Orcas and seals compete for a limited number of chinook salmon

It’s always been troubling to me that the Southern Resident killer whales, which frequent Puget Sound, have struggled to maintain their population, while other fish-eating resident orcas seem to be doing much better.

Killer whale chases a chinook salmon
Photo: John Durbin, Holly Fearnbach, Lance Barrett-Lennard

Now several researchers have analyzed the energy needs of all the seals, sea lions and killer whales that eat chinook salmon along the West Coast, from California to Alaska. The study provides a possible explanation, one that is consistent with what many scientists have suspected all along. Here’s how I explained it in a story written for the Encyclopedia of Puget Sound:

“Puget Sound’s endangered killer whales are waiting at the end of a long food line for a meal of chinook salmon — basically the only food they really want to eat.

“Ahead of them in the line are hundreds of salmon-craving killer whales in Alaska and British Columbia. Even farther ahead are thousands of seals and sea lions that eat young chinook before the fish have a chance to grow to a suitable size for orcas.”

My story contains plenty of numbers to explain what this is all about.

This issue of competition for food is not a simple one to discuss or resolve. But the new paper, published in the journal Scientific Reports, adds an important perspective when trying to answer the question: “Do we have too few salmon or too many marine mammals?”

From a historical viewpoint, the answer must be that we have too few salmon. But from a management perspective, we might have to conclude that the ecosystem is out of balance and that we have been restoring some marine mammal populations faster than we are restoring the salmon that they eat.

In an intriguing study published in March in the journal Nature Ecology & Evolution (PDF 840 kb), a group of West Coast researchers investigated whether it is better to recover populations of prey species first, followed by predator species, or if it is better to recover predator species first, followed by prey species.

Protecting predators first — which is usually the way humans do things — may slow the growth of prey species or even trigger a population decline, the report says. That creates a problem for predators that specialize in that one kind of prey as well as for those that have no access to alternative prey.

It may seem logical to rebuild the prey species first, the authors say. But, with some exceptions, recovering prey species first causes the combined predator and prey populations to peak at high levels that are unsustainable in the overall ecosystem.

“In the real world,” the paper states, “transient dynamics like these that result from eruptions of prey populations can lead to surprising cascades of ecological interactions and complex but often mismatched management responses.”

The authors conclude that the fastest way to restore depressed populations is through synchronous recovery of predators and prey by carefully rebuilding two or more populations at the same time.

Management tactics may include culling predators even before optimal population numbers are reached. Such actions require careful study, as culling may produce unexpected consequences, according to the report.

Other options include protecting multiple species within protected geographic or marine areas or focusing on single species by protecting select habitats or reducing human exploitation.

For Southern Resident killer whales, the question will be whether populations of other marine mammals — particularly harbor seals in Puget Sound —should be controlled. If so, how would people go about doing that?

One related issue that needs more study is the effect that transient killer whales are having on the Salish Sea population of seals and sea lions. As the Southern Residents spend less time searching for chinook salmon in the inland waterway, the seal-eating transients are being spotted more and more by people along the shores of Puget Sound.

Some studies estimate that the transients need an average of one to two seals each day to maintain their energy needs, although we know these whales also eat smaller sea otters and larger California and Steller sea lions, as well as an occasional gray whale.

Are the transients culling the population of harbor seals in Puget Sound or at least limiting their growth? Even before the transients were showing up frequently, biologists were telling us that the overall harbor seal population appeared to be peaking and perhaps declining.

It would be interesting to create a future-looking computer model that could account for populations of salmon and marine mammals under various scenarios — including possible management actions by humans and the ongoing predation by transient killer whales.

If we want to keep things more natural while helping out the Southern Residents, maybe somebody could come up with a strategy to attract and maintain a healthy population of seal-eating transient orcas within the Salish Sea.

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