A group of dolphin spotters had a shock when they witnessed a pod of the animals killing a porpoise for sport.
The tour company, based in New Quay, Ceredigion, said it was “morbidly amazing” to witness something so rare.
They said the “glamorised” bottlenose dolphins people think of were actually “pretty brutal”.
They believe it could be the best documented case of the behaviour, known as porpicide, which has ever been recorded.
The trip, led by wildlife guides Josh Pedley and Andy Walsh of SeaMôr Dolphin Watching, set out on the company’s regular 18:35 session on Monday with nine passengers.
Mr Pedley said they had a seen a group of about eight bottlenose dolphins on their usual route, having a typical 10-minute encounter before carrying on.
As the boat made its way to New Quay headland, the group spotted a porpoise.
“We thought ‘this could get interesting’ – half joking initially,” said Mr Pedley.
“We’ve talked about porpicide, and we hear about it, but it was almost spoken about mythically.”
Porpicide is the name given to the deliberate slaying of a harbour porpoise by bottlenose dolphins.
“There was just an eruption of white water and it was evident that the dolphins had picked up on the porpoise’s presence basically as soon as it entered the bay,” said Mr Pedley.
He explained to the boat passengers that dolphins kill porpoise for sport, not food.
“I said that we were going to see something we’ve never seen before, and it might not exactly be nice but it is such a rare occurrence that it is worth watching and documenting because you will never see it again,” he said.
“I can quite confidently say it is probably the best documentation of this behaviour to ever be recorded.”
Mr Pedley said they had “unparalleled views” of the encounter as the porpoise tried to hide under the boat.
“All the passengers were fantastic, we had the right people on,” he said.
“We had a couple on board and one of their first questions was about porpicide, so it was amazing timing really.”
Brett Stones, who owns the tour company, described the situation as “exciting but sobering all rolled up into one”, adding he had never seen it in more than 27 years of running trips.
“Everyone was just in shock,” he said.
“We see dolphins a lot, and we see porpoise a lot, but we don’t see dolphins killing porpoise. It might not be a rare occurrence, but it’s a rare occurrence to be witnessed by humans.”
The team said reaction on social media had been mixed, with some commenters suggesting they should have tried to save the porpoise.
But Mr Stones explained that would not be possible, both because it would have been interfering with nature but also because there would have been no safe or practical way to move the “slippery marine mammal”.
“It looked so vulnerable lying there, it shocks you to the core. I’m pretty unshakeable and it was almost too much for me to watch,” he said.
“You’re almost willing it on-board so you can cwtch (hug) it, but you couldn’t do that.”
Mr Pedley added: “I’m not Aquaman – I’m not sure what I was expected to do in that situation.”
Mr Stones said dolphins were intelligent, but with that came “other traits”, with people being fooled by their “permanent smile”.
“They are also a lot bigger in the UK than in Florida. When you see behaviour like that – a porpoise is 1.5m (4.9ft) long, a lot of humans aren’t much more than that,” he said.
“We have people saying they want to swim with these animals. A lot of that is coming from places like Sea World where its a caged, captured animal, not the real beast,” he said.