Vembanad Lake fish count indicates decline in backwater salinity: Study

While the two-day-long fish count has spotted 43 fin fishes and five shellfishes in Vembanad Lake, a vast majority of them belonged to the freshwater category.
Team members collecting fish samples from Vembanad Lake as part of Fish Count 2022
Team members collecting fish samples from Vembanad Lake as part of Fish Count 2022

KOTTAYAM: The lives of people inhabiting the banks of Vembanad Lake are deeply connected with the brackish water system. Fishing and black clam collection are the only source of livelihood for a predominant number of people here. Fishermen pushing country-made boats through backwaters and tourists arriving to enjoy the beauty of backwaters seen enjoying fish delicacies of the region are common sights for the people of Kottayam and Alappuzha districts.

However, unseasonal rains and climate change appear to have cast a shadow on the brackish ecosystem of the Vembanad backwaters. As per the findings of the Vembanad Fish Count-2022, organised under the aegis of the Ashoka Trust for Research in Ecology and Environment (ATREE), the nature of the lake water has changed with a sharp decline in its salinity.

While the two-day-long fish count has spotted 43 fin fishes and five shellfishes in Vembanad Lake, a vast majority of them belonged to the freshwater category.

According to ATREE officials, it was a surprise finding which made this year's fish count different from the previous 14 years. "This is for the first time that the presence of 'Chela Fasciata', a purely freshwater fish has been reported from Vembanad Lake. This indicates the freshwater nature of the lake," they said.

Meanwhile, Jojo T D, project coordinator at ATREE Community Environment Resource Centre (CERC), said though a delay in opening the Thanneermukkam Bund (Saline water barrier) and torrential rains might be the reasons for a change in the nature of lake water, a detailed study is needed to ascertain such assumptions. According to him, a change in the nature of water would badly affect the fish and black clam (Villorita cyprinoides) deposit in the backwater. "Usually, salinity level is up to 16 ppt during the time of fish count being conducted in the month of May every year. However, this time it is zero, which is a surprising finding," he said.

He added that the decline in salinity would affect the breeding of black clams and the arrival of migratory fishes from sea. "The black clams usually have two breeding seasons that fall in December-January and April-May. With Thanneermukkam saline water barrier being closed during the December-January season, the breeding of black clams happens in April and May. Moreover, 10 to 12 ppt of salinity is needed for the breeding of clams. If the lake water loses its salinity, it would affect the population of clams and their reproduction is likely to decrease in the following season," Jojo said.

Jojo added that the presence of migratory fish from the sea could not be found in the survey. "Usually, lake water is more brackish during the summer season and migratory fish are common here during the fish count," he said.

The survey was conducted by collecting samples from 15 points dividing the entire lake into West, East and Riverine sections. The survey was conducted in association with Kerala University of Fisheries and Ocean Studies, Department of Fisheries and Aquatic Biology, Kerala University and Vembanad Kayal Samrakshana Samithy. The survey also spotted babies of 'Oreochromis niloticus', a fish variety that has been included in the Global Invasive Species Database. Following this, the organisers suggested a detailed study into the presence of invasive fishes which may affect the deposit of natural fishes.

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