The tiger prawn boom
In the early 1980s, commercial aquaculture was a new concept in Sri Lanka. The promise of high returns from the international market for shrimp drove rapid, often uncontrolled, development around Negombo Lagoon. The species of choice was the giant tiger prawn, Penaeus monodon, a fast-growing crustacean native to the region's coastal waters. By the mid-1990s, nearly 1,400 farms, many of them small-scale and built without official approval, crowded the coastal belt, converting over 4,500 hectares of land into aquaculture ponds.
This explosive growth came at a significant ecological cost. Large areas of mangrove forests, nursery grounds for wild fish and shrimp, were cleared to make way for the ponds. The intensive farming method, which packed ponds with 30 to 40 shrimp per square meter, required huge inputs of fresh water, feed, and chemicals. The farms drew water directly from the lagoon and adjacent canals, and then discharged untreated, nutrient-rich effluent back into the same water bodies. This practice of "self-pollution" created a feedback loop of deteriorating water quality.
An ecosystem collapse
The constant influx of waste from the shrimp ponds fundamentally altered the lagoon's chemistry. Effluent loaded with uneaten feed, feces, and fertilizers caused nutrient levels, particularly nitrogen and phosphorus, to spike. This led to eutrophication, a state of excessive richness of nutrients that stimulates dense plant growth and death of animal life from lack of oxygen. Algal blooms became common, and the overall health of the ecosystem declined. The changes stressed the wild shrimp populations that depended on the lagoon as a nursery, including commercially important species like Penaeus indicus and Metapenaeus dobsoni. As farmed shrimp production soared after 1994, the catch of wild shrimp began a steep decline.
The crowded, high-stress conditions in the ponds, combined with poor water quality, created a perfect breeding ground for disease. In 1996, an outbreak of White Spot Syndrome Virus (WSSV) tore through the farms. This contagious and lethal virus can cause mortality rates of up to 100% within three to ten days. The outbreak was catastrophic, causing an estimated loss of 1,000 million Sri Lankan Rupees in foreign exchange and rendering about 85% of the farm areas non-functional. The industry collapsed, leaving abandoned ponds and a severely degraded lagoon ecosystem.