Since April 20, orcas have killed four gray whale calves in eight days, Nancy Black said Friday.
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Black, who co-owns Monterey Bay Whale Watch, says a family of nine killer whales has taken part in all of the attacks, but the first killing involved 33 orcas.
It's not uncommon for orcas to prey on gray whale calves, says Shawn Johnson, director of veterinary science at California's Marine Mammal Center. He hadn't heard about the latest killings.
Black called their frequency unprecedented.
"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."
The calves and their mothers are migrating up the California coast from Mexico. The arrival in Northern California was a few weeks later than usual this year, so a lot of hungry killer whales were waiting, Black said.
Adults in the orca family pod also might be teaching the youngsters how to hunt.
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"They (killer whales) learn different methods of hunting from different areas so it's passed on through the generations," she said. "And this particular group ... is very good at it."
The nine-member pod, dubbed Emma's group, includes a matriarch, her daughter Emma and a granddaughter, along with some juveniles, including one dubbed Little B who is less than 6 months old.
"They learn early because it's pretty dangerous for the killer whales to hunt a gray whale because the mother gray whale can slam them with their fluke," Black said.
Orcas share their prey with the rest of the group, including members that didn't take part in the hunt, Black said.
She noted the orcas in the latest attacks also killed with unusual speed. A Wednesday attack took just 20 minutes, compared with the several hours it usually takes a pod of orcas to separate a calf from its mother and drown it.
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