Environment Agency bosses have begun a criminal investigation after identifying the source of the pollutant which resulted in thousands of fish dying in Doe Park reservoir at Denholme.

It comes as the agency warns local residents and businesses to dispose of chemicals carefully and not to pour them down drains.

The agency was called to the reservoir six weeks ago after reports of dead and distressed fish at the site, and found about 2,500 fish, including roach, bream, perch and pike.

Fisheries officers said they believed the cause of the pollution was organic matter that had been washed into the water through flooding.

Ten barrels of hydrogen peroxide were pumped into the reservoir to help break down organic matter which can cause poor water quality. At the time oxygen levels in the water were found to be good.

Shortly afterwards the agency said its investigations, along with support from the local community, had revealed the potential source of the contaminant.

Louis Harvey, agency environment management team leader, said: “We have identified the source of the pollutant which caused this serious incident, and have now started a criminal investigation.

“Levels of pollutant in the reservoir have decreased greatly, and we have been keeping Bradford Council’s Environmental Health department informed. Our staff continue to check the reservoir regularly.

“It’s important that chemicals are disposed of carefully, and not poured down drains. Residents and businesses wanting to dispose of chemicals should contact their local authority, which may be able to help or advise.”

Bradford Council, which operates a water activities centre at the reservoir, was forced to postpone an open day as the centre remained closed for three weeks. It has since reopened for activities and the rearranged open day has been held.

The reservoir is a compensation reservoir and is used to top up river levels.