While environmentalists and scientists agree that pollution is the killer, a scientific explanation on why it happened on one day is yet to come. Environmental Activists said that a broken Metrowater pipe that runs along the Adyar bridge has been discharging sewage into the river for several months now.

"We made complaints complained to Metrowater and the state pollution control board, but no action was taken," said Pooja Kumar of NGO Coastal Resource Centre.

"Fish kills of kind are routine, especially after monsoon when Metrowater releases untreated sewage because its infrastructure is unable to handle the shock-loads of sewage," she said.

Metrowater denied , but agreed that they have to look at the sewage pipelines. "We got a complaint that a pipe was broken last week, and we fixed it. It seems to have developed cracks again. We will look in to it on Thursday morning," said a Metrowater engineer.

With the surge of sewage, environmentalists say, the 'biological oxygen demand' must have crossed the tipping point. "When happens, fish and other aquatic life suffer," said V Arun, coordinator for Student Sea Turtle Conservation Network.

Experts say it could be a combination of factors . "Since the rain stopped, there could have been a sudden drop in oxygen content in the water causing these deaths," said T K Ramkumar, former member of the Madras high court monitoring committee on development of Adyar Creek, Chennai. Tamil Nadu Pollution Control Board officials say they are not aware of incident.

Estuaries like mangroves and coral reefs are important for marine ecology as they act as a breeding ground for marine animals. "Beyond the Kotturpuram bridge, the water is too polluted for any life to be present. The conditions are better closer to the sea, but is a shocker," said Ramkumar.

Species found in estuaries are also unique, thriving on water that is neither too saline nor fresh. They include eels, mullets and several species of catfish. "Common eels live in fresh water and breeds in the sea. Elvers or baby eels live in the estuaries for a while before going to fresh water," said Ranjit Daniels of Care Earth. Estuaries are important for several other reasons as well. "Several species of mussels, crabs, prawns and fish live in the estuary that serves as indication of pollution levels in the water. Some are so sensitive and can be used to track climate change," added.

Birds like black-winged stilts - which are residential - are also found in estuaries. "Several species of sandpipers also come calling from the arctic and northern Europe to our estuaries," said Daniels.

Area: 300 acres from Thiru-Vi-Ka bridge to the river mouth

Why is it important: It is a breeding ground for some marine species and home to many unique species

What lives here: Eels, prawns, several fish species, mussels. More than 200 species of birds - both residential and migratory - are found here