UPDATE, October 27, 2010
In a new development, resident killer whales have been seen toying with harbor porpoises, according to a story by reporter Larry Pynn, who writes about the phenomenon earlier this month in the Vancouver Sun.
He included comments from Joe Gaydos, who was my source on this blog post. He also quoted John Ford, a researcher with the Canadian Department of Fisheries and Oceans, who said females may be treating them like their own offspring:
“It could be a maternal-driven behavior that is misdirected towards another species. These animals (porpoises) are often sort of carried about on their backs or heads, pushed around. It’s almost like a behavior you’d see with a distressed or dead calf of a killer whale. We’ve seen a still-born calf pushed along or carried along by the mother.”
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We normally think of predator-prey relationships as being highly efficient systems with little waste. But Puget Sound researchers are finding that some transient killer whales seem to be killing sea lions for no apparent reason.
I’ve always thought that predators kill and eat what they need for survival, thus holding in check the prey population. Predators would never kill more than they need, I assumed, because they would risk eventually wiping out their food source.
Well, it’s time to rethink how some predators think.
Joe Gaydos of the SeaDoc Society tells me that transient killer whales, which eat marine mammals, have apparently killed three Steller sea lions and one California sea lion within a month’s time in Puget Sound and the San Juan Islands.
Examinations of the animals show that they died from blunt trauma, including broken chest bones and abdominal cavities filled with blood. No gunshot wounds or other complicating factors were seen.
Joe told me that boat strikes can cause similar injuries, but it seems unlikely that this many boat injuries could occur in the same precise way with no other injuries being observed. Instead, it appears likely that transient orcas killed them without even taking a bite.
“Cold-blooded killers,” I suggested. I should have asked him
about a recent incident in which a
group of transients attacked a much-loved gray whale near
Whidbey Island before letting it go.
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