Thousands of dead fish were found on the banks on the Brahmaputra river, raising suspicions that fishermen may have used explosives and chemical fertilizers to net a large catch.

The district administration has cautioned people against consuming the dead fish for fear of poisoning after they were washed ashore at the Kacharighat area of Guwahati under mysterious circumstances, official sources said.

"Since yesterday the banks of the Brahmaputra had been lined with dead fish," they said.

"This is an unnatural phenomenon. Something is happening, but without tests, we can't say anything. It is new to us," said forensic scientist Dr D J Hazarika.

District officials have rushed to the spot and the Pollution Control Board of Assam, Forensic Laboratory, Guwahati Municipal Corporation and the Directorate of Fisheries have been asked to probe the matter.

The Pollution Control Board and other agencies have already collected water samples from the river and started an inquiry.

A senior district official said that it could either be a case of poisoning and water pollution or fallout of an explosion to catch fish in large numbers.

Miscreants often use explosives and chemical fertilizers to net a large catch in the shallow waters of small rivers or streams during winter, but it is usually confined to small water bodies and has probably not taken place in the Brahmaputra before, the official said.