They can be as heavy as a horse and dive to incredible depths.

Atlantic bluefin tuna that were once rare in Scottish waters, are thought to be arriving off the Western Isles in increasing numbers.

Experts are now exploring whether fish numbers will ever be enough to sustain the kind of sport tourism seen in other parts of the world.

In the Caribbean and off North America, powerful boats with anglers strapped into their seats, giant rods secured to the deck as they wrestle with their thrashing quarry have long been a common feature.

Tourism chiefs and marine scientists are now working with local fishermen to ascertain whether there is sustainable future for both the fish and for businesses.

One man chosen to help in the survey is skipper Angus Campbell from Harris, who runs cruises out to St Kilda and provides specialist support for organisations doing marine surveys west of the Hebrides.

With support from Highlands and Islands Enterprise (HIE), Comhairle nan Eilean Siar (Western Isles Council) and European Fisheries Fund (EFF) funding, he has been working with Marine Scotland to tag tuna with satellites to track them.

He said: "Our ultimate goal is to be able to offer fishing trips for the tuna. which could extend our tourist season by may be six or eight weeks in the autumn. "We only find the fish here in September and October, but it would work in well with our Kilda Cruises, allowing us a few more weeks."

He already caught three last year, brought them alongside his boat for tagging before releasing them again.

"We didn't know if they were America fish coming over to the UK or whether they were coming from the Mediterranean. One tag came off 200 miles west of Ireland. The second fish the tag came off in the Bay of Biscay and the third came off in the Azores, which was strange as it had passed the Mediterranean.

"So was it heading across to South America, we just don't know. But they are incredible fish which can go about 40mph and the one we had measured was 115 inches, almost 10 feet and about 900 pounds. We have got the all the gear to catch them having bought it from the US. All we need now is the weather."

Francis Neat, a researcher from Marine Scotland, said: "Marine Scotland needed to learn more about blue fin tuna in Scottish waters - how many there might be, how long they reside here, where they come from, and where they spawn.

"By working with Angus we were able to satellite tag three of these giant fish last year. The information we received suggests that the tuna swam thousands of miles and dived to depths in excess of 1000 metres.

"This study is aligned with the tagging programme of the International Commission for the Conservation of Atlantic Tuna and is an important first step toward understanding blue fin tuna behaviour in Scottish waters and assessing in the longer term if a recreational catch-and-release sport fishery could be sustainable."

Mr Campbell caught a 515lb Bluefin tuna in Hebridean waters back in 2013 and as a result recently starred on the Outdoor Channel's Trev Gowdy's Monsterfish programme, a channel is watched by 39 million viewers.

During the programme, the presenter, American fishing expert Fred Lavitman, expressed his doubt about finding tuna in Scotland, before finally hooking a 500lb tuna, which broke his line.

Large Bluefin tuna have become more noticeable over recent years, chasing shoals of mackerel off the coast of the Outer Hebrides. It is thought that they are gradually moving north as herring and mackerel stocks recover and the water temperature rises.

Mr Campbell said: "After I caught the first rod-line caught tuna in Scotland, I was approached by the American TV producers. The advertising the Outer Hebrides has received from the programme is incredible. The programme featured highlights in the islands, such as Calanais Stones and Harris Tweed, strengthening the area's position to attract more visitors to benefit from the important tourism sector in these islands."