Last month, inland fishers found parts of Ashtamudi teeming with mackerels, a surprise spotting that was first considered more of an accident. But the shoal did not vanish, and the fact that it remained in the brackish water for several days indicates an alarming change in the wetland ecosystem — escalating salinity of water.
With a considerable dip in the piscean population and its clam beds shrinking fast, the protected Ramsar site of Ashtamudi is already a matter of serious concern for environmentalists and authorities alike.
“Saline incursion during high tide makes it possible for marine shoals, especially pelagic ones, to survive. Some years back, oil sardines entered Ashatamudi and stayed there for some six months. Usually, marine varieties cannot survive in low-salinity levels, but now the water has become similar to that of the sea. If the marine regime continues, the entire aquatic environment will be affected,” says K.K. Appukuttan, marine fisheries expert and former scientist with Central Marine Fisheries Research Institute (CMFRI).
While the increasing salinity and the presence of marine fauna will drive some estuarine fish species to the upper reaches of the lake, some others will perish. “But there are also some inland species that can tolerate both conditions and they will survive. At the same time, there can be competition for food among plankton feeders,” he says.
Freshwater inflow down
Over the years, the salinity of the water has steadily increased, mainly owing to the blocked inflow of freshwater after the construction of the Kallada dam.
Another major threat faced by Ashtamudi now is the presence of an invasive bivalve from South America. It started appearing in Vemband during early 2019 and currently it has colonised many creeks in Ashtamudi. The bivalves are around two centimetres long with very little meat, but they have been monopolising and altering the habitat.
“They are slowly eradicating the stock of Ashatmudi’s short-necked clam (Paphia malabarica), the only Marine Stewardship Council-certified fishery in India. This year, the proper settlement of the clam has not taken place so far and one reason can be this invasive species. They have occupied the entire area and there is no space left for the Paphia malabarica larvae to settle,” he says.
Invasive bivalves
Though the invasive bivalves are edible with a chemical composition being almost similar to that of native ones, meat percentage and market value are low. “They must have reached Indian ports attached to ship hulls, but they multiply fast. They start early spawning and cover the area like a carpet, blocking the settlement of native clams and mussels. They can also cause a deficit of microalgae, the main food.”
It is not easy to control the new menace as the bivalve spreads too fast and has high tolerance threshold.
The variation in salinity level and the presence of invasive species can also affect the fish sanctuaries being set up in Ashtamudi and Vembanad by the Fisheries Department as part of a project to revive the inland fisheries sector. The department had identified a number of sites to conserve pearl spot and yellow clam, but the recent changes in the habitat may not be ideal for fish seed ranching and clam relaying.
