Video by Josh Pedley. Warning: Some people might find this video disturbing.
A group of people on a dolphin watching tour around New Quay, Ceredigion were left shocked when they saw the animals viciously attack a porpoise.
SeaMôr Dolphin Watching had taken a group of dolphin spotters out on the water when they witnessed for the first time what has been described as a "porpicide".
Josh Pedley, who was leading the boat tour, described how rare it is to see this sort of attack, saying: "It is a behaviour I had always (morbidly) hoped to encounter as the concept is fascinating."
He said the moment could be "some of the best documentation of this behaviour to ever be recorded."
The pod of eight bottlenose dolphins repeatedly chased, slammed and threw the porpoise out the water for over ten minutes.
The behaviour is said to have been for "fun or enrichment," rather than for food.
Porpoises appear similar in appearance to dolphins, but they are more closely related to narwhals and belugas than dolphins.
The harbour porpoise is one of six species of porpoise and is one of the smallest marine mammals.
Describing what the group watched, Josh said: "The porpoise stood no chance. The dolphins eventually got bored and we watched on in disbelief as the porpoise took its last breaths along side our boat."
He said: "Sad for the porpoise, but an incredible wildlife moment to witness and possibly with it some of the best documentation of this behaviour to ever be recorded."