ENVIRONMENT specialists are still in the dark over what killed hundreds of fish in the River Colne.
Fish along a four-mile stretch of the river between Slaithwaite and Aspley started dying over the weekend.
Initial investigations showed the problem could have been caused by a discharge into the river from a blocked sewer.
But then fish started dying upstream.
By Monday it was estimated that several hundred stone loach had died, along with dozens of trout and some grayling.
The Environment Agency believes a slow-acting pollutant had got into the water near Slaithwaite – but cannot pinpoint where.
The agency is treating the deaths as a category one incident, the most serious level for this kind of river pollution.
A spokesman said: “The situation in the River Colne is improving, and the number of fish dying on a daily basis has dropped.
“Although a significant number of trout have died there has not been a total wipe-out of the species in the river.
“We are still unable to trace the source of the pollution, which we believe started last Friday somewhere near Slaithwaite, and we would welcome any information from the public on this.”
Paul Dukinfield, 35, and his son Jamie, 11, had been fishing in the river at Milnsbridge on Wednesday.
They decided to return home to Paddock after failing to catch anything in two hours.
Mr Dukinfield said: “We were stopped by a man who said that all the fish in the river right up to Slaithwaite were dead.
“On a half-mile stretch of the river we counted 96 dead trout and roach. This was a very distressing thing to witness. The sight of all those dead fish brought a tear to my eye.
“I hope that the culprits of this devastating act are brought to justice as soon as possible.
“Those responsible have ruined a very popular spot on that river.”
Anyone with information is asked to call the Environment Agency Hotline on 0800 807060 or email firstname.lastname@example.org