Marine experts say rarely seen Risso's dolphins spotted near Anglesey are one of the largest pods ever witnessed off the Welsh coast.
More than 50 of the marine mammals were sighted at the weekend in the sea between Point Lynas and Porthwen.
Sea Watch Foundation director Dr Peter Evans said he came across several groups on Saturday and Sunday.
He described the numbers gathered in Welsh waters as "exceptional".
The dolphin species is not a common visitor to UK shores, with only a handful normally seen in this area every year.
"They do come to the same areas every year - but normally there are no more than about 10 or a dozen at most," said Dr Evans.
The Risso's dolphin is the largest of the species, with adults appearing almost white, which has seen them sometimes mistaken for killer whales.
There were also large numbers of younger animals in the groups, which are a darker grey and brown.
"These were engaged in lots of playful behaviour, swimming in a tight bunch often in direct contact with one another," said Dr Evans.
The mammals feed on squid, octopus and cuttlefish, which experts believe are the clue to why so many of the dolphins have appeared.
"I imagine there is plenty of food around at the moment, and that is what has attracted them," he said.
The sightings will help provide vital scientific data for the Sea Watch Foundation and researchers, as the migration patterns of the dolphins are still shrouded in mystery.
Some images snapped by Dr Evans show known individual dolphins, identified by scars, and will be compared with photographs of Risso's dolphins seen recently near the Isle of Man.
Dr Evans said it will help establish the movements of the groups over the autumn months, before they disappear into deeper westerly waters for the winter - returning to the Irish Sea in the spring.
He also urged anyone spotting dolphins off the Welsh coast to let the Sea Watch Foundation know, so they can continue to monitor and help protect dolphin populations.