2.1 Site of the experiment The present experiment was conducted in Brackishwater Station of Bangladesh Fisheries Research Institute (BFRI) situated in Paikgacha sub-district (upazila) is a part of Khulna city and located between longitudes 220 28´ and 220 43´ North and latitudes 890 09´ and 890 23´ East of Bangladesh.
2.2 Experimental design The experiment was conducted in nine on-station earthen ponds of 0.1 ha each following the design furnished in the Table below.
2.3 Preparation of shrimp pond The ponds were prepared by complete drying, liming CaCO3 @ 250 kg/ha of soil and then filled with the tidal water by complete screening with fine meshed net up to a depth of 1.0 m. The entire farm was encircled up to 1.0 m height with fine meshed nylon net that ensures a biosecurity. Water was treated with chlorine @ 20 ppm to disinfect water and kill all animalcules those are the carrier of pathogen and disease. Molasses @ 80 kg/ha were applied to develop colour of water to prevent penetration of sunlight and then fertilized with urea and TSP @ 25 and 30 kg/ha, respectively for quick development of colour of water and production of plankton. 2.4 In-pond nursery preparation and stocking of postlarvae (PL) An in-pond nursery was constructed at one corner of each pond by encircling nylon net fastened in bamboo frame. After seven days of fertilization required quantity of shrimp PL were stocked with acclimatization with pond water and reared for 14 days before releasing into the whole pond. In the nursery the stocked PL were fed with CP (Charoen Pokphand) nursery feed (NASA) twice daily.
2.5 Post stocking management After two weeks of nursery rearing they were allowed to spread over the whole pond by opening the nursery enclosure. In the grow-out ponds, the shrimp were fed with CP feed depending on the biomass of shrimp. The feeding behavior and well-being of shrimp were checked every 1-2 days intervals through cast netting. Growth of fishes was monitored at weekly interval and feeds were adjusted accordingly. The water of the ponds was treated with CaMg (CO3)2 (dolomite) @ 15 ppm on monthly basis and fertilized with inorganic fertilizer whenever necessary. In the grow-out ponds Zeolite @ 4 ppm was applied whenever needed. Aeration has been provided to the ponds whenever necessary through agitating water by paddle wheel/airjet aerator. 2.6 Monitoring of water quality The different hydrographical parameters viz., surface water temperature (oC), depth (cm), transparency (cm), salinity (ppt), pH, DO (mgl-1) and Total alkalinity (mgl-1) were monitored at seven days interval using a Celsius thermometer, a graduated pole, a secchi-disk, a refractometer, a portable pH meter (HI 8424, Hanna Instruments, Portugal) and a portable dissolved oxygen meter (HI 9142, Hanna Instruments, Portugal) and Total alkalinity was determined following the titrimetric method according to the standard procedure and methods. Dissolved oxygen (DO) was monitored almost daily after 50 days of culture.
2.7 Harvesting of shrimp After 63 days (9 weeks) of grow out period, shrimps of all ponds were harvested, firstly with repeated cast netting and finally by complete dewatering with pump. Total weight and number of shrimp in each pond were recorded for data analysis. The survival rate as well as production, specific growth rate (SGR % day), feed conservation ratio (FCR), average daily gain (ADG g/pcs/day) and final weight gain (%) were also estimated. Specific growth rate (SGR % day) and feed utilization efficiency were calculated respectively as follows:
a. ADG (g) = Final weight (g)/Duration of culture (days) b. Final weight gain % = (Final wt.-Initial wt. / Final wt.) X100 c. SGR (%/day) = 100 x [(LnWt-LnW1)/t] Where, Wt. = Final weight W1 = Initial weight t = Days of culture d. Survival rate % = 100 x (No. of shrimp harvested / No. of shrimp stocked) e. Feed Conversion Ratio (FCR) = Dry weight (g) of feed supplied / Live weight (g) of fish gained.
2.8 Data analysis The final data were expressed as mean value ± Standard deviation (SD) and analyzed by one-way analysis of variance (ANOVA) to find the level of significance of difference among different treatments and the significance was assigned at the 5% level (P>0.05). All statistical analysis was done by using the Microsoft Excel and SPSS (Statistical Package for Social Science) version-20.