What a survey tells us about the peculiar lives of Kerala’s fish

Divided by a bund, the once-uniform ecology of the Vembanad lake now harbours a varied marine life. In the 11th edition of the annual Vembanad Fish Count, researchers counted a total of 145 species

Why was the scientist in the same boat as the fisherman?
Because they were participating in the Vembanad Fish Count!

The annual survey in Kerala's largest lake is a democratic affair, says Priyadarsanan Dharmarajan. The 54-year-old is the man behind the first of its kind survey, which started in 2006 as a bottom-up approach to create awareness about and study the fisheries of India's longest lake. Each year, between 130 to 170 people, including local residents, scientists, fisherfolk, researchers and volunteers participate in the survey, says Rajan, who grew up by the backwaters. He is a Senior Fellow at Bangalore-based non-profit Ashoka Trust for Research in Ecology and the Environment (ATREE) and the Convenor of the Community Environmental Resource center (CERC), exclusively set up for conservation of the Vembanad lake.

The Trust partners with a host of organisations, including the Lake Protection Fora, the Kerala University of Fisheries and Ocean Studies (KUFOS), the Kerala State Biodiversity Board, Cochin University of Science and Technology, and others, for the survey. Although the weekend effort does not present a full picture of the lake's fisheries (a task that would require year-round monitoring), it offers baseline data of the marine life of the Ramsar site (a protected wetland of international importance ). The 2018 edition took place on May 24-25. Equipped with nets, baskets and dip nets, seven teams comprising 20-40-members each, fanned out to 15 points in the southern side of the lake and 25 points to the north of the Thanneermukkam bund.

This is the first time the lake's northern side was being surveyed, thereby vastly increasing the survey's sample sites. At the end of the day's fishing trips, the participants counted 96 distinct fish and crustaceans in the northern side and 49 species in the southern side, says Professor KV Jayachandran, former head of research at KUFOS, who has been involved with the Vembanad Fish Count since its inception. Some of the species found in the northern side are Commerson's Glassy Perchlet (Ambassis ambassis), Giant Trevally (Caranx ignobilis), Java Rabbitfish (Siganus javus), Indian Ricefish (Oryzias setnai), while Filament Barb (Dawkinsia filamentosa), Horadandia brittani (one of the smallest fish and endemic to coastal floodplains of south India), Malabar Labeo (Labeo dussumieri; endemic to southern Kerala and Sri Lanka), Green panchax (Aplocheilus blockii) are prevalent in the southern side.

Prof Jayachandran attributes the huge difference in the number of species found in the northern and southern sides to the varying salinity levels in the water. Vembanad lake, he says, was a vast expanse of lake connected to the Arabian Sea. Saline water incursion into the lake would bring with it different fish and crustaceans, even as it purified the water and washed out to the sea during low tide. Most of Kuttanad, the rice bowl of Kerala is reclaimed from the Vembanad lake and saline intrusion during high tides was a threat to the paddy cultivation on the lake's isles. This is why the Thanneermukkam bund was built in 1974 to increase paddy production. Ever since, Vembanad's northern side has had brackish water while the southern side, fed by a riverine system, has freshwater. “The bund has severely disturbed the ecology here. It is the greatest tragedy in the wetland system,” says Prof Jayachandran. “Many species, such as the giant freshwater prawn (Macrobrachium rosenbergii) will migrate down to estuaries where salinity is desirable for them to breed, without which they will not breed at all. As a result, the population of many such species is continuously decreasing all along Vembanad and Cochin backwaters.”

Equally worrying is the issue of invasive species, points out Rajan, such as the aquarium escape Atlantic molly (Poecilia mexicana). Among the invasives are euryhaline species i.e. they are able to tolerate a range of salinity, and are therefore found in waters on both sides of the Vembanad. There are 15 such species in the lake, including Kerala's state fish peralspot (Etroplus suratensis) or karimeen as the locals call it in Malayalam, the giant freshwater prawn, the yellow catfish (Horabagrus brachysoma), colloquially called manjakoori, and the Orange chromide (Pseudetroplus maculatus), or pallathi.

“Another euryhaline is the Tilapia (Oreochromis mossabicus), which is an exotic fish,” says Prof Jayachandran. The widely consumed African native species was introduced in Kerala decades ago for farming and is hugely popular because it proliferates “anywhere and everywhere”. It feeds on plankton, grows up to 20-30 cm and can weigh up to 450-500 grams within 6-7 months. “Once it is given to farmers for stocking, this fish can migrate to the lake areas,” he says. “Data indicates that the number of Tilapia is increasing in the Vembanad. This is an acute problem because once this species establishes itself, it displaces other species. As its numbers increase, it requires a lot more space, food, and so on. This leads other species to move away.”

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