Sainsbury's has announced it is moving to a traffic-light system for its fresh fish to indicate the sustainability of each species.
By the end of the year, it says only amber and green-rated fish will be on sale in its stores. It has already stopped selling skate and huss, because they are threatened species.
Reports suggest the move has cost the supermarket as much as £1.5m in lost sales.
Suppliers will be required to prove that they are fishing responsibly. Any fishery with Marine Stewardship Council accreditation would get instant green rating, allowing suppliers to skip detailed questions.
An amber rating gives suppliers the go-ahead,
provided corrective action is taken. Red is a closed door, Sainsbury's said.
It admitted that getting the right data from suppliers would be a challenge and has promised to resort to independent specialists where information was not up to scratch.