COIMBATORE: After the rejuvenation project, most of the lakes in the city look nothing but spectacular with lots of amenities for the visitors to unwind. But don’t be hoodwinked by the outer beauty. These lakes are increasingly dominated by invasive fish species, threatening aquatic biodiversity and disrupting ecological balance.According to reliable sources, invasive species like African catfish, tank-cleaner fish, Tilapia (Jilebi) and common carp (Catla) today outnumber indigenous fish species, leading to long-term ecological imbalances.Experts point out that invasive fish could displace native species for food and habitat, prey on indigenous fish and other aquatic organisms, and alter food chains. The presence of pollutants, including heavy metals entering lakes through sewage inflows, adds another dimension to the problem, raising concerns about contamination within aquatic food webs and the potential accumulation of toxic substances in fish.Environmentalists are, hence, seeking measures to regulate the spread of invasive fish species and control fishing practices that have also led to the dominance of exotic varieties.They also raise concerns over the continued sale of African catfish in the city despite a ban on the species. This, they say, speaks volumes for the gaps in enforcement. “The civic body should pitch in to restore native fish populations.“During TOI’s visit to Ukkadam Periyakulam, Selva Chinathamani and Perur Periyakulam, it’s noticed that African catfish is among those kept for sale, with Tilapia being the dominant breed.The Tamil Nadu Wetland Mission has identified and listed Tilapia, Mosquito fish, Guppy, Sailfin Catfishes/Sucker mouth, African Catfish and Common Carp as major invasive species recorded in and around the wetlands in the state.R Manikandan, founder of Kovai Kulangal Pathukapu Amaipu, says all tanks linked to the Noyyal river system are filled with exotic species. “Tilapia could be seen even at the foothills of Kovai Kutralam. Many people aren’t aware of indigenous species because of exotic dominance. The govt should survey all the waterbodies and reinstate indigenous varieties.“Pointing out that Kurichi lake had crabs some 30 years ago, he says there’s none today. “The exotic and invasive fish species will prey on these organisms, breaking the aquatic food chain.”According to a researcher at the World Wide Fund for Nature, most of the lakes in the city are dominated by tilapia, an exotic breed. “It could survive when the dissolved oxygen is at least four, or even less. Ecologically, it is a threat.“Some bird population observations over the period will reveal the availability of its prey, he says. “For instance, darters could consume only smaller fishes. Tilapia could grow up to five kilograms, compared to the smaller native species around 10cm. So, if these bird sightings have been reduced, it means the availability of their prey has declined. Similarly, there have been no major biomagnification studies to prove that these contain heavy metals or toxic elements when consumed.”Another concern is the introduction of Gambusia, commonly known as mosquito fish, into water bodies to control mosquito larvae, even though studies have identified the species as invasive and warned of its adverse effects on native aquatic ecosystems.S Raja, assistant professor, department of zoology, Kondunadu College of Arts and Science, emphasizes on the need for a stock assessment in lakes to analyze the dominance of exotic species. “There have been studies that document the presence of varieties of exotics and their equivalent indigenous species, but a stock assessment will provide the ratio of the species in a particular waterbody. The invasive varieties actually create ecological stress, creating a gap in the food chain.“
