Reports of a large number of fish dying in a river in West Bengal's Jalpaiguri district have raised concerns about the toxicity of the water, particularly as it joins the Teesta river further downstream. “The reports of the deaths of large numbers of fish in the Karola river first came in on Monday morning. We immediately began disposing of the dead fish. All the fish found floating in the waters as well those found by fishermen have been disposed of,” District Magistrate Smaraki Mahapatra told The Hindu over telephone on Tuesday. Ms. Mahapatra said samples from the Karola had been collected and were being analysed by the State's Pollution Control Board (PCB) to determine the reason for the death of the fish in such large numbers. The part of the Karola river that flows by Jalpaiguri town has been contaminated, but the upstream parts of the river have not been affected, she said, adding that water supply to the town will not be affected. On concerns that the toxic waters from the river will run into the Teesta threatening the water supply of a large part of north Bengal, Ms. Mahapatra said that was unlikely to happen as the volume of water flowing in the Karola was not comparable to that of the Teesta. Besides, the flow of the Karola is very less in this season, she added. “Experts of the PCB and other local experts have told us that it is possible that the toxicity of the water may have gone up leading the oxygen level to dip, and hence become insufficient to sustain the fish,” Ms. Mahaparta said. She said it was possible that the water was contaminated by pesticides that may have overrun from the fields near the river. Superintendent of Police Sugata Sen said the police were conducting investigations into who or what may have caused the waters to turn toxic.