(Vancouver Sun)
A third of young grey whales eaten by killer whales, scientists say
The mothers will often defend their calves and, if the mother is very aggressive, the killer whales will often give up
BY JUDITH LAVOIE, POSTMEDIA NEWSJANUARY 22, 2011
A group of transient killer whales are killing and eating almost one-third of eastern Pacific grey-whale calves born each year, a groundbreaking four-year study has found.
The research, conducted by a team of Canadian and U. S scientists, also found the killer whales are storing the carcasses of grey whales in relatively shallow water and returning to feed for several days.
It is the first time such food-storing behaviour has been documented.
The killer whales congregate around Unimak Island, Alaska, and for about one month during the grey-whale migration eat grey-whale calves and yearlings almost exclusively, even though the adult greys are much larger than killer whales and their calves are almost as large as the predators, said Lance Barrett-Lennard, Vancouver Aquarium research scientist and one of the study's authors.
"The killer whales use a bunch of techniques to separate [grey-whale] mothers from the calves," he said.
"The mothers will often defend their calves and, if the mother is very aggressive, the killer whales will give up. The very pugnacious females seem to rule," he said.
It is difficult to watch, Barrett-Lennard said, as the grey females try to get the calf to shallow water or try rolling over, nestling the calf in their flippers or on top of them.
"It's a real battle to the death and, if the female releases the calf, it's very one-sided and they kill it very quickly by grabbing it by the snout or the pectoral flipper and pulling it under until it drowns," Barrett-Lennard said.
Transient killer whales have previously been seen killing grey whales in Monterey Bay in California and off the west coast of Vancouver Island, but not in the organized fashion or in the quantities documented in the study.
Grey whales have rebounded from overhunting and the population is now believed to be between 18,000 and 20,000, so the predation is not likely to adversely affect populations, Barrett-Lennard said.
It is possible other groups of transients are practising similar behaviour, but this group appears isolated and distinct, and have characteristic welts on their bodies, he said.
The discovery that killer whales are storing the carcasses was unexpected, Barrett-Lennard said.
Usually, if they kill in deep water, they have only a short time to eat before the carcass drops to the bottom of the ocean, and killer whales are not deep divers.
"Sometimes they hold up the carcass and get a few more feedings out of it, but it's very hard work. I had speculated that another way would be to drive [the prey] into shallow areas, but we hadn't previously seen any evidence of that," Barrett-Lennard said.
The researchers found the killer whales would kill greys in depths of 10 to 20 metres and would then leave for 24 hours or more before returning to feed again.
A spinoff discovery was that killer-whale predation also provides food for Alaskan brown bears and sleeper sharks, and the populations of both are extremely healthy around Unimak Island.
"Scraps of grey whale wash ashore and the bears are on it in a flash," Barrett-Lennard said.
One observation was a whale carcass floating ashore and 19 brown bears immediately gathering around it, he said.
The research also shows the very specific tastes of different groups of killer whales, Barrett-Lennard said.
Resident killer whales like chinook salmon and a recent study found offshore killer whales like to eat sleeper sharks.
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