From aquarium into rivers: Invasive China kholisha, sucker fish threaten native species with extinction

For nearly a decade, Shaheen has fished the canals of Feni and Noakhali, but since last year's floods, his nets tell a troubling story. Each haul now yields five to six kilograms of almost a single species, locally called "China kholisha" or "hybrid kholisha."

"Now, whenever I throw the net, this fish comes up," Shaheen said. "Other fish hardly appear anymore. I sell it for Tk70 to Tk100 per kilogram, but people barely want it."

The invasive species has been identified as snake-skin gourami, or Thai gourami, an ornamental aquarium species native to Southeast Asia, including Thailand, Cambodia, and Laos. Highly adaptable, it survives in low-oxygen water and reproduces rapidly. 

While not toxic for consumption, Prof Md Manjurul Kibria of the University of Chittagong warned it poses a serious ecological threat. "Because its breeding capacity is so high, native species cannot survive competition with it," he told The Business Standard. 

He said the fish likely spread from Feni's aquarium nurseries after last year's floods and is now common in Noakhali, Cumilla, Feni, and Mirsharai, though it has not yet reached Kaptai Lake.

Researchers say the trend mirrors patterns seen elsewhere. Citing Kaptai Lake, they noted that smaller species like kachki and tilapia are increasing while native carp such as rui and katla are declining. "Rui and katla fall behind in competition with kachki and tilapia," he said, calling it a natural ecological outcome. 

The average gourami size has also grown rapidly. Initially small after the floods, larger fish are now common, with eight to ten weighing a kilogram. In Feni markets, prices have fallen to Tk100-150 per kg as supply surged. "We once sold it for Tk300-350," said trader Salahuddin. "Now it's everywhere, so the price has dropped."

Ornamental fish trade under scrutiny

Bangladesh has around 1,000 species of ornamental fish, according to traders and hatchery operators, with some imported and others produced locally. Common gourami varieties used in aquariums include blue, golden, kissing, pearl, and snake-skin gourami, with blue and golden gourami being especially popular. Several gourami hatcheries operate in Feni. 

Gourami fish became widespread in Bangladeshi aquariums in the late 1990s, and a single snake-skin gourami can produce up to 2,000 offspring. Most ornamental fish hatcheries are located in Feni, Dhaka, Mymensingh, and Jashore, leading researchers to suspect Feni hatcheries as the source of gourami now found in open waters. 

A Feni hatchery manager said snake-skin gourami were widely bred, often by hobbyists. Katabon trader Jahid Hossain noted the species was used in aquariums since 1995 but has since lost demand; after escaping hatcheries, its rapid breeding fueled an uncontrolled spread.

A broader invasive species crisis

Fisheries researchers warn that snake-skin gourami is only one of several invasive species introduced into Bangladesh's waters from aquariums. Others include sucker fish, croaking gourami, piranha, and alligator gar.

Among them, sucker fish posed the most severe threat. Widely found in rivers, canals, wetlands, and ponds, they compete aggressively with native species, consume eggs and larvae, damage embankments by burrowing holes up to five feet deep, and reduce water body productivity. Researchers have also detected sucker fish in the Halda River, Bangladesh's only natural carp breeding ground.

According to the Department of Fisheries, sucker fish were illegally imported from Brazil in the 1980s as ornamental fish and later escaped into natural water bodies. In response to their rapid spread, the government banned their import, breeding, farming, transport, sale, distribution, marketing, and storage in 2023.

Weak regulation, urgent warnings

Experts say weak quarantine enforcement has allowed invasive species to spread. Professor Kazi Ahsan Habib of Sher-e-Bangla Agricultural University said aquarium hobbyists often release foreign fish into natural water bodies when tanks become overcrowded. "As a result, these alien and invasive species spread rapidly, reproduce in large numbers, and damage aquatic ecosystems," he said.

Halda River researcher Md Manjurul Kibria warned that imported aquarium fish should undergo strict testing and quarantine. "But due to weak quarantine practices, invasive species like sucker fish and gourami are spreading. We are now finding sucker fish in the Halda as well. This is a serious threat to our fisheries sector," he said.

Md Anwar Hossain, acting director of the Department of Fisheries in Chattogram division, said gourami were seen occasionally in the Feni–Cumilla region before the floods but in small numbers. "Due to prolonged waterlogging, they are more common in Feni and Noakhali. They are not yet widespread in Chattogram," he said, stressing that fish imports must not be allowed without expert approval.

As invasive aquarium species continue to spread through Bangladesh's rivers and canals, researchers warn that without tighter regulation, effective quarantine, and public awareness, the country's native aquatic biodiversity faces a growing and potentially irreversible threat.