Hide and seek on the Stillaguamish

How WDFW is working to better understand predation impacts on one of Puget Sound’s most-threatened salmon runs

WDFW Biologist Sarah Colosimo uses a spotting scope to scan the tidal flats where the Stillaguamish River empties into Port Susan, looking for signs of predators that may be preying on threatened salmon.

On an overcast day in late summer, Sarah Colosimo took a short walk through the woods, emerging on a raised footpath overlooking a mile of tidal flats. This is where the Stillaguamish River comes to its final destination, spilling first into Port Susan — a bay on the east side of Washington’s Camano Island — then eventually dissipating into Puget Sound and out to the Pacific Ocean.

She’s been here before. In fact, Colosimo, a wildlife biologist with the Washington Department of Fish and Wildlife (WDFW) specializing in marine mammal investigations, has been routinely coming to this site for months. It’s just one of a dozen or so stops she makes on a weekly basis at various points along the Stillaguamish River, home to one of Puget Sound’s most threatened Chinook salmon populations.

Colosimo sets up a spotting scope and scans the flats. She’s looking for seals, sea lions, and birds that might be eating those Stillaguamish salmon, a key step to better understanding how predation impacts the vital, but troubled, Chinook run.

The effort is just one piece of a very large puzzle; researchers and managers with tribal, state, and federal entities are all working to increase their understanding of how avian and marine mammal predation are affecting salmon throughout the Sound, and how that predation fits into the many other factors negatively impacting salmon across the region.

Recovery and decline

The passage of the Marine Mammal Protection Act (MMPA) in 1972 was a milestone for struggling marine mammal populations in the U.S. In 1975, the estimated population of California sea lions on the West Coast was just 75,000; today, that number is estimated at more than 250,000. The eastern North Pacific population of Steller sea lions has also seen a strong recovery, from about 18,000 animals in 1979 to more than 70,000, and was delisted from the federal Endangered Species Act (ESA) in 2013.

But those species’ recoveries have coincided with the decline of another beloved species: the salmon of the Pacific Northwest.

Harbor seals swim near the mouth of Whatcom Creek in fall 2020.

Anecdotal stories abound of harbor seals and sea lions — both California and Steller — consuming fish returning to rivers across Puget Sound. Stories of marine mammals ripping fish from the hooks of anglers, or gathering at river mouths to feast on salmon returning from their time in the ocean.

But the full picture is more complicated than that — while marine mammals and predatory birds certainly include Puget Sound’s iconic salmon as part of their diet, quantifying exactly how that predation impacts salmon runs and understanding the impact in context of other threats to salmon recovery is challenging.

“We’re still in the early phases of decoding the scale of impacts that pinniped and avian predation have on salmon in Puget Sound,” said Casey Clark, lead marine mammal researcher with WDFW. “Seals and sea lions can travel widely and follow migrating prey, and with the size and sheer number of rivers throughout Puget Sound, it can make tracking predation remarkably challenging.”

Unlike on the Columbia River, where sea lions have been closely observed for two decades at pinch points like Bonneville Dam, the rivers of Puget Sound are widely dispersed and may not have areas where salmon bottleneck in the same way they do at the major dams in the Columbia Basin. There are some areas — like the Ballard Locks at Lake Washington in Seattle, where sea lions in the 1980s played a role in the decimation of the steelhead run there, and where they remain a presence even today — but many of the rivers of Puget Sound are harder to police for predators.

A run in trouble

Puget Sound Chinook salmon were listed as threatened under the ESA in 1999, and runs have continued to decline in the decades since. The Stillaguamish stock is in particular trouble: an average of less than 1,000 fish have returned to spawn in the river in each of the past few years.

It is considered one of the main “constraining” stocks in Puget Sound, meaning that when Stillaguamish Chinook are present in the Sound on their journey back to their home spawning grounds, they limit the ability of anglers to fish other, healthier runs of fish due to ESA constraints. The Stillaguamish itself has closed to fishing at times and locations where Chinook might be present, to help protect wild spawners.

Sarah Colosimo gathers data during a predation survey along the Stillaguamish River in late summer 2021.

This summer’s predation survey is one step to better understanding the factors affecting this troubled salmon run.

“This project is really meant to give us a baseline for understanding what types of animals are preying on salmon in the Stillaguamish, and beginning to calculate the effect that those predators might be having on the population,” said Colosimo, the biologist conducting the survey.

After making a series of notes on a tablet using an application developed specifically to track data from the project, Colosimo packs up her spotting scope, and heads back to her vehicle toward the next stop.

