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.

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