Water Quality and Phytoplankton Communities in Intensive Shrimp Culture Ponds in Kung Krabaen Bay, Eastern Thailand

Size: px
Start display at page:

Download "Water Quality and Phytoplankton Communities in Intensive Shrimp Culture Ponds in Kung Krabaen Bay, Eastern Thailand"

Transcription

1 Water Quality and Phytoplankton Communities in Intensive Shrimp Culture Ponds in Kung Krabaen Bay, Eastern Thailand SIRI TOOKWINAS AND PuTIH SONGSANGJINDA MariM Shrimp Research GIld Development Institute. Departmeht of Fisheries. Chatuchak,. Bangkok Thailand Abstract Water quality and phytoplankton communities were studied in 20 intensive shrimp culture ponds and in the Inlet aad drainage canals at Kung Krabaen Bay, Eastern Thailand during the first shrimp crop or The grow-out ponds were categorized into two groups: low stocking density «60 PUm2) and high stocking density (>60 PUmJ). The results showed that there were no significant differences in water quality between the two stockin~ density groups. Phytoplankton couected using a 6O-fLm mesh net consisted or 79 genera with concentrations ranging rrom 1,822 to cellsll rrom the first month up to the time or shrimp harvesting. Water quality deteriorated In high and low stocking density shrimp ponds, which had an Influence on abundance and diversity or net phytoplankton communities. Biochemical oxygen demand and ammonia-nitrogen were most closely related to abundance or net phytoplankton communities during the shrimp grow-out period. Salinity, temperature, and dissolved oxygen appeared to play an important role in phytoplankton community variation in Inlet and drainage canals. Phytoplankton community structure In drainage canals showed more variation than In ponds. The management implications or the results and recommcndations ror rurther studies are also considered.. Marine shrimp fanning has been practiced in Thailand for the last 80 yr. However, only recently has intensive shrimp fanning expanded in the country's coastal area. In 1995, shrimp farms covered approximately 70,000 ha and produced 200,000 metric tons of shrimp ~r year {Tookwinas 1995). Expansion of shrimp farming in Asia and the Americas has led to concern about potential environmental impacts (Phillips et ai. 1993) and a need for better understanding of environmental interactions as a basis for improved management (Barg 1992). Nutrients added to intensive shrimp ponds can affect environmental conditions within ponds and surrounding marine environments through effluent discharge. Shrimp pond effluent has two important components: 1) discharged water and 2) accumulated sediment. Shrimp feed represents the largest single source of nutrients in intensive shrimp ponds: only 21 % of nitrogen and 6% of phosphorus added into shri-np grow-out ponds are incorporated into harvested shrimp flesh (Stapornvanit 1993). The remainder is dispersed in the pond as uneaten food, shrimp feces or excreted mittter. and thcy arc thcn.wailaolc to support the rapid growth of phytoplankton and heterotrophic organisms. To understand the impact of shrimp farm effluent on the surrounding environment. it is important to assess the effect of these nutrients on phytoplankton communities in the shrimp pond and waters rcceiving shrimp pond effluent. This study analyzed water quality and net phytoplankton communities in coastal ponds with different shrimp stocking densities and in assoc.ated water supply and drainage canals. Relationships between water quality and phytoplankton communities were then explored using multivariate techniques. Material and Methods Study Site Kung Krabaen Bay is located at lat 12"33'-12"36'N. long 10Io53'-10Io55'E in

2 T ABLF. I. Background infotmdtion and productivity of shrimp ponds used in d8is study. Stocking Pond :Irca Gmw-OUl dcnllity Harvellt Si7.c Production Pond no. (1I1~) period (d) (PUm2) (ind./kg) (kglha) FCR Survival (%) , , / / / / / / / ' 42/ / / / , / SO.58 49/ / , / , Min Max 2.1! Mean II Chanthaburi province in Eastern Thailand. The bay is fringed in the inner parts by mangrove forest which is about m wide from landward to seaward edge (Raine 1992). Intensive shrimp farm ponds are located around the bay behind the mangrove area with a total pond area of 160 ha. Fanning Practices and Data Collection. Two groups of grow-out' ponds were selected for the study: a low stocking density group (L) with shrimp present at <60 postlarvae (PL}/m2 and a high stocking density group (H) with shrimp present at >60 PU m2. A total of 20 were studied. Pond preparation, stocking density. water quality management, water exchange, and other culture techniques were managed according to normal Thai practices. Fanners used the low water exchange system. This system involves no water exchange during the first 2 mo of culture (on top-up) and an estimated 5-10% of pond volume exchanged during the high tide period every 2 wk in the third, fourth, and fifth months. Pertinent culture information for study ponds are shown in Table 1. Shrimp Penaeus monodon post-larvae were stocked in January Estimates of stocking rate for each pond were derived from fann records. Shrimp were fed after stocking with several brands of commer-. dally available shrimp feed. Shrimp feed input was regulated by farmers, who kept daily records of feed input. Shrimp were harvested during pond drainage by a net trap attached to the pond water. gate Farmers recorded the total weight and size of shrimp. This information was used to calculate the survival rate, production, and feed conversion ratio (Table 1). Water Sampling and Analysis Water samples were collected from water supply canals (inlet water, designated as I),

3 Data Analysis Net Phytoplankton Community Sampling and Analysis. Shrimp Results Production

4 Water Qualitv Net Phytoplankton Communities

5

6

7

8

9

10 Literature Cited