The coho variety of the fish had not previously been found in Denmark.
But in recent months, over 20 of the species have been caught in waters off Jutland's Djursland and in Horsens Fjord, reports news agency Ritzau.
Researchers from the Natural History Museum of Denmark in Copenhagen, who identified the fish, will now attempt to find out where they originated, according to a press statement released by the museum.
“Reports of unusual-looking trout or salmon from several areas in eastern Jutland have flourished on the Internet over the last few months,” Henrik Carl, project leader with the museum's Fish Atlas (Fiskeatlas) project, told Ritzau.
“We have therefore collected some of the fish from the amateur fishermen that caught them. Thorough investigations have confirmed our suspicions that we are dealing with coho salmon,” Carl said.
The salmon species is farmed at several locations in Europe, making it probable that the animal has slipped the net at such a farm, writes Ritzau.