This “fearsome beast” has been found washed up on Black Isle beach, prompting concerns that the waters around the Highland coast could be warming up.
Gwyn Phillips 60, of High Street, Rosemarkie, discovered the 3ft long tuna while walking his dog on the village shore.
Tuna appear to be becoming a more common sight around the Scottish coast, and Marine Scotland has started researching how many there are and why they are here.
Mr Phillips, who runs a yacht business, said: “I usually walk the dog along the beach to the first set of caves every day.
“I noticed it on Wednesday, I didn’t know what it was to start with, it was just this large fish. It looks quite a fearsome beast.
“The back end is all eaten away. It is about 3ft long, quite bulky and just lying there above the high water mark, so it will stay there for quite a few days until the tide comes up.
“I took a photograph because I didn’t know what it was. I was curious because I have never seen anything like it around here.
“I was at a British Legion meeting the other night and showed the photo around, and everyone said, ‘oh, that’s a tuna’.”
A Marine Scotland spokesman said: “There is currently little information about the blue fin tuna and its movements around the Scottish coast, and Marine Scotland wishes to obtain sufficient data to determine the numbers of this species seasonally visiting Scottish western waters.
“Three blue fin tuna fish are reported to have been tagged so far this year. The satellite tags utilised will stay with the fish for up to a year and then automatically detach. Marine Scotland will then analyse the data and consider the next steps in developing the project.”
The first bluefin tuna caught in Scotland was by Angus Campbell of Kilda Cruises, off the coast of Harris, in September 2013.
Tuna are usually found in warmer water, but it is thought they are gradually moving north as herring stocks recover.
Mr Campbell said: “They are becoming more and more common. We are seeing quite a lot of tuna fish. 2012 was the first year we saw them.
“It is hard to say why, that is why Marine Scotland is doing research to find out where they are coming from.”