They were found in the River Colne, near Huddersfield - hugely popular with anglers - in the last few days and the Agency has appealed to the public for information to help them find the cause of the mystery outbreak.

First reports came when anglers noticed that smaller fish were dying off and then more dead fish - and larger ones - were reported a few days later. One theory is that the problem could have been caused by an industrial discharge into the river from a blocked sewer.

However, the dead fish started to appear upstream from there and by Monday it was estimated that several hundred stone loach had died, along with dozens of trout and some grayling, in the four-mile stretch of the Colne between Slaithwaite and the Aspley basin, near the town centre.

An Environment Agency spokesman said: "At first it was only the smaller fish that were hit but as time has passed the larger fish are now also becoming sick and dying.

"This suggests it is a slow-acting pollutant that has somehow got into the water, and if anyone has any evidence or information we'd welcome it."

He estimated that many hundreds of fish had died, and the toll was continuing to rise. So far the Agency has been unable to trace the exact cause, but investigations are continuing. The Agency is treating the matter as a category one incident – the most serious level for this kind of river pollution.

The news is a blow for this part of Yorkshire because only recently a once polluted stretch of the River Aire has seen the re-appearance of salmon in increasing numbers. During a routine Environment Agency fishery survey, two salmon of around three pounds were caught at Chapel Haddlesey weir, which is at the tidal limit of the Aire.

Scales from the fish were taken for analysis, and the agency's fish experts believe they will show that the fish have been away at sea for one year before returning to the Ouse system, which includes the Aire. Salmon have also previously been caught on the Aire by anglers, but these were the first to be caught by an Environment Agency survey team.

Just a couple of months ago, a large salmon weighing more than ten pounds was also caught during a routine survey carried out last year on the River Don at Sprotborough near Doncaster.

www.fishupdate.com is published by Special Publications. Special Publications also publish FISHupdate magazine, Fish Farmer, the Fish Industry Yearbook, the Scottish Seafood Processors Federation Diary, the Fish Farmer Handbook and a range of wallplanners.

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