The aerial footage was taken by passengers on a Monterey Bay Whale Watch trip on April 30 off the California coast. As can be seen in the video, the orcas start to make their move toward the calf, encircling the adult and her baby. The battle lasted about an hour, according to the onlookers.
The mother thinks quickly and swims with her calf toward shallower water closer to shore, which orcas tend to avoid. Local authorities said the calf was hurt during the attack but should survive its injuries.
Gray whales typically migrate north from Mexico each spring. When they swim through Monterey Bay, the deep waters set the scene for dangerous encounters.
It's not uncommon for orcas to prey on gray whale calves, said Shawn Johnson, the director of veterinary science at California's Marine Mammal Center.
Nancy Black, who co-owns Monterey Bay Whale Watch, says killer whales are on an unprecedented killing spree in the bay, attacking and feeding on gray whale calves.
"Usually the killer whales come in and out. They aren't here every single day," she told The Monterey Herald. "We see them more often in April than May by far, but they just seem to be hanging around and waiting for more gray whales to come through."