The Dorset Wildlife Trust is urging anglers not to kill and Bluefin tuna they catch off the south coast of England.
Numbers of the endangered species have been sighted around Portland Harbour, and several are reported to have been landed by both spear anglers and rod and line fishermen.
The Trust's Peter Tinsley said: "It would be irresponsible to intentionally kill one of these fish and it would be sad to see another killed in Dorset."
The Southern Inshore Fisheries and Conservation Authority also urged anglers to release any they catch.
The largest of the tuna fish, the bluefin, has been known to reach about 4.3m (14ft) in length and to weigh 800kg (1,800lb).