Salmon being sold door-to-door in the north west could have been illegally caught by poachers targeting the protected species, fisheries conservationists have warned.
Lower quality farmed salmon is also being sold in the area as wild-caught fish, raising concerns over hygiene and food safety.
The Ulster Angling Federation and the Loughs Agency have issued a joint appeal asking households and businesses to refuse to buy salmon that is being offered for sale door-to-door in the Foyle area as it could be the result of criminal activity.
"These may well be the product of illegal fishing and therefore represent a significant threat to the future well-being of salmon stocks in the Foyle," the statement said.
Salmon which have been legally caught in the Foyle will carry the proper black plastic tag with a number beginning 'FFA' attached through the gill and mouth.
The statement added: "However, the public must be aware that it is a criminal offence to offer for sale or purchase any such wild salmon that have been caught in the Foyle catchment.
"All salmon without a tag are illegal and fish offered headless or filleted should also be rejected in order to help protect our native wild salmon stocks.
"In addition, there are reports that inferior farmed salmon are being offered for sale as wild fish. All this raises substantial issues of poor hygiene, improper storage, and food health and safety of these suspect salmon being offered for sale."
The two organisations said they were working very hard in many ways to conserve Northern Ireland's salmon stocks to ensure their long-term survival and also for the benefit of the community.
"Continued vigilance is needed against the trade in illegal salmon so we hope the good sense of householders and business people will prevail and that salmon will be allowed to run our local rivers unhindered allowing succeeding generations to appreciate the beauty of this most wonderful of fish," the bodies said.
In recent years Loughs Agency staff have been attacked by poachers as they attempted to seize illegal salmon nets in the Foyle.
On one occasion the poachers even went as far as to drop concrete blocks onto the boats used by fisheries officers.
The Atlantic salmon is extinct or in critical condition in about one-third of the rivers in its range and is endangered or vulnerable in a further third.
According to NI Museums website Habitas, substantial decline has taken place in our rivers due to pollution, habitat degradation, overfishing, creation of barriers to migration, predation and increased mortality at sea.
Most commercial netting of salmon has now stopped due to buyouts, and anglers have been asked to practice catch-and-release in populations below their conservation limit.