The fish of all varieties, mostly young ones, were found dead at a one-km stretch, according to residents.
The sea around Rameswaram, which is part of the Gulf of Mannar biosphere, is rich in marine life including marine mammals like dugongs, dolphins and endangered turtles.
Destructive fishing methods carried out by fishermen and sewage let out to the sea from several lodges in Rameswaram threaten the marine ecology of the region.
Municipal chairman A Arjunan said, “It could be either a natural phenomenon or that some fishermen might have abandoned their catch,” he said. Trawl netting, especially bottom trawling, carried out by local fishermen destroyed fish wealth, he added.
Fisheries officials at Rameswaram ruled out any chemical pollution and oil spilling.
“We are investigating the cause,” an official said.