Transient orcas may be leaving uneaten food behind

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.”

—–

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.

Observers say transient orcas often begin their attacks on seals and sea lions by bumping them, sometimes even throwing them out of the water.

This kind of excessive killing behavior has been seen in other predators enough times to be given a name: “surplus killing” or sometimes “surplus predation.”

An entry in Wikipedia says surplus killing has been associated with weasels, wolves, orcas, red foxes, spotted hyenas, brown bear, lynx, and mink, as well as spiders, zooplankton, damselfly naiads and predaceous mites.

In 2005, Gaydos and others published an article in Northwest Naturalist, which described five harbor seal pups found decapitated with bite marks from a killer whale, but the animals were not eaten. That was chalked up as a likely case of surplus predation.

Why predators act this way remains somewhat of a mystery, but I have heard of examples in Kitsap County of foxes killing all the chickens in a hen house and cougars killing every goat in a barnyard. One explanation I’ve been given by wildlife biologists is that predators get worked up during a kill and may no idea that they have exceeded their needs.

“This is something that scientists have been asking for a long time,” Joe Gaydos said. “At one time, it was considered aberrant behavior, since it does not seem to have an ecological benefit.”

For killer whales and perhaps other animals, it may at times be a matter of training for the younger animals if prey can be easily taken.

More than a few cat owners know how their pets will bring them a mouse or a bird as a gift with no intention of eating it. Their hunting instinct just seems to take over.

Joe said the killer whales may kill to share the food, since it is well established that fish-eating Southern Resident orcas will share food with members of their pod.

One theory developed in Australia is that predator-prey relations that have evolved over millennia may be more balanced, whereas new predators in an ecosystem may kill more than they need.

A study described in Ecos magazine suggests the dingo has evolved in Australia, and its total predation may be limited by its hunting style, preferred prey size and sparse numbers.

On the other hand, the red fox, which came to the island ecosystem just 140 years ago, has been known to kill many more animals than needed for food. As pointed out in the article:

“In one case, a single fox gained access to the Heirisson Prong conservation site at Useless Loop in Shark Bay and killed more than 100 of the 350 burrowing bettongs in three months before it was destroyed. This occurred despite the presence of many rabbits, the fox’s normal prey.”

I asked Joe if he believed the surplus killing might be a common, but undetected, practice among transient orcas. He said he did not know, but marine mammal stranding networks have collected lots of information about dead seals, sea lions and porpoises. No pattern has ever been noted, he said, but by looking back over the records it might be possible to see whether strikes by transients could have been an unidentified factor.

Amy Traxler, coordinator of the San Juan County Marine Mammal Stranding Network, credited local beach-walkers with rapid reporting that helped the researchers.

“Private citizens can make a difference and contribute to science,” Amy wrote in a note. “We rely on the public to let us know where these stranded animals are. The faster we get to them, the better chance we have of determining cause of death and learning important information.”

To report a dead or injured animal, one may call the national hotline, 800-853-1964. For Kitsap and Pierce counties, the direct pager number is 253-589-7235. For King and Snohomish the number is 206-526-6733. (Listen, then leave a page.)

5 thoughts on “Transient orcas may be leaving uneaten food behind

  1. Interesting. Minnie, our first rehomed Siamese always brought little critters home…twelve years of occasional little deceased animals carefully laid on the doorstep for us to find.

    The only dog I’ve ever had actually hunt, kill and eat prey is a Standard Poodle. Other than watch him follow a scent, I’ve never caught him at it…only found the evidence on his cream colored jaws. I added bells to his collar to warn the little birds and squirrels but within a week or two, the evidence was back on his jaws … and he leaves nothing behind.
    He is not ‘hungry’…he gets free choice every night and breakfast every morning. He has a high activity level and entertains himself physically fit.

    Is it possible the whales sensed an overpopulation and simply rebalanced nature?
    Sharon O’Hara

    He is well fed and fit.

  2. I saw a special on TV a while back that where they were trying to discover the reason so many porpoises had been found smashed and dead. Unfortunately, I can’t recall where this was happening. The answer was truly shocking. Apparently, dolphins were attacking the porpoises by using their heads to butt the porpoises up out of the water and ram them to death. I’m not sure they ever found out the why of it.

  3. In 1970 while in college, I attended a Cetacean Symposium during which this “kill and leave” behavior was reported for Orca’s in Alaska. The speaker presented a photo of on Orca attacking an elephant seal by driving it completely out of the water and then leaving the dead seal.

  4. There have been multiple first-hand observations, including photos and video, of Killer Whales living up to their name. On several occasions this spring off the north shore of Orcas island, observers have followed ‘the action’ for hours from shore and from boats. Observers have no doubts about who the killers are.

  5. 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.”

Leave a Reply

Your email address will not be published. Required fields are marked *

Before you post, please complete the prompt below.

Is water a solid or a liquid at room temperature?