| Abstract | Bandon Bay located at Surat Thani province of southern of Thailand is one of the most productive coastal areas in the country. More than 80 km of coastal belt is utilized mainly for shrimp and mollusc culture. Molluscs, especially oyster, Crassostrea belcheri (Sowerby) and cockle, Anadara granosa (Linnaeus), have been cultured in Bandon Bay for more than 20 years. In 1998, mollusc farms occupied about 3,000 ha of intertidal mud flat of the shore area. These farms produced 7,800 and 20,000 t of oysters and cockles, respectively, which contributed 42 % of total mollusc production in Thailand that worth 17.3 million USD. Beside mollusc culture in the bay, intensive shrimp culture is practiced along the coast. The current production is about 16,000 t from 1,006 intensive shrimp farms with areas of 3,500 ha. Intensive shrimp culture which primarily depends on highprotein pelleted feeds is practiced around the bay. Only 20 - 25 % off nitrogen and 10 - 15% phosphorous from feed are retained in the biomass and the rest would be exported to the bay during water exchange and drainage. Exported nutrients from shrimp culture to the natural environment enrich coastal waters and enhance plankton blooms. It is known that molluscs are efficient filter feeders capable of depleting phytoplankton in the water column. Hence, it is possible that eutrophication process in this coastal area from shrimp farming effluents could probably enhance mollusc production. But, as mollusc culture is an important livelihood option of local communities in this area, it is necessary to reduce any negative impacts from shrimp culture, if any, to ensure its sustainability. This research study, therefore, is conducted to investigate the current situation in mollusc culture in Bandon Bay in terms of culture practices, socio-economics, and to identify constraints and prospects. Furthermore, it was also investigated whether shrimp culture activities poses any positive or negative impact on mollusc production. Finally, the potential for integrating mollusc and shrimp culture to enhance mutual benefits for the farmers in coastal areas and ways in which to minimize coastal eutrophication are discussed. A socio-economic and ecological survey using farmer interviews, SWOT analysis and water quality parameter measurements were conducted during January to December 1999. Monthly water samples from 29 sampling stations in the river mouth, mollusc culture, and open bay areas were collected for 12 months and analyzed for ammonia-nitrogen (NH₃-N), nitrite (N0₂-N), nitrate (N0₃-N) and soluble reactive phosphorus (P0₄³⁻), and chlorophyll-a (Chl-a). Secondary information on phytoplankton abundance and shrimp culture was also collected from local research institutions. In addition, information on water level, water discharge, wind, and nutrients and Chl-a concentration in Bandon bay were used to determine Chl-a distribution around the mollusc culture area using hydrodynamic and water quality modules of MIKE21 model. The results revealed that both oyster and cockle culture provided adequate net returns of 4,905 and 1,257 USD ha⁻¹ year⁻¹, respectively. Nutrient concentrations in river mouth and shrimp culture areas were significantly (P<0.05) higher than the open bay area and Chl-a in mollusc culture areas was significantly (P<0.05) lower than that in the river mouth and shrimp effluent discharge areas. There was a significant positive relationship (P<0.05) between shrimp culture area around Bandon Bay and Chl-a concentration. Effluent discharging period of shrimp farms coincided with Chl-a peaks in the bay. There was a significantly positive relationship (P<0.05) between shrimp and oyster yields. The simulated Chl-a concentration was fitted with observed Chl-a concentration outside of the mollusc culture area. Since Chl-a concentration inside mollusc culture area is low, difference between simulated and observed values were used to evaluate Chl-a content reduced by molluscs in the culture area. It was found that MIKE21 model can be successfully used to determine Chl-a distribution in relation to nutrient loading, water discharge and wind conditions in Bandon Bay. The modeling results showed that the simulated Chl-a concentration in November in mollusc culture area, were ranged from 11.5 - 37.5 ug 1⁻¹ probably due to relatively higher water discharge from river and shrimp farm effluents. The results also showed that Chl-a concentration was approximately 72% lower in the mollusc culture area than non-mollusc cultured areas. These simulated and observed results indicated that mollusc farming can be effectively used to recycle the waste nutrient discharged the river and shrimp farms to control eutrophication in Bandon Bay. The results revealed that as mollusc and shrimp culture in Kanchanadit district coastal areas can be integrated. This sustainable coastal aquaculture therefore is possible. In the meantime, according to water samples analysis and model simulation results to increase mollusc production in Bandon Bay, the coastal areas of Thachang and Chaiya district which located at the northwest and north part of the bay, respectively, were recommended for mollusc culture extension due availability of phytoplankton in the water and shrimp farms on its coast. |