Watching Our Water Ways

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.)

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Tags: killer whale, Marine mammals, Predation, Predators, Surplus killing, surplus predation, transient killer whales, transient orcas

This entry was posted on Friday, May 14th, 2010 at 7:46 pm by cdunagan and is filed under Marine mammals, Recreation, Research, Waterfront residents. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.