An "unprecedented" rise in grey whale killings by orca in California's Monterey Bay area has seen four grey whales killed in eight days.

Marine biologist Nancy Black said orca regularly hunting the grey whales was unusual behaviour and believes this particular pod may have significally evolved predatory instincts.

"Usually the killer whales come in and out. They aren't here every single day," said Black. "I've been studying them for 30 years here, this has really never happened before."

Black said a family of nine killer whales were involved in all attacks – the first of which involved 30 whales.

She added she was not surprised by the orca's increased numbers this month, as they usually see a decline in appearances in May. But she was struck by the way they have been "hanging around" waiting for an opportunity to pounce on their prey.

"We see them more often in April than May by far, but they just seem to be hanging around and waiting for more grey whales to come through. It's pretty unprecedented just because the same group of killer whales has been feeding on them each time."

Black claimed a killer whale is taught different ways of subduing its prey depending on its upbringing, and that they must adapt to killing baby grey whales in the correct manner or face the risk of a counterattack from their protective mothers.

Grey whale
Grey whales are being hunted by hoards of orcas in the Monterey Bay. OMAR TORRES AFP

"They (killer whales) learn different methods of hunting from different areas so it's passed on through the generations. And this particular group… is very good at it," Black explained. "They learn early because it's pretty dangerous for the killer whales to hunt a grey whale because the mother grey whale can slam them with their fluke."

In addition to grey whales, orca often hunt humpback whale calves, seals, sea lions, dolphins, tuna and great white sharks.

They act swiftly to separate mothers from their calves in order to avoid the power of her enormous tail, or fluke, a blow from which is strong enough to kill them.