran and yeast were mixed and heated and kept for two days then these mixture was applied at the rate of 80–100 kg/ha for plankton production. PCR tesed Shrimp fry (15–20 days old post larvae) were collected from hatchery and transfered into the rearing ghers. Stocking in the rearing ghers was done after plankton production. Then feeding starts and feeding frequencies was 4 times in a day. Aeration started after 2-3 hours of administered feed in the Gher. Bio-security and hygiene practices were maintained properly. The tiger shrimp, Penaeus monodon is the main target species for semiintensive farming. The amount of feed supplied was calculated based on shrimp biomass. The other forms of poststocking management included only periodic liming of the ponds as a measure of disease prevention.
2.3 Water quality parameters Water samples were collected twice in every month from the selected farms using 500 ml plastic bottles between 09:00 AM to 10:00 AM. After collection of the samples dissolved oxygen (mg /l) was measured immediately in the sampling site. Other parameters of the water samples such as pH, Alkalinity (mg/l), nitrite nitrogen (NO2–N) Ammonia (mg/l) and Iron (mg/l) were measured by using HACH test Kit (Model FF-1A Cat. 2430-02). A water temperature (0 C) and salinity (ppt), were recorded directly on the spot by a Celsius thermometer and a refractometer (Atago, japan).
2.4 Soil quality parameters After collection of soil samples from the selected ghers, samples were air dried and crushed without any granules. Then the tagged samples were sent to Soil Resource Development Institute (SRDI) Khulna for analysis of its parameters. Their analyses procedures are described below. Samples were air-dried and results were expressed as the total dry matter, T-DM (g/kg). Soil pH was determined from a soil suspension in distilled, de-ionized water (soil: water ratio of 1:5, using a digital pH-meter. Phosphate concentrations were determined by shaking the soil samples with 0.5 M NaHCO3 solution (pH 8.5). Phosphorus in the extract was determined by developing blue color using stannous chloride reduction of phospho-molybdate complex and measuring the color spectrophotometrically at 660 nm wavelength. All colorimetric examinations were done using standard calibration curves. Total-P (mg/100g) was measured by ascorbic acid method (APHA, 1998).To determine the total nitrogen, and sulfur, samples were oven-dried at 450 C for 2 h and crushed with a mortar. The total nitrogen was determined by using the Kjeldahl method To determine, sulfur, the samples were treated first with water and then with 6M HCl and the parameters were determined by using an elemental analyzer. The organic content of the soil (also called loss of ignition) was determined by combustion of samples in porcelain crucibles at 550 0C for 12h in a muffle furnace and the final product of the combustion was expressed as the ash content of the sediment.
Plankton study For Qualitative and quantitative estimation of plankton were samples were collected at fortnightly intervals from the ghers by passing depth integrated water samples through finemeshed plankton net (0.025 mm). After collection The samples were preserved immediately with 5% buffered formalin in the plastic bottles. Then Plankton density was estimated by using a sub-sampling technique, A Sedgwick– Rafter (S–R) cell was used under a calibrated binocular compound microscope for plankton counting. Plankton were identified to genera level and were counted using the formula proposed by] and was expressed as the number of cells per liter of water. Statistical analysis For all sampling techniques, three replicates were analyzed and means and standard deviations were calculated and expressed as mean (+SD). Significance of variations in the water quality parameters within ghers were tested using one way analysis of variance, ANOVA, which was followed by Duncan’s multiple range test (Duncan, 1995) for significant values. Significance of correlation coefficients was calculated according to Zar (1999). Values were considered at 5% level of significance.