Mystery of poisoned fish is solved

ONE OF Britain's worst ever freshwater pollution incidents was apparently a natural event caused by algae, the Environment Agency said yesterday.

The Government's green watchdog said a population explosion of a microscopic, single-celled plant called a diatom wiped out a fish farm and hundreds of thousands of coarse fish along two miles of the Kennet and Avon Canal in Berkshire. The costs of the damage, investigation and clean up are around pounds 1m. When the diatoms died they leaked huge quantities of starch- like polysaccharides. These are not themselves toxic, but they appear to have clogged the gills of the fish and prevented them taking oxygen from the water.

The agency is convinced that this, rather than some man-made pollutant, was to blame for the incident which began just over three weeks ago. the contamination flowed from the canal into the River Dun and into the Berkshire Trout Farm three miles downriver. There it wiped out 150 tonnes of young trout worth about pounds 500,000.

Back in the canal, fish continued to die as the contamination spread. The levels of "toxicity" fluctuated which, said the agency, ruled out a single chemical spillage as the cause.

No mammals, birds, insects and other invertebrates appear to have been affected, and the agency believes the contamination is harmless to people. The agency could only find one other case where suffocation due to polysaccharides killed large numbers of fish. That was in New Zealand in 1994. ''We're on the verge of something quite new here,'' said Geoff Brighty, the agency's head of environmental toxicology. ''It has been a big surprise.''

Michael Stevenson, owner and manager of the trout farm, is not convinced. ''I just don't believe it,'' he said. While the later stages of the fish kill may have been due to algae, he believes that it began as a man-made disaster - although he will not say who or what he suspects was behind it.

The agency will pour large quantities of corrosive hydrogen peroxide into the canal over the next few days to break up the polysaccharide molecules.