But it was not an entirely unfamiliar sight. Scores of dead fish had washed ashore at the same spot in January 2015, prompting government officials and journalists to rush to the village. A month later, laboratory tests concluded that hazardous levels of sewage in the waters of the Adyar river had killed the fish.

Pollution seems to be the killer this time too, according to the research body Central Institute of Brackishwater Aquaculture, which is studying the incident. The institute’s director, K Vijayan, said the fish had suffocated because of a reduction in oxygen levels. The New Indian Express quoted him as saying this was “either due to substantial increase in pollutants entering the water body or low tide minimising sea water ingress into the estuary or a combination of both”.

However, Tamil Nadu’s Fisheries Minister D Jayakumar blamed the fish deaths on the presence of too much fresh water in the estuary, reportedThe News Minute. “Good fresh water passed through this water body and due to this, the salt content in the water reduced,” he said. “That is why the fish have died.”

The two incidents in the space of three years have left fishermen in the village worried about the impact on their livelihoods. Palayam, president of the Ururkuppam Fishermen Cooperative Society, said he was particularly anguished to see several dead sand whiting fish, or kezhanga – an economically important variety that sells for Rs 30 to Rs 50 per fish. “It usually survives even in shallow water and moves with the current,” the 54-year-old said. “This is all because of the chemical water let out by industries in the river. This water has poison.”

Polluted river

The waters of the 42-km-long Adyar river are highly polluted. Residents said the Chennai Metropolitan Water Supply and Sewerage Board releases large volumes of sewage into the river every monsoon. Tamil Nadu is currently in the middle of the North-East Monsoon cycle, which sets in around the end of October.

According to Palayam, industries further upstream also release chemical waste into the Adyar.

In addition, surplus water from hundreds of tanks, lakes, small streams and stormwater drains in Chennai drain into the river, adding to pollution levels.

“The estuary does not have clean water or enough food for fish,” said marine biologist Rahul Muralidharan. “There is little oxygen for fish to breathe.”

A 2014 study published in the Journal of Threatened Taxa recorded 42 species in the upriver stretch, 25 in the middle stretch and only five in the lower stretch. “This lack of diversity in the lower stretch of the river can be directly linked to pollution, especially the lower reaches from Nandambakkam Bridge to Kotturpuram, which exhibit anoxic conditions [depletion of dissolved oxygen] for most of the year,” said the study.

Despite the high pollution levels, numerous fishing communities depend on the Adyar for their livelihoods. Palayam said fishermen here make Rs 10,000 to Rs 20,000 a month.

‘Extreme event’

The fishermen usually fish along the mouth of the estuary when the sea is rough, as they had been doing for the past few weeks. Palayam said they managed a good catch on Monday, only to be disheartened by the sight they saw on Tuesday.

“Thousands of people are dependent on these fishes, and they all died,” he said. “My stomach is burning because of this.”

At least five species – including the mullet, tilapia and Indian goat fish – were found among the dead fish. Palayam said many more lay under the river bed where they could not be easily seen.

He deduced the fish had been swimming southwards following the currents from Andhra Pradesh before they moved closer to shore. He explained that they usually do this when they sense some kind of disturbance in the sea, like a tremor or a storm.

Muralidharan, too, pointed to the presence of the tilapia, or Egyptian mouthbreeder, an invasive species that survives upstream of the river and is not known to travel this far down the river. “Tilapia is actually a very hardy species that comes to the surface and breathes through its mouth,” said Muralidharan. “So this is sort of an extreme event.”