Study Area: The experiment was conducted in 6 selected shrimp ghers (ponds) of Shymnagar thana under Satkhira district for a total period of 6 months.
Experimental Ponds: The ponds, located on the banks of the Munshiganj and Kadamtoli rivers in the district of Satkhira, had both inlets with feeder canals and outlets on the opposite direction. Water was pumped from the feeder canal to ponds through axial pumps and discharged by gravitational forces through PVC pipes laid at the bottom of the ponds and taken across the embankments. The ponds were designed as P1 through to P6. Ponds P1 and P2 were used for treatment-1 (T1) and considered as the controls where no feed and fertilizers were used. Ponds P3 and P4 were used for treatment-2 (T2) where only fertilizer was used, while those designated as P5 and P6 were used for treatment-3 (T3), where both feed and fertilizers were used. The areas of the experimental ponds are shown in Table 1. All the ponds were rectangular in shape. The dikes were firm and high enough to hold 1 m deep water throughout the study period.
Pond Preparation: The ponds were sun dried and treated with stone lime (CaCO3) at the rate of 200 kg/ha. After complete sun drying, water was allowed to enter into the ponds during high tide through fine meshed net to prevent the entrance of undesirable organisms. After 2 to 3 days of watering, fertilizers at the rate of urea 40 kg/ha and TSP 20 kg/ha were used and allowed to grow sufficient natural feed for the next 7 days, until the ponds were ready for stocking post larvae (PL) of Penaeus monodon.
Stocking of PL: Fingerlings collected from natural sources with the mean initial weight of 0.04g (±0.01) were kept for 5 to 10 days in the pre-stocking tanks (locally called Goi) made by the side of the inlets for acclimation before releasing them to the stocking ponds. The stocking density in different ponds varied from 20,000 to 33,000 per hectare.
Feeding: Pelleted diet (Grower, Saudi-Bangla Fish Feed Ltd.) containing 36% crude protein level was used in T3 from the third month. Initially feeding rate was 0.5% of the body weight of the shrimp, which was increased to 1% for the last month of the experimental period. Feed was given twice daily before sunrise and after sunset, and the amount given was recorded for subsequent calculation of growth parameters.
Management of Water Quality: Efforts were made to keep the water quality of the pond as good as possible, since in most cases shrimp disease are caused by poor water quality management. No rigid schedule for exchange of water could be maintained, as it was dependent on the tidal condition; however, about 25% to 30% of the total water was exchanged every day. The pH was always maintained at or above 7.3, and seechi disc readings were kept within 30 to 35 cm by applying stone lime whenever necessary at the rate mentioned earlier. Temperature, salinity, DO, CO2 and pH was monitored fortnightly by using a Centigrade thermometer, hand refractometer, DO meter and a combined digital CO2/pH meter, respectively.
Measurement of Shrimp Growth: Random sampling was done fortnightly to monitor the growth of the shrimp. Sampling was done in the early morning to avoid the stress from the sunlight. Bulk weight was taken with ordinary balance and the amount of feed to be given was adjusted accordingly. At the end of the culture period, the shrimps were harvested by completely draining out the ponds. Depending on the size of the ponds complete harvesting took 3 to 4 days.