Study site: The present study was conducted at Kathaltala, Dumuria Upazila of Khulna district of Bangladesh, throughout the year 2004. Two stations from the Teligati river (semi diurnal tidal system), two from canal (river connected) and ten shrimp ponds were selected for weekly sampling. Surveys in the greater Khulna region (Khulna, Satkhira and Bagerhat) were also conducted to verify the results.
Shrimp farming techniques and management: The quantity and quality of inputs (feed, fertilizer and water exchange) and production were recorded during the study period without any interruption to the farm activities. Various issues related to management, growth and disease prevalence were closely monitored and observed with a participatory approach.
Pond preparation started in January after completion of paddy culture (from August to December). A periphery canal near the border of the farms was dug and the soil was used for making dike to store water for their farm. The canal was used as nursery pond (locally known as tob) for shrimp fry. The farmland was ploughed and left for 7-10 days under sunshine and then lime (CaO) was applied. After 7 to 10 days of liming inorganic (mainly Urea and TSP) and organic fertilizer (cow dung) was applied.
After 7-10 days of fertilization water was introduced by allowing the water level of high tide to enter into the ponds during the new or full moon. No particular stocking density was maintained. Stocking varied from farm to farm and average stocking density was 13,782 PL ha-1(1.37 PLm-2). PL stocking continued from March to June with every lunar phase by 6-8 spells and stocking completed before the rainy season. Monoculture of shrimp was not observed in the site. Though tiger shrimp, Penaeous monodon was the target species, some other species, especially fresh water prawn, Macrobrachium rosenbergii (during rainy season) and finfish species like tilapia, Oreochromis mossambicus and persia, Liza parsia were used to stock. A number of other shrimp and finfish species also enter into the ponds with tidal inflow of water. Fry nursing in nursery pond (tob) was a common practice especially at early spells i.e., before the rainy season. Duration of nursing was varied and usually it was for one month. Fry acclimatization was not practiced properly.
There were inlet and outlet facilities used for receiving and discharging the water where spring tide used for water inlet and neap tide for water outlet. Most of the river or canal side farms exchange farm water frequently with tide, but water exchange was rare in the farms located far from the river or canal. Some farmers occasionally added only river water allowing high tide to maintain water level in the ponds.
Frame survey in the wider area: A questionnaire survey considering 60 randomly selected shrimp farms was carried out in the wider area (3 coastal districts - Khulna, Satkhira and Bagerhat) with a view to cross check the managerial issues recorded in the study area.
Water sampling and monitoring: Weekly water quality monitoring throughout the year was conducted in 10 selected shrimp ponds and 4 stations of river (station 1 and 2) and water supply canal (station 3 and 4) to observe water quality dynamics. Separate sampling during both high and low tide was also considered. The parameters studied weekly were temperature (°C), depth (cm), salinity (ppt), pH, dissolved oxygen (mgl-1), biological oxygen demand (BOD) (mgl-1), ammonia nitrogen (NH3-N) (mgl-1) and total phosphate (TP) (mgl1). Water samples were collected fortnightly from pond inlets and outlets throughout the shrimp growing cycle to determine total nitrogen (TN) (mgl-1) and total phosphorus (TP) (mgl-1).
Water pH was recorded directly by using a digital pH meter (pHep+ by HANNA). Salinity was measured by a hand refractometer (ATAGO S/Mill-E). Dissolved oxygen (DO) was measured on the spot by a digital DO meter (DO-5510, Lutron). The NH3-N and TP were measured by using a water test kit (HACH). Total nitrogen was estimated by micro Kjeldahl method (APHA, 1998) and total phosphorus by digestion followed by direct spectrophotometric reading. Biological oxygen demand (BOD5) was analyzed in the lab by Winkler titrimatric method.