The red-eared slider, an aggressive non-native turtle, appeared in large numbers in Dhaka’s Hatirjheel lake after heavy rains last July, raising fresh concerns about the capital’s fragile waterways, already under pressure from invasive sucker fish.

The threat is not confined to Dhaka. Thai gourami, known locally as “China khalisha”, has spread through water bodies across the country, joining banned piranha and African catfish.

Experts warn that foreign species bred for home aquariums are increasingly entering Bangladesh’s natural waterways, threatening the biodiversity and food chain of the Ganges delta. Without action, the government could face costly efforts to contain them, along with possible public health risks.

Changing urban habits have helped drive the spread. Exotic fish, once mostly kept by wealthy households, have become popular with the middle class. When they outgrow their tanks, breed or owners move house, they are often dumped in nearby lakes, drains, canals and ponds. From there, they can enter the country’s wider river system.

Sucker fish show how quickly an aquarium species can spread once it reaches open water. Popular because they eat algae and organic waste, they have lived in the Buriganga River for nearly two decades now. They breed rapidly and have spread from the Buriganga to the Shitalakshya and Balu rivers and to water bodies elsewhere in the country. Piranha and African catfish also spread into water bodies across the country before the government banned both species.

Thai gourami are now spreading rapidly in Dhaka and other parts of the country. Local markets are also selling them for consumption. Reproducing quickly and their ability to survive harsh conditions allow them to compete with native species and prey on fish and their eggs, putting biodiversity under stress.

The effects can spread through the Ganges delta, which contains 20 to 25 agro-ecological zones where native aquatic life has maintained an ecological balance for thousands of years, said Dr Md Moniruzzaman, a professor of geography and environment at Jagannath University.

“Non-native species disrupt these systems through aggressive feeding and other behaviour, pushing indigenous species towards extinction,” Moniruzzaman told Bonik Barta. “Native fish depend closely on local plants, animals and agriculture. So the loss of one species can affect the wider food chain. But there’s still not enough public awareness of the issue.”

Experts say invasive species can destroy native habitats as well as alter feeding and breeding patterns. By eating local fish and their eggs, they reduce native populations and upset the balance of local ecosystems.

The range of invasive species is also widening. Dr Niamul Naser, a professor of zoology at the University of Dhaka, told Bonik Barta: “Piranha and African catfish once spread widely through Bangladesh’s water bodies. The government then banned both species. New invasive species such as Thai gourami, alligator gar and non-native turtles are now spreading through open waters.”

The full damage may take years to become clear. But if these species continue to spread at the current rate, numerous native fish could be at risk within two decades, Naser warned.

He called for an immediate ban on imports of invasive aquatic species. “There are many native fish species suitable for aquarium keeping. Non-aggressive exotic species can also be kept. But the government can’t remain passive as fish and other aquatic species that threaten biodiversity and ecological balance spread through the country’s waters.”

The aquarium trade is not the only way these species reach the wild. Wildlife trafficking is another source, according to wildlife experts. They said Bangladesh is used as a major wildlife trafficking route. Traffickers bring various species into the country, some of which are sold in domestic markets while others are smuggled across the border into neighbouring countries. During law-enforcement raids, smugglers operating in Bangladesh sometimes dump exotic animals into nearby water bodies to avoid arrest, potentially releasing foreign fish, turtles and other non-native species into the ecosystem.

A Forest Department raid in Dhaka’s Mirpur area on July 3 illustrates the scale of the trade. The Wildlife Crime Control Unit (WCCU) recovered 1,104 wild animals, including 846 red-eared slider hatchlings and 56 common snapping turtles, both invasive non-native species.

Unit officials said smugglers bring the turtles into Bangladesh from China and Thailand through Dhaka airport to supply a growing aquarium market among middle- and upper-class urban households. As the animals mature, owners often release them into nearby ponds, lakes and other open water bodies.

WCCU Inspector Asim Mallick said: “Besides Hatirjheel, red-eared sliders have been spotted in several ponds at Dhaka University as well. This omnivorous turtle could pose a serious threat to native aquatic ecosystems if it spreads into rivers, canals and other water bodies.”

The common snapping turtle poses a greater threat, Mallick said. Adults can weigh up to 40 kg, compared with about 6 kg for red-eared sliders. “Both species are omnivorous and highly harmful to local biodiversity.”

Governments in Japan and Australia spend heavily to control non-native species that prey on native wildlife and destroy habitats, said Professor ASM Saifullah of Mawlana Bhashani Science and Technology University’s Department of Environmental Science & Resource Management.

He told Bonik Barta: “Some foreign species are so aggressive they prey on native wildlife or destroy their habitats. That’s why Australia banned tilapia and is working to eradicate it. Japan spent nearly a decade controlling mongooses introduced from Bangladesh. Bangladesh must act now to curb invasive species and protect its ecosystems before they pose a severe environmental threat.”

Md Masudur Rahman, a wildlife research officer with the Forest Department’s Wildlife and Nature Conservation Circle, Dhaka, told Bonik Barta: “Invasive plants and animals pose serious threats to the local ecosystems. The Forest Department regularly conducts raids to stop the smuggling of exotic fish and other animals. It also raids markets to seize such animals and take action under the law. Stopping the entry and spread of invasive species remains critical to protecting domestic biodiversity.”