A council in Cheshire is investigating rogue door-to-door fish sellers who are conning residents.
High-pressure techniques are being used to encourage people to part with their cash for sub-standard seafood, Ellesmere Port and Neston Council said.
One victim in the Whitby area paid £400 to a door-to-door fish seller who left him with poor quality fish and much less than he thought he had paid for.
An investigation is now under way into the seller by trading standards.
The council said that some reputable fishmongers do trade door-to-door with regular customers, but urge caution.
Lyn Collins, the council's environmental protection unit manager, said: "I would urge residents to be cautious before buying fish from door-to-door sellers and would recommend that people should buy from suppliers known to them."