Keywords: Species and food webs, Mammals, Nearshore habitat, Harbor seals, Killer whales, Salmonids, Salish Sea, Implementation Strategies, Salish Sea Currents magazine

Last month's federal authorization to kill more than 700 sea lions to protect salmon runs along the Columbia River is prompting discussions of similar actions for harbor seals in Puget Sound. But experts say the situations are very different with many unanswered questions. 


It is no longer a question of whether Puget Sound’s harbor seals are eating salmon in significant numbers. Some experts fear that seals may be hampering efforts to recover the threatened Puget Sound Chinook and, in turn, the endangered southern resident killer whales.

How to manage seal and sea lion populations — including the potential lethal removal of animals — is the focus of much discussion, according to Michael Schmidt, deputy director of Long Live the Kings, a group focused on environmental restoration, research and education. But it is just one of many concerns in the long-term effort to recover salmon and steelhead, he said.

Seals and sea lions are protected under the federal Marine Mammal Protection Act. Consequently, extensive scientific evidence must be provided before federal agencies consider actions to control or reduce their numbers, noted Schmidt, who is the U.S. coordinator for the Salish Sea Marine Survival Project. That project, which involves U.S. and Canadian researchers, is investigating why so many salmon are dying in marine waters.

In August, three Northwest states and six tribes were issued a federal permit to kill up to 540 California sea lions and 176 Steller sea lions along a 180-mile stretch of the Columbia River. The five-year authorization followed a 2018 amendment to the federal law. As a result, it is no longer necessary to observe and identify individual sea lions responsible for eating threatened and endangered salmon downstream of McNary Dam.

Unlike the situation on the Columbia River, where problem animals are a small portion of the coastal population, the threat in Puget Sound appears to be the overall seal and sea lion population, which grew rapidly after the 1972 passage of the marine mammal law.

Experts throughout the region acknowledge that scientific questions regarding Salish Sea harbor seals — and to a lesser extent sea lions — must be sufficiently answered before population controls are implemented. Answers are needed, they say, not only to meet federal requirements but also to protect the Puget Sound ecosystem from unintended consequences.

According to recent reports by the Washington State Southern Resident Orca Task Force and the University of British Columbia Marine Mammal Research Unit, certain questions must be answered with greater certainty as part of any plan to control seal or sea lion populations.

Questions yet to be fully answered

 

How many harbor seals are living in the Salish Sea, and where are their numbers most concentrated?

Occasional aerial surveys account for seals hauled out on land to provide an overall census with local observations. Better estimates could be provided with more aerial surveys, including counting seals in rivers. Some say it is time to develop more accurate correction factors, which are numbers used to expand the haul-out count to include seals swimming in the water or otherwise unobserved. New approaches, such as satellite imagery or genetic analyses, could help refine population estimates, which are essential to understanding the effects of seal removal.

A harbor seal skins a chum salmon, Whatcom Creek Estuary, Washington. Photo: Andrew Reding (CC BY-NC-ND 2.0)

A harbor seal skins a chum salmon, Whatcom Creek Estuary, Washington. Photo: Andrew Reding (CC BY-NC-ND 2.0)

How is the seal population changing, and what may be limiting growth?

There is evidence that populations have leveled off or even declined after growing at about 6 percent per year during the 1970s and ‘80s. Researchers say the seal population may have reached carrying capacity. If so, how many seals can be supported given the varying fish abundance?

What exactly are seals eating at various times and in various places?

Research based on fecal samples has greatly increased scientific knowledge about the species of fish consumed by maie and female harbor seals. But some researchers are concerned that fecal samples taken on shore may not fully represent overall consumption.

Seals in different locations in Puget Sound may be eating different things, and some salmon populations are more at risk of collapse than others. One answer is to increase sample sites and figure out what seals are eating at various locations throughout the year. 

How does predation by seals fit into the total ecological picture?

Seals are known to prey on a variety of species, including hake, herring and anchovies, as well as salmon. If available, would seals choose another prey species over salmon?

The food web is complex. If people were to take steps to dramatically increase the herring population, could greater numbers of herring help feed both seals and Chinook salmon?

What other species of fish, birds and marine mammals are eating salmon?

A lack of data about other predators could be creating a bias against seals, some experts say. Accounting for the full extent of salmon predation by all the major predators, including a growing number of harbor porpoises, is considered a critical issue. If seals weren’t around, would the number of salmon increase substantially, or would other predators just get more to eat?

How would killer whales adjust to fewer seals?

While southern resident killer whales may be competing with seals for salmon, transient killer whales eat seals as part of their diet. If people took action to reduce the number of seals, would the transient orcas be able to find enough food? Would transients shift their predation to other marine mammals, such as porpoises and large whales?

What are the effects of salmon hatcheries on seal predation?

Some people speculate that seals may be eating more hatchery salmon, as opposed to wild salmon, because seals are attracted to high densities of salmon during mass releases from hatcheries. If seals target hatchery fish, does that reduce predation on wild salmon, or does it teach seals to eat more salmon in general? Experiments with hatchery releases could provide some answers.

How are man-made structures affecting seal populations and predation?

Haul-out sites, such as log booms and docks, are known to bring seals together. Bridges, locks and other man-made obstructions help to concentrate fish. Artificial lights are known to affect fish behavior. All can lead to increased predation on nearby salmon and steelhead runs. Structural modifications and seal deterrents may not require extensive permitting to improve salmon and steelhead survival.

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About the author: Christopher Dunagan is a senior writer at the Puget Sound Institute and the Encyclopedia of Puget Sound.

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Implementation Strategies

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