Water, sediment and mussel samples were collected and subjected to various analyses to explore the cause of invasion and ways to eradicate or utilise them for other purposes.
Water, sediment and mussel samples were collected and subjected to various analyses to explore the cause of invasion and ways to eradicate or utilise them for other purposes. 
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Ecological disaster brewing inside Pulicat bird sanctuary

SV Krishna Chaitanya

CHENNAI: Jaya (47), a traditional fisherwoman who earns her living picking prawns from the Pulicat lake, says the quality and quantum of shrimp availability is drastically declining. “Earlier, if I spent two hours in the water, I used to catch prawns worth Rs 200 to Rs 300, but now I am spending four hours to earn Rs 150 per day,” Jaya said and blamed it on “Kaaka Aazhi”, a common Tamil name for Charru mussels (Mytella strigata), an invasive South American species.         

TNIE travelled across the Pulicat bird sanctuary, which is home to India’s second largest brackish water lake, and found that the invasive species has reached an alarming level extending up to several kilometers. If left unchecked, it would soon turn into an ecological disaster compromising the entire habitat, depleting native life and having ramifications on thousands of migratory birds that visit Pulicat.

Fishers have already begun complaining of reduction in prawns, crabs and fish life, besides wiping out the locally prevalent and commercially valuable yellow clams (manja matti) and green mussels (pachai aazhi).

Chandrasekharan, owner of a boat and a fifth generation fisherman, says things are becoming dire. He fears the entire Pulicat wetland ecosystem would collapse because of this invasive species. “The government must act immediately. We spotted it in 2022 in one or two locations and ignored it, but now it is spreading in the sanctuary areas and adjacent wetlands.”

In Pulicat, fishermen use the padu system, a sustainable community-based fisheries management. Unlike open ocean fishing where the net is casted from the boat, here they get down into the water and set up the net. “Our legs are getting cut and bleeding because of this invasive mussel. We have some 40 padus (fishing grounds) in Pulicat, of which at least half are affected,” Chandrasekaran said.

John Bosco from Nadumadhakuppam said Charru mussels are hampering the reproduction of prawns and crabs, which are bottom feeders. These mussels cover the lake bed like a thick carpet, not allowing prawns to lay larvae and crabs to dig their boroughs. “We are suffering heavy economic losses. Many fishermen are forced to take daily wage jobs and join ports as unskilled labourers to make a living.”

Need for comprehensive study

The first reported invasion of these mussels in India was from similar tidal wetlands — the Vembanad and Ashtamudi wetlands in Kerala. A few years ago, the problem emerged in the Ennore wetlands in Tamil Nadu.

Joe Kizhakudan, a senior scientist from the Central Marine Fisheries Research Institute (CMFRI), told TNIE: “Once a wetland is infected by this mussel species, it is almost impossible to control. Regular manual removal and dredging will help, but use of predatory species that feed on these mussels will be the long-term solution. Usually, crustaceans like crabs and prawns feed on them, but fishermen are saying their population is declining. A comprehensive study is needed to understand the species better to evolve remedial measures.”   

Charru mussels

This species will throw other commercially valuable green mussels and yellow clams into extinction as they are all competing for the same space, food and oxygen. Charru mussels are more dominant, spread rapidly and multiply every two months, he said. 

A team of experts from Dr MGR Fisheries College and Research Institute, Ponneri visited Ennore Creek in January last year, to inspect the invasion site along with officials of the State Fisheries Department and CMFRI. Water, sediment and mussel samples were collected and subjected to various analyses to explore the cause of invasion and ways to eradicate or utilise them for other purposes.

Their study concluded that the Charru mussel invasion has depleted phytoplankton availability required for shrimp larvae, thereby affecting shrimp production in these waters. It also supported the apprehension of fishermen that the invasion of this species have affected the other commercially important edible oysters and mussels in Ennore, besides prawns, and if it is left unattended, it might spread to other coastal areas and pose a serious threat to its fauna also.

Scientists saw promise in the utilisation of Charru mussel meat as an alternative for fish meal. “Measures need to be taken by the government to remove them from creek waters by dredging operation; or harvest them by fishermen for utilisation as fish feed or fish fertilizer preparation; as mussel meat did not contain toxic heavy metals,” the study recommended.

Will act immediately: Govt

With invasion spreading from Ennore to the more ecologically sensitive Pulicat bird sanctuary, the state government said it would take immediate measures to arrest its spread. Supriya Sahu, Additional Chief Secretary, Environment, Forests and Climate Change Department, told TNIE: “We will discuss the issue with the fisheries department and Water Resources Department to expedite the proposal to remove Charru mussels from both Ennore and Pulicat wetlands.”

Chief Wildlife Warden Srinivas R Reddy also said the problem is alarming and all options will be explored. “Dredging a large waterbody like Pulicat lake is neither advisable nor feasible, but we will look at other options.”

Deepak Srivastava, member secretary, Tamil Nadu State Wetland Authority, said when the invasion in Ennore was reported, WRD prepared a project report for habitat enhancement for fishermen in which dredging the affected areas was also included, but the project was not implemented.

“I will follow up on the project with the WRD and work with the forest department to contain the spread in Pulicat under the wetland mission, which was launched by Chief Minister M K Stalin to protect important wetlands in the state. We will start removal of Charru mussels from next week on a pilot scale,” he said.

M Jaya, a traditional fisherwoman in Pulicat

Ballast waters primary source?

It is suspected that Charru mussels arrived in the region through alleged unchecked discharge of ballast water and the failure of the Kattupalli port authorities to prevent migration of invasive species from the hulls of visiting ships. The first reported invasion of these mussels was from similar tidal wetlands — the Vembanad and Ashtamudi wetlands in Kerala that are tidally connected to the Kochi port and harbour. Experts say when ballast water is pumped into a ship, sediment and microscopic organisms are also transferred into the ballast tanks. These organisms include bacteria, microbes, small invertebrates, eggs, cysts and larvae of various species. Many of these organisms are able to survive for extended periods in inhospitable environments, including a ship’s ballast tanks. When the ballast water is discharged, the organisms are released into the local marine environment.

Similarly, species from one part of the world may piggyback on ships’ hulls as foulants to other parts of the world. Given the right conditions, they not only survive but can also flourish, becoming invasive and threatening and even eliminating resident populations, like in Ennore and Pulicat.

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