Biologists are worried that hatchery-raised brown trout - which have less genetic diversity than wild populations - are escaping or being released, interbreeding with the locals, and thus lowering diversity among the natives.
There are also concerns because rainbow trout - which are not indigenous to Europe - have escaped from hatcheries, although most of these fish have been treated to make them sterile.
The Norwegian scientists selectively bred hatchery-raised brown trout until they had a strain which had a finer pattern of spots on their scales and fins than the wild fish, making their descendants easy to identify.
They released 160 modified fish into two rivers containing 100 wild brown trout and after three years determined that the two groups had thoroughly mixed.
English Nature said last week that it is worried that such inter-breeding will wipe out native fish, and has called for wild and farmed populations to be kept strictly separate.
Mary Gibson, a scientist who studied the experimental results on behalf of English Nature, said: "A precautionary approach should be taken with respect to stocking of any water containing the populations of wild, native species."
The Norwegian scientists have also warned that interbreeding is jeopardising the wild population. A summary of their research, published by the Ministry of Agriculture, said: "It is highly probable that escape and release of cultivated organisms in large numbers will affect wild populations ecologically and genetically."
The danger in Britain is that fishery owners are stocking rivers with hatchery fish to improve catches for anglers paying up to pounds 180 a day on prime stretches of streams in the south.
But Simon Barnes, chairman of the British Trout Farmers Re-stocking Association, defended the practice. "Rivers have been stocked ever since man has fished: they have to be," he said.
"With today's fishing demands the rivers couldn't sustain a stock of wild trout and the only alternative to that would be for people to fish in rotas.
"Habitats have to be improved but that's a long term measure and you'd still have to stock in the meantime."Reuse content