A great blue heron scans the tidal flats near Port Susan in summer 2021. (Sarah Colosimo photo)

She and Clark worked with the Stillaguamish Tribe of Indians to identify the ideal sites for the study, where Colosimo has a good view of the river and predators might gather to intercept outmigrating salmon smolts in the spring, or returning spawners in the summer.

She visits all of the sites several times each week, making sure to arrive at different times of day, and takes a few moments to survey each section of river. Today, it’s quiet, with no sign of any predator activity at any of the stops. But, as Colosimo notes, it’s all data, and she enters it into the app.

It’s actually been surprisingly quiet for much of the summer, Colosimo said.

“We expected to see a lot more activity at some of these locations, but that hasn’t necessarily been the case,” she said. “We saw quite a bit of avian activity in the spring when the smolts were moving out to sea, but fewer pinnipeds.”

That doesn’t mean they haven’t been there, though; Colosimo was surprised during her surveys to spot a seal as far upriver as the Cook Slough Weir, not far from where I-5 passes over the river, about 6 miles from salt water.

A Stillaguamish Chinook salmon. (Casey Clark photo)

To supplement the shore-based survey data, biologists this summer conducted aerial and boat surveys to observe potential marine mammal haulouts near the mouths of several Puget Sound rivers. Colosimo and Clark also participated in summer broodstock collection with the Stillaguamish Tribe, looking for scars and other signs of attempted pinniped predation on returning spawners. Biologists also continue to collect scat samples to analyze predator diets at locations across the Sound.

One piece of the puzzle

It’s important to note that predation is just one part of the equation when calculating the many factors impacting Chinook salmon in Puget Sound. In addition to predation, partners from state, tribal, federal and non-governmental entities are all exploring and working to address the effects of factors like degraded salmon habitat, barriers to fish passage, the region’s booming human population and increasing activity throughout Puget Sound and along its shores.

Habitat recovery is particularly critical for the Stillaguamish River, especially the restoration of estuaries and riparian habitat. The Stillaguamish Tribe has focused in recent years on purchase of lands focused on estuary, upriver, and channel migration areas, to allow for habitat improvements along the river. Stillaguamish Tribal Chair Shawn Yanity said in early 2021 that approximately 1,700 acres of land had been acquired to support this effort.

The Stillaguamish Tribe in 2016 completed the Zis a Ba restoration project, which restored 87 acres of estuary habitat for salmon.

“We’ve seen seals following Chinook upriver, following coho upriver,” said Yanity. “We know they’re here, and it’s important for us to understand how predation is affecting these runs. But if we find out that maybe predation isn’t having that big of an impact, we still need to ask where we should focus our efforts. It’s important we look at all the factors affecting the river.”

Salt marsh on Leque Island, part of the Skagit Wildlife Area, in August 2021.

In 2019, another large habitat restoration effort for the Stillaguamish was completed when WDFW, in collaboration with local and tribal partners, restored 250 acres of tidal marsh habitat on the Leque Island Unit of the Skagit Wildlife Area west of Stanwood (adjacent to the Zis a Ba site). The Eide Road Preserve on Leque Island was among the sites included in this summer’s predation survey.

What’s next

With this year’s survey effort coming to an end, WDFW biologists will now begin analyzing the data to determine the potential impacts of predation on the Stillaguamish run.

“This is a first step in better understanding how predation affects this particular population,” Clark said. “Chinook spend most of their lives at sea, and travel widely, so we can’t account for every instance of predation these salmon might face. But this data could help us determine if there are specific deterrence measures that might be effective throughout the river, or even in specific locations.”

Those measures could include non-lethal deterrence of predators at key pinch points at specific times of year, devices to keep predators out of critical areas, habitat alterations to reduce available nesting or haulout habitat, changes in the timing or method of releasing juvenile salmon from hatcheries, or a variety of other recommendations.

The hope is that this survey work will continue into the future, and potentially expand to other Puget Sound watersheds. Understanding the scale of avian and pinniped predation on these salmon should also help managers better understand the role of this important prey for another, more embattled marine mammal population: The endangered Southern Resident killer whales.

This effort is part of a broader appropriation made to WDFW by the Washington State Legislature to examine pinniped abundance in Puget Sound and their diet. Additionally, the Legislature provided to contract the Washington State Academy of Sciences to develop and provide policymaker with a report on pinniped predation of salmon, focused on the Salish Sea and Washington’s outer coast.

Some anglers are eager to point out that a sea lion culling program like the one on the Columbia River may be necessary for Puget Sound