Tag Archives: Harbor seal

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.

Finding answers for dangerous decline of Puget Sound steelhead

Harbor seals have become prime suspects in the deaths of millions of young steelhead trout that die each year in Puget Sound, but the seals may not be working alone.

Trends

Disease and/or various environmental factors could play a part, perhaps weakening the young steelhead as they begin their migratory journey from the streams of Puget Sound out to the open ocean. Something similar is happening to steelhead on the Canadian side of the border in the Salish Sea.

More than 50 research projects are underway in Puget Sound and Georgia Strait to figure out why salmon runs are declining — and steelhead are a major focus of the effort. Unlike most migratory salmon, steelhead don’t hang around long in estuaries that can complicate the mortality investigation for some species.

The steelhead initiative was launched by the Washington Department of Fish and Wildlife and Puget Sound Partnership with funding from the Legislature. The steelhead work is part of the Salish Sea Marine Survival Project, which is halfway through its five-year term, according to Michael Schmidt of Long Live the Kings, which coordinates the effort in the U.S. The larger project involves at least 60 organizations, including state and federal agencies, Indian tribes and universities.

A new report on research findings for steelhead (PDF 9.8 mb) describes the most significant results to date for our official state fish, which was listed as “threatened” in 2007. While steelhead populations on the Washington Coast and Columbia River have rebounded somewhat since their lowest numbers in the 1980s, steelhead in the Salish Sea remain at historical lows — perhaps 10 percent of their previous average.

“Because steelhead are bigger and move fast through the system, they are easier to study (than other salmon species),” Michael told me. “It has been a lot easier to feel confident about what you are finding.”

Abundance

Steelhead can be imbedded with tiny acoustic transmitters, which allow them to be tracked by acoustic receivers along their migration routes to the ocean. It appears that the tagged fish survive their freshwater journey fairly well, but many soon disappear once they reach Puget Sound. The longer they travel, the more likely they are to perish before they leave the sound.

While steelhead are susceptible to being eaten by a few species of birds, their primary predators appear to be harbor seals. These findings are supported by a new study that placed acoustic receivers on seals and observed that some of the transmitters embedded in steelhead ended up where the seals hang out, suggesting that the fish were probably eaten.

In a different kind of tagging study, Canadian researchers placed smaller passive integrated transponder (PIT) tags in a large number of coho salmon and attached devices to read the PIT tags on coho salmon.

“What is most interesting to date,” states a new report from the Pacific Salmon Foundation,“ (PDF 4 mb), “is that we only have confirmed feeding on tagged coho salmon by four of the 20 seals equipped with receivers. This suggests that feeding on juvenile salmon may be an opportunistic behavior acquired by a limited number of seals.”

New studies are underway to confirm steelhead predation by looking at fecal samples from seals in South Puget Sound. Researchers hope to figure out what the seals are eating and estimate steelhead consumption.

As I mentioned at the outset of this blog post, it may be more than a simple case of seals eating steelhead. For one thing, seal populations may have increased while their other food choices have decreased. Would the seals be eating as many steelhead if Puget Sound herring populations were close to their historical averages?

Other factors may be making young steelhead vulnerable to predation. A leading candidate is a parasite called Nanophyetus salmincola, which can infest steelhead and perhaps increase their risk of predation. The parasite’s life cycle requires a snail and a warm-blooded animal, as I described in a story I wrote for the Encyclopedia of Puget Sound — part of a larger piece about disease as a powerful ecological force. Anyway, the snail is found only in streams in South Puget Sound, which might help explain why steelhead deaths are higher among these South Sound populations.

Experiments are underway to compare the survival of two groups of identical steelhead, one group infested with Nanophyetus and one not.

Depending on funding and proper design, another experiment could test whether treating a stream to temporarily eliminate the snail — an intermediate host — could increase the survival of steelhead. If successful, treating streams to remove these snails could be one way of helping the steelhead. For these and other approved and proposed studies, check out the Marine Survival Project’s “2015-2017 Research Work Plan” (PDF 9.3 mb).

Other factors under review that could play a role in steelhead survival are warming temperatures and pollution in Puget Sound, which could help determine the amount and type of plankton available for steelhead and salmon. Could a shift in plankton result in less food for the small fish? It’s a major question to be answered.

I’ve mentioned in Water Ways (3/15/2010) that transient killer whales, which eat seals, sea lions and harbor porpoises, may be helping their distant cousins, the Southern Resident killer whales, which eat fish. Those smaller marine mammals compete for the adult salmon eaten by the Southern Residents. By clearing out some of those competitors, the transients could be leaving more salmon for the Southern Residents.

It may be too early to draw any firm conclusions, Michael Schmidt told me, but transient killer whales may be helping steelhead as well. Last year, when transients ventured into South Puget Sound and stayed longer than usual, the survival rate for steelhead from the nearby Nisqually River was the highest it has been in a long time.

Were the whales eating enough seals to make a difference for steelhead, or were the seals hiding out and not eating while the whales were around. Whether there were benefits for the steelhead, we could be seeing what happens when a major predator (orcas) encounters an abundance of prey (seals).