Vancouver Aquarium researchers have discovered a unique pack of killer whales that are plundering massive grey whales with hunting methods never seen before.

Dr. Lance Barrett-Lennard says the newly discovered group of over 150 transient killer whales return to ambush grey whales every spring near Unimak Island, Alaska, as the whales migrate north to the Bering Sea.

It has long been known killer whales, or orcas, can take down pretty much any size sea mammal, Barrett-Lennard says. But this specific group has developed a unique set of strategies, including stashing carcasses of calve and yearling whales on the ocean floor and returning 24 hours later to devour the prey.

“The most surprising thing was the storing of carcasses at a consistent depth so they could return to feed,” Barrett-Lennard said.

Sitting in a small fishing boat in stormy seas, Barrett-Lennard observed groups of about 40 orcas “hang out in the background,” as packs of four to six killers attempted to separate mothers and calves.

The attackers ram mother whales and constrain them by grabbing on flukes or the tail, in battles that can last about 45 minutes. Some mothers fight so ferociously within the first 20 seconds, thrashing their tales and ramming the orcas, that the killers give up.

When a mother is separated though, almost every time, a brutally efficient death by drowning awaits the younger whale. In one hour the killers will send an eight tonne, 15-metre long yearling to the bottom.

“It’s awesome, but hard to watch,” Barret-Lennard said. “These killer whales are so good at what they do. I find myself cheering for the grey whales.”

The attack pack of four killers gets first dibs on the carcass, “but the whole mob” will return the next day and continue to feed, which is a pattern never observed in any other group of orcas, Barrett-Lennard said.

Barrett-Lennard found that this group of killer whales feeds almost exclusively on the grey whales for extended periods. The finding indicates they’ve picked up on the grey whales’ annual migration patterns and know when and where to hunt them in the perfect strategic location.

In fact, Barrett-Lennard and his colleagues predicted they would see killer whales hunting grey whales at the “choke point” of the northern migration route, off the shallow black sand beaches and crashing white surf that borders Unimak Island, the easternmost of the Aleutian Islands.

Transient killer whales are one of three distinct types of orca in the northeastern Pacific Ocean. Resident orcas eat fish and make a lot of noise during the hunt, unlike stealthy transients. Another new report says mysterious offshore orcas, which are difficult to find and study, actually feed on Pacific sleeper sharks.