Tag Archives: Pinniped

Increase in harbor porpoises shifts Puget Sound’s food web

Most of us have heard that harbor seals eat Chinook salmon, which are the preferred food for our beloved Southern Resident killer whales, an endangered species whose long-term survival could hinge on getting enough Chinook.

The number of harbor seals in the inland waters of Washington state now totals somewhere around 10,000 or slightly higher, according to the latest estimates by Steve Jeffries, a marine mammal biologist with the Washington Department of Fish and Wildlife.

Harbor porpoise surfing in a boat wake in Burrows Pass, off Fidalgo Island.
Photo: ©Cindy R. Elliser, Pacific Mammal Research

But did you know that harbor porpoises, which eat many of the same things as harbor seals, now number around 11,000 in the same general area? That’s according to a recent study for the Navy led by research consultant Tom Jefferson.

I have to say that those numbers came as a major surprise to me, and I began to ask questions about what all these porpoises in Puget Sound might be doing to the food web, which involves complex interactions between salmon, seals, porpoises, orcas and many other species.

The result of my inquiry is a story published this week in the Encyclopedia of Puget Sound.

I won’t repeat all that I learned here, since the story lays out the facts as told to me by top experts in the field. But one of the key points is that harbor porpoises have flown largely under the radar, as their population has grown by up to 10 percent per year. One major reason seems to be that harbor porpoises don’t appear to eat many salmon — although more work needs to be done to understand their actual diets.

Even if harbor porpoises don’t eat salmon, however, they must be taking a major chunk out of the food web by eating fish that might be preferred by harbor seals and even consumed by Chinook salmon themselves.

More than a few people have proposed reducing the harbor seal population in the Salish Sea by one means or another. Among them are members of the Southern Resident Killer Whale Task Force, appointed by the governor to save the orcas from extinction. But task force members appear to have ignored the impact of harbor porpoises.

Draft recommendations (PDF 342 kb) from the task force call for determining how much effect seals and sea lions are having on the population of Chinook salmon available to the killer whales. The task force is likely to call for a scientific panel to be convened for evaluating predation and considering possible management actions, such as eliminating haul-out areas used by seals or directly removing animals that eat too many salmon.

Experts say the number of seals appears to be declining in our inland waterways, likely the result of various factors — including the increasing presence of marine-mammal-eating transient killer whales. The transients are known to eat seals, sea lions, porpoises and other marine mammals.

Since forage fish consumed by salmon are also taken by harbor seals and harbor porpoises, understanding the entire food web seems like a critical step in determining where human intervention would be most successful — whether that means decreasing the number of harbor seals, increasing the number of forage fish, or even boosting the right kind of plankton that feed the forage fish.

Scott Pearson of the Washington Department of Fish and Wildlife has proposed a project that would map the locations where major predators — such as harbor seals and harbor porpoises — tend to hang out. That’s likely to be locations where large numbers of schooling fish congregate, he told me. Tidal currents and channel bathymetry may concentrate food and create hot spots of predation, where people could potentially influence which species gets eaten by which predators. Given existing conditions, saving salmon and their prey at the expense of seals and porpoises could be the preferred choice.

One interesting aspect about the recent rise in the number of harbor porpoises is an associated decrease in the number of Dall’s porpoises — the reverse of what happened from World War II up into the 1990s, when Dall’s porpoises seemed to increase as harbor porpoises declined.

Dall’s porpoises tend to prefer open-water habitats, including the ocean, whereas harbor porpoises are likely to be found closer to shore, as their name implies. Dall’s and harbor porpoises tend to avoid each other, according to observers, and some speculate that female Dall’s porpoises don’t like being around male harbor porpoises, which are known for their sexual aggression among females of their own kind.

Hybrid Dall’s/harbor porpoises — the result of male harbor porpoises impregnating female Dall’s porpoises — have been observed in the Puget Sound region. Check out “Harbor Porpoise in the Salish Sea” (PDF 4.4 mb) by Jacqlynn Zier and Joe Gaydos. See also “Disappearance and Return of Harbor Porpoise to Puget Sound” by Joe Evenson and associates.

As harbor porpoises increased (blue lines), Dall’s porpoises decreased (red lines) in the inland waters of Washington state.
Graphic: “Disappearance and return of harbor porpoise…,” Evenson, et al.

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.

Seals and sea lions may be undercutting chinook and orca populations

Seals and sea lions can no longer be ignored in the effort to recover our threatened Puget Sound chinook salmon or our endangered killer whales.

A new study shows that seals and sea lions are eating about 1.4 million pounds of Puget Sound chinook each year — about nine times more than they were eating in 1970, according to the report. Please read the story I wrote for the Encyclopedia of Puget Sound, also published in an abridged version in the Kitsap Sun.

Harbor seals rest on the breakwater at Poulsbo Marina. // Photo: Meegan Reid, Kitsap Sun

Seals and sea lions in Puget Sound get the first chance to catch the chinook as they leave the streams and head out to the ocean. Since they are eaten at a very young age, these small chinook, called “smolts,” never grow into adults; they never become available for killer whales or humans.

Based on rough estimates, as many as one in five of these young fish are getting eaten on their way out of Puget Sound. If they were to survive the seals and sea lions and one factors in the remaining mortality rate, these fish could translate into an average of 162,000 adult chinook each year. That’s twice the number eaten by killer whales and roughly six times as many as caught in Puget Sound by tribal, commercial and recreational fishers combined, according to the study.

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Amusing Monday: Humoring a friendly leopard seal

In today’s featured video, National Geographic photographer Paul Nicklen calmly describes his underwater encounter with a massive leopard seal in the Antarctic.

I guess Nicklen was not so calm at the time, as he tells in his narration, but he stayed in place and kept shooting as the leopard seal made moves toward him that could be interpreted in various ways. Nicklen, who has plenty of experience around wild animals, said the seal acted aggressive at first but later tried to make a connection, perhaps by offering the diver a penguin to eat.

Nicklen, who has been working in the polar regions for 17 years, had a “unique childhood among the Intuit in Canada’s Arctic,” according to his bio. He has shot some amazing and exciting scenes, and I’m an admirer of his images of the spirit bear, which is another unique story. See the spirit bear photos on his webpage, and check out the National Geographic story by Bainbridge Island writer Bruce Barcott. Nicklen lives on Vancouver Island.

As for leopard seals, they are pretty amazing creatures, though not always amusing. Take a look at this series of videos by BBC Nature. You can also swim with a leopard seal via a “crittercam” in this National Geographic video, which features the work of biologist Tracey Rogers. (The crittercam part starts about halfway through.)

Another crittercam captures the movements of an Australian sea lion as it hunts for and eventually eats an octopus. The National Geographic footage is from a project designed to figure out what the sea lions are eating. Australian sea lions were once hunted to near-extinction but are now protected by the Australian government.