More than 1,500 dead fish have been found floating in the river, lifeline of India's northeast this week, fuelling fears that toxic chemicals are being used by fishermen.

Groups of fishermen sometimes use explosives and toxic chemicals to net fish in large quantities, authorities said.

"We have already collected water samples and some dead fish for a thorough investigation and tests," said a state pollution board official, asking not to be named because he was not authorised to speak to the press.

The state forensic science laboratory is examining the fish.

"This is something unnatural and we can be sure only after proper tests," D. J. Hazarika, a scientist at the laboratory said.

Locals in the area said they believed water pollution in the Brahmaputra, not poisoning, was behind the fish deaths.

Authorities have cautioned people against eating the dead fish.

The 2,906-kilometre (1,816-mile) river -- one of the longest in Asia -- traverses Tibet, India and Bangladesh before emptying into the Bay of Bengal.

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