Traces of the anti-depressant Lofepramine have been found in pike on the Grand Union Canal.
Scientists studied fish samples taken from 39 rivers and waterways close to sewage works, to test for the presence of anti-depressants and other drugs.
Ninety per cent of them were found to be positive, Fisheries Minister Ben Bradshaw said in reply to a parliamentary written question earlier this year.
"Pike are a predatory fish and so exhibit the highest levels of any residues likely to enter the food chain," he said.
