When surfers in Cornwall saw a huge, iridescent bluefin tuna rising from the water almost in touching distance, they screamed with delight.
This sight is becoming more common in British waters due to climate change, conservationists say - and they are asking for the endangered fish to be protected.
Currently, EU countries which have a bluefin tuna quota, including Spain and France, can hunt the fish in British waters due to a loophole - when the rules were created, they so rarely ventured this far north that there was no point making our seas exempt. British fishermen are not allowed to catch and sell the fish.
Now, there are calls to make our waters a sanctuary for the "tigers of the sea", which are persecuted everywhere they swim due to the high price of tuna flesh. One large bluefin can sell at auction for over £1 million - but the stunning predator is at risk of extinction.
Senior Government sources told The Telegraph they "absolutely love" this idea for when Britain leaves the EU and that they are looking into making our waters a haven for the rare animals.
Megan Hemsworth, a surfer who took the amazing shot, told The Telegraph: "I was shooting on the land with a zoom lens, the tuna jumped out and all the girls surfing were just screaming with delight and taking it in, I got back on the camera and luckily it did jump again so I got the shot. All the girls in our group of surfers who saw it jumping in front of the group were blown away by how large the fish was."
She said she had never seen one in the flesh before, and that local fishermen had seen them out to sea, but never quite so shallow.
Ms Hemsworth added that at first they thought it might be a dolphin, explaining: "It definitely seemed bigger than a dolphin, we thought it was a big dolphin at first but it was very big and very aggressive in how it moved and then when we looked at the photo we realised it was a tuna."
They have only recently come back to the UK after they disappeared 40 years ago due to overfishing. According to scientific reports, warmer waters due to climate change have lured some back. In the 1930s, the species was a common sight in the seas off Scarborough and was highly prized by big-game fishers. Now, sightings off Cornwall have become more common and in the last year, tourist boats have started taking people out on tuna-watching expeditions. This year also marked the first tuna sightings off parts of the Sussex coas
The Blue Planet Society, which was founded to protect the rare creatures of the sea, is pushing for the UK Government to protect the rare fish, as we would an endangered mammal.
John Hourston, founder of the group, told The Telegraph: "There's a misconception they're protected in UK waters because our fishermen don't have quotas for them. But there are quotas for EU countries and if you have one you can wander into UK waters and catch them."
He added that the fish should be given the same respect as a lion, as it is a beautiful and endangered predator.
The campaigner explained: "This is a globally endangered fish, it is wildlife, it has returned to British waters, this is like lions arriving suddenly, it's back in British waters after a long absence and everyone's clamouring to kill them. The anglers want to catch them for sport, and youve got the fishing industry clamouring to catch and sell them."
Mr Hourston said the UK should be "world leading" and protect the fish in our waters: "We want the UK to be the first country in the world to give them full protection as you would an endangered land animal. If this was a Scottish wildcat you wouldn't allow people to go round catching them for fun would you. We need to be giving marine life parity to land life. When has a large wild endangered animal come back into the British ecosystem. We need to protect this animal which is persecuted wherever it goes.
"It would be world leading and really quite innovative of the British Government to say right here's an opportunity, and it's only an opportunity because it's come back. We need to give them a safe haven in British waters."
A government spokesperson said: "We have a strong track record in championing sustainable fishing and we are committed to ensuring that effective international and domestic management measures are agreed for Bluefin tuna.
“We are aware of increasing sightings of this iconic species in UK waters - and are giving full consideration to what measures are necessary to give them sufficient protection in our waters.”