DT Explains: How Kakka Aazhi species invaded Kosasthalaiyar and is now spreading to Gulf of Mannar

CHENNAI: Kosasthalaiyar, one of the three rivers criss-crossing the metropolis and only the cleaner river among the three, is the life line of several fishermen villages in and around Ennore. Even as the fast-paced industrialisation has been threatening the river for decades, fishermen are faced with another new challenge – invasion of Charru mussels (Mytella Strigata), locally known as Kakka Aazhi.

Native to south and central American coasts, fishermen and the Tamil Nadu Water Resources Department allege that the invasive species reached the Ennore shores through ballast water carried by the ships reaching the city ports from the Americas.

For uninitiated, ballast water is the water that ships carry in their tanks and cargo-holds to help them maintain stability and manoeuvrability during a voyage.

Fishermen first spotted Kakka Aazhi around 15-20 years ago but the infestation has become more rapid after Cyclone Vardah (2016). Also, structures constructed inside the river for thermal power plants, and debris dumped inside the river provide haven.

Kakka Aazhi spreads like a carpet over the river bottom preventing prawns from grazing or burying themselves in the river sediment. As per fishers’ accounts, the alien species is wiping out the locally prevalent yellow clams (manja matti) and green mussels (pachai aazhi). In the past, fishermen from one village would catch around 1 tonne of prawns from the river, which has been reduced to 100-200 kg/day.

Apart from spreading on the river bed, the mussels would grow over the water-level resembling rocks. This affects movements of boats and damages fishermen’s nets.

Meanwhile, a study titled ‘Invasion of Alien Mussel Mytella strigata (Bivalvia: Mytilidae) in the Gulf of Mannar, India and Possible Threats to the Native Biodiversity’ conducted by researchers of the Centre for Marine Living Resources and Ecology in Kerala, Suganthi Devadason of Marine Research Institute (SDMRI) in Thoothukudi, and University of Kerala, have identified the invasion of Kakka Aazhi into ecologically sensitive Gulf of Mannar. The study explains how they can survive large fluctuations in salinity for long and short periods, and tolerate a wide range of salinities as low as 2 ppt (parts per thousand) and as high as 40 ppt.

One of the extensive studies on the molluscan diversity of the Ashtamudi estuary in Kerala did not record Mytella Strigata from the region from February 2017 to January 2018. But, the species spread rapidly after the very severe cyclonic storm Ockhi (2017).

The southern bench of National Green Tribunal (NGT) is hearing a case pertaining to the invasion in Ennore and directed the government to curb them. The government has proposed controlling the spread biologically using Rapana Rapiformis (whelk gastropod).