POLLUTION experts were left scratching their heads after a mystery phenomenon killed hundreds of fish along a stretch of river in Watford.

Despite extensive tests to the water alongside Riverside Road, officials from the Environment Agency failed to establish exactly what was responsible for wiping out the scores of fish, including chub and perch.

Even more mysterious was the fact that the mystery pollutant failed to kill off small fry such as gudgeon and roach.

The agency was alerted to the problem on Monday after neighbours along Riverside Road became concerned at the smell rising from the river. They feared chemicals from nearby factories had flooded in.

Mrs Jeanette Ward said: "Our garden backs on to the river and our dogs swim in it. The children have a dinghy and are often out there. It is a major concern to us because chemicals in the water could obviously affect them, but nobody is telling us anything.

"It was exactly the same five years ago when the fish died - no one seemed to be able to tell us what was going on then either -and it's very worrying."

Three men in a boat from the agency took to the water on Tuesday afternoon with rakes and spades to collect the dead fish.

For almost two hours they carefully paddled up and down the 250ft stretch of river and their catch resulted in two large bagfuls of stinking fish.

One official said: "That was one of the smelliest jobs of the day. The fish have got their eyes popping out and they absolutely stink."

The men were also surprised that only the larger fish appeared to have been affected and only in this part of the river.They pointed out that it might have something to do with oxygen levels as bigger fish need more than smaller fish.

This clue to the mystery was also picked up by the agency, whose tests revealed oxygen levels were lower than usual but not considered enough to have caused death on such a scale.

An agency spokesman said: "We are baffled by it - whatever caused the pollution has been so diluted that we are unable to identify it. It could be anything. Often we have these situations where something gets into the water and kills the fish but we cannot prosecute because we cannot trace it."

Suggestions made by the agency as to possible pollutants include something as simple as milk or orange juice.

The official said people could have emptied a bottle into the water without realising milk is one of the most dangerous pollutants in rivers.

This is because milk mixes with the water, absorbing oxygen, unlike pollutants such as sewage or oil which sit on the water and can easily be dredged off.

Unhappy residents believe incidents like this will continue to happen as long as the river remains unpatrolled.

Keen fisherman Richard Stewart, 15, from Riverside Road, said: "You never see a dredger on the river clearing it and occasionally you see this horrible green algae-like substance on the water which is never cleaned up. The banks are overgrown and the agency doesn't seem to care, even though I have phoned them up at least four or five times.

"It's made worse by kids in the summer holidays who act stupidly and throw things like shopping trolleys and other things into the water."

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