SELF-POLLUTION : A MAJOR THREAT TO THE PRAWN FARMING INDUSTRY IN SRI LANKA

Shrimp farming is rapidly expanding over a coastal belt of about 120 km in the northwestern province of Sri Lanka. The Dutch Canal Mundel lagoon system, a shallow semienclosed coastal water system with very limited tidal mixing due to sandbar formation, act as the major brackish water source and a receiving body for the effluents discharged for 70% of the shrimp farms in Sri Lanka. Farms are mainly developed on intertidal areas in land-use categories of mangroves, salt marsh and scrubland; occupying more than 60% of the natural flood plains. Floods during intermonsoons have affected 40% of the farms directly and retension of flood water in lagoons has resulted in unusually low salinities (2 ppt) for 2-5 weeks, restricting water exchange in farms. Concentrations of sulfides (5.2-8.0 mg L-1) and nitrites (0.58-0.93 mg L-1) were at suboptimal levels in culture ponds during rainy seasons. Contamination of water source by discharged effluents is evident due to unplanned location of inlet and outlet canals. Highly acidic water (pH 3.5) with high concentrations of aluminum (1.8-2.5 mg L-1), manganese (0.16-3.6 mg L-1) and iron (1.7-19.3 mg L-1) were observed in water discharged to the major waterbodies from ponds built on acid-sulfate soils. Self-pollution caused by farming activities has resulted in environmental degradation which in return has caused major problems to the shrimp farming industry. High incidence of disease symptoms, growth retardation, poor shrimp quality, socioeconomic problems together with environmental problems have caused major threats to the industry. Better planning with due consideration to environmental and socioeconomic problems when locating and constructing shrimp farms would help to improve and ensure sustainability of the industry.