Super pod of 1,500 Risso's dolphins spotted off Monterey Bay

The large mix of adult and juvenile dolphins was probably the result of several pods coming together and swimming south, she said.

A huge gathering of Risso's dolphins was spotted off the coast of California on Friday, with a drone capturing the incredible sight of the dolphins cavorting around a boat.

Captain Evan Brodsky, with the Monterey Bay Whale Watch, said they were on the horizon as far as he could see.

"They were just having a great time. So they were breaching everywhere ... tail slapping, coming right over to the boat. They looked like they were having a big party," said Colleen Talty, a marine biologist on the trip to track the annual migration of gray whales heading to breeding lagoons off Mexico's Baja California coast.

Risso's dolphins typically travel in groups of only 10 to 30 animals, according to the National Oceanic and Atmospheric Administration, making this sighting particularly rare.

The large mix of adult and juvenile dolphins was probably the result of several pods coming together and swimming south, Talty said.

The Monterey coastline is a particularly ideal location to spot the dolphins because they prefer extremely deep water.

The area's underwater submarine canyon means they may swim much closer to shore than elsewhere along the California coast, Talty said.

The team did not initially grasp the sheer size of the super pod that appeared on Friday morning, thinking there were only several hundred dolphins.

"Once we put the drone up, I was just blown away ... I kept saying, 'Look at my screen. Look at my screen. Look how many there are'," said Capt Brodsky.

"It just blows my mind every time. It never gets old."

Ms Talty described one particularly memorable moment on Friday in which she stood at the front of the boat and looked down to see several dolphins turning on their sides and staring straight back at her.

As the dolphins continued to interact with the boat, the team drove straight ahead, making sure to stay out of their way.

"You don't want to make any abrasive manoeuvres and potentially injure an animal. That's of course the last thing you ever want to do," Capt Brodsky said.

"So we just cruise along with them. It was an incredible experience."