Dozens of killer whales in California’s Monterey Bay were observed last week in an unusual attack on two adult gray whales, video shows.

Researchers and guests on March 30 witnessed “five hours of an attack by over two dozen Bigg's killer whales on two adult gray whales,” tour agency Monterey Bay Whale Watch said.

Bigg’s orcas, formerly known as West Coast Transient orcas, are a subtype described by killer whale researcher Michael Bigg, which feed exclusively on marine mammals and tend to travel in smaller groups than so-called resident orcas.

“Usually killer whales will hunt gray whale calves as they head up to their northern feeding areas with their moms. But these were not calves: they were huge adult gray whales,” Monterey Bay Whale Watch said.

Videos shared by the tour agency show multiple orca pods joining together for the relentless attack. “We were able to observe the unique hunting strategies of the pods and the rarely seen defensive strategies of the two Grays,” the agency said.

“The battered gray whales eventually made it to shallow water, and the orcas broke off,” the agency said.

The agency said that in more than 30 years of documenting encounters for the California Killer Whale Project, this was the first observed attack on adult gray whales in Monterey Bay.