'Guests' are gulping Bengal's fish

Calcutta, Jan. 5: A carnivorous African catfish is among several exotic fish species that have slipped into the Ganga and other rivers and threaten the future of traditional Indian freshwater carps such as the catla, mrigal and rohu, scientists have warned.

Researchers at the National Bureau of Fish Genetic Resources, Lucknow, say these exotic species were introduced by aquaculture farmers over the past decade in violation of the Centre's guidelines, and have dispersed into streams and rivers through various routes.

Local populations of the catla, mrigal, rohu and other native species have declined after the introduction and spread of some of these alien species, a senior scientist from the Lucknow institute told the Indian Science Congress here today.

Surveys by the institute over the past five years suggest that the catch of these native species has declined by up to 70 per cent along select stretches of the Ganga and its tributaries.

"We're witnessing a biological invasion," Atul Kumar Singh, a principal scientist at the Lucknow institute, told The Telegraph on the sidelines of the convention. "The exotic fish species spread rapidly. We now find them in the Ganga, Godavari, Gomti and elsewhere."

The African catfish, the bighead carp from China, and the Amazon pacu are among the species to have been introduced in India without an adequate risk-benefit analysis of the impacts of their entry into local freshwater habitats, Singh said.

"The carnivorous African catfish eats our native fish species, while the other exotic species are breeding and spreading faster in Indian rivers than the native species."

Aquaculture farmers find them attractive because they grow fast and in plenty, Singh said. Since their introduction, the total annual catch along sections of the Ganges has increased by about 20 per cent.

"But they are low-value fish and are replacing traditional high-value native species," Singh said.

While fish reared in aquaculture farms are expected to be isolated from free-flowing streams and tributaries of rivers in the neighbourhood, scientists believe the exotic fish have had opportunities to escape because of the carelessness of the farmers or during floods.

Two years ago, Singh and his colleague Wazir Lakra surveyed field studies of exotic fish from across the country and documented the presence of 291 ornamental fish and 31 aquaculture fish species introduced in India with or without appropriate approvals.

The field studies indicate "the widespread occurrence of some of the world's worst invasive species", Lakra and Singh had reported in the journal Reviews in Aquaculture.

Although the exotic fish possess some attractive features, they reduce the availability of local species.

The Lucknow institute has developed guidelines for rearing alien fish to minimise the risk of escape into local freshwater habitats. The sutchi catfish from Vietnam and the white-head shrimp are examples of exotic fish being currently reared under these guidelines.

Scientists say the spread of exotic fish isn't a problem unique to India. The bighead carp is viewed as a nuisance in the US, and environmental researchers in Britain had pointed out in 2005 that 25 populations of the Asian topmouth gudgeon were threatening native fish.

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