The trout farm was found guilty of negligence and causing nuisance to members of the Savernake Fly Fishing Club in Wiltshire.
Judge Mark Dyer at Swindon county court said that the nuisance had been a substantial one and not just a fleeting incident. The farm had been negligent in not preventing escapes.
The fishing club's secretary, Roy Broderick, had brought the action following the escape of the voracious rainbow trout into the river Kennet from the farm at Mildenhall, near Marlborough, Wiltshire, in early 1990.
He claimed that they ruined 40 per cent of the 1990 season, as the hungry rainbows became a pest to fishermen attempting to catch wily brown trout.
In earlier evidence, the judge had been told by John Hounslow, the club's river keeper, that catching a rainbow trout each time you cast was not sport. 'Everybody complained that they were catching small rainbows. It was impossible for them to fish for the brown trout.'
In his ruling, the judge said that the fly fishing rights to the stretch of river had been given to the club by Crown Estate Commissioners. He said that the brown trout was considered to be the aristocrat of fish among anglers, because it was cunning and put up a fight when hooked. It was for that reason that fly fishing on the river was at a premium. Club members paid pounds 500 a year to fish there.
There had been an escape of 1,000 rainbow trout in 1989, followed by further break-outs of 300 large rainbows, followed by a further 1,000 baby fingerlings. The judge said that trout farmers should take stringent measures to protect against escapes.
The farm had compounded the risk by putting fingerlings into the same pond from which large trout had escaped. The farm had not taken adequate steps to make effective repairs.
The judge awarded special damages of pounds 10,200 against the owners of the farm, Gale and Ainslie. He also ordered the defendants to pay costs of pounds 10,000 and interest on the damages award which Simon Jackson, solicitor for the Anglers' Co-operative Association, which backed the prosecution, said would be in the region of pounds 2,750.
After the hearing Allen Edwards, director of the association, said the judgment allowed it to go ahead with similar prosecutions involving the river Nidd in Yorkshire and the river Eamont in Cumbria.