Killer whales return to cache of food days after kill
VANCOUVER - Who knew squirrels and killers whales had so much in common?
New research by a team of experts reveals that orcas in Alaskan waters leave a cache of their kill and return later to feed, like the furry forest dwellers.
The team also found that transient killer whales congregate for the annual northern migration of grey whales and cull up to one-third, or about 300, of the calves born annually to the Eastern Pacific grey whales.
The four-year research project has also established the role the orca plays in controlling the grey population, and the support their kills give to scavenging Pacific sleeper sharks and brown bears.
"This is the first time that any whales have been documented to cache prey, to store it up for the future," Barrett-Lennard said in a telephone interview Friday from Perth, Australia.
Barrett-Lennard, an adjunct professor in zoology at the University of B.C., said he was especially pleased with the findings because he earlier wrote a paper noting killer whales have a problem with larger kills in the ocean because the carcasses sinks and they aren't deep swimmers, leaving much of the kill to go to waste.
The study, by Lance Barrett-Lennard and a team of researchers, was published this week in the journal Marine Ecology Progress Series.
In it, researchers detail the behaviour of a newly-discovered transient population of killer whales on the edges of the Pacific Ocean and the Bering Sea. Normally transient killer whales are loners or travel a few to a pod, so finding 150 transients together off Alaska's coast was "mind-boggling" for Barrett-Lennard and his fellow researchers.
The whales gathered in a cove off Unimak Island, Alaska, which is the largest and most eastern of the Aleutian Islands.
They are there for the return of thousands of grey whales heading to their Bering Sea feeding grounds from the winter breeding grounds around Baja, Mexico.
Transient killer whales are one of three different populations of orcas, the others being resident and offshore populations. Most studies have been done on the resident killer whales in the waters off British Columbia and the states of Washington, Oregon and Alaska.
While resident whales eat mainly fish, transients dine exclusively on marine mammals.
Barrett-Lennard said Unimak Island seemed to be a perfection location for the killers to prey on the greys and the ocean isn't deep there, so they gorge on their catch over several days.
"For a month or more, the killer whales hunt and feed exclusively on grey whale calves and yearlings," the report said.
The grey whale's only defence is to move to very shallow water, where the killers are reluctant to press the attack.
The orcas also seem to give up if mothers aggressively defend their calves.
Researchers watched as the orcas hunted the grey whales, using stealth and specialized techniques with highly developed group co-ordination. But it's what the whales did with their kill afterwards that was especially interesting to them.
"After an initial feeding period, killer whales leave the site for 24 hours or more before returning to feed again -- the first time such food storing behaviour has been reported in whales," said the report summary.
A fully grown grey whale can be up to 15 metres long and weight 33,000 kilograms, while a baby is an average of 4.5 metres long and weights from 500 to 680 kilograms.
The stored carcasses leave a distinctive oil sheen on the water above and what's left of the carcass is often scavenged by sleeper sharks in the water and then Alaskan Brown bears when the remains wash ashore.
While researchers have only known about these hunting grounds for a few years, Barrett-Lennard said it's clear this has been going on for a long time.
The island is covered with a healthy population of brown bears, he said, and every time the killer whales near the shore the bears run down as if they're being called for dinner.
"It's perfect timing for the brown bears, they're just out of their dens, the hibernation is over, they're famished," he said. "We've seen up to 19 brown bears on a single (grey whale) carcass."
Barrett-Lennard has been working since 2002 with Alaskan biologist Craig Matkin on the killer whale research project along the Alaskan Peninsula.