Rupert Kirkwood, 60, was out kayaking when he saw the super expensive bluefin tuna in a 'feeding frenzy' with the whales and dolphins.
Bluefin tuna, which are also known as giant tuna, are the most valuable species of fish in the world and have been known to sell for millions in Japan - just last year one sold for a whopping £2.5m.
It's illegal to catch the fish in the UK unless you are licenced by the Fisheries Administration.
Rupert was about three mile off the coast of Plymouth in Devon and says he's never seen so many tuna that far east in British waters.
He said: "There was a constant roar of a tuna feeding frenzy for about two hours. I saw about 20 different feeding frenzies over that time.
"They pin the shoaling fish - sand eels and herring - against the surface then hit them from below with such force that the tuna often breach the surface themselves.
"If they don't breach the surface there's a huge splash of spray, it's much more violent than a dolphin breaching the surface. They will be travelling two or three times the speed of a dolphin.
"They're about the same size as a dolphin. There were dolphins in the same feeding frenzy.
"They were Atlantic bluefin tuna, also known as giant bluefin tuna. They are the biggest tuna species in the world.
"They've been seen down in Penzance the last couple of weeks, I saw a handful at Fowey the other day, and I've seen one as far east as Torbay.
"I'm not aware of anyone who has ever seen this sort of remarkable sight, this level of activity this far east, near Plymouth.
"I certainly never have, I've seen the occasional flurry and tuna jumping - but getting a photo of them is virtually impossible because they are so fast.
"The reason I did is because so many were erupting from the surface."
Rupert reckons he was able to make the incredible sighting because he always makes sure to pay attention, whereas many people who go on wildlife tours end up spending too much time on their phones.
He added: "There's a lot of boats that go out to the Eddystone, I spend all my life looking.
"People in the boats don't notice these because they are so busy looking at their phones.
"I think people don't believe what I see but it's because I'm looking the whole time. That's the wonderful thing about a kayak.
"That day it was so calm that I could hear porpoises puff in the distance, for about three hours I heard almost constant porpoise breathing, splashing dolphins, splashing tuna, and breathing minke whales - it's unbelievable."
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