It has been learnt that in the low-lying outskirts of Dhaka, African catfish are being cultivated in specially constructed tubs or submerged bamboo cages in ponds and swamps. But during floods, the fish spill out from the farms and take refuge in the sewerage system, different ditches in the city, and also in the natural water bodies.

Fish traders and city residents said African catfish are being bred in ponds at Bashabo, Badda, Rayerbazar, kamrangichar, Khilgaon and Mirpur.

Fish farmers said African catfish multiplies rapidly within just two to three months, which makes them popular among fish cultivators.

"A lot of people are cultivating African catfish. If someone releases 10 maunds of fry for Tk 24,000, he will get 70 to 80 maunds of fish after three months which he will be able to sell for Tk 15,00 per maund," said a worker at Siraj Mia's fish farm at Rayer Bazar embankment.

"The size of a fish depends on feeding. The more you feed them the more they grow, ten maunds of fry usually grow to become 100 maunds of fish," he said.

When asked about what they feed the fish, workers of different farms said rotten carcasses of fowls, cow and chicken intestines, rotten food and garbage from restaurants and what not.

But fish traders said these days the city residents do not purchase African catfish due to its ugly look and food habit.

"So we send them to Cox's Bazar, Chittagong, Khulna and India," said a worker of a catfish farm at Kamrangichar, as he was feeding the fish rotten carcasses of fowls in a submerged cage.

Some traders in the city sell small African catfish passing them as native catfish as the price of native catfish is very high. The price of native catfish is around Tk 400 per kilogram while the price of African catfish is Tk 80 to Tk 100 only.

While the impact of other foreign fish species seems to have been negligible, African catfish is noticeably wiping out indigenous fish population in ponds and water bodies. It has been learnt that Piranha, another carnivorous species of fish from Africa, are also being bred in the city secretly. Both of these species even prey on humans, experts say.

Experts expressed their concern in 2004, when a 20kg African catfish was caught from a sewer at Panthapath. Sewer cleaners who caught the huge catfish claimed there were more African catfish in the sewers, hinting that the sewerage system might have become a home to the species.

Ironically, officials of the fisheries department introduced African catfish to Bangladesh. It is one of the 14 fish varieties introduced by the authorities since 1953.

A high official of the Department of Fisheries said now a days the department is not encouraging cultivation of African catfish.

"But we have not enacted any law regarding it. So officially we can not do anything if someone cultivates it," he said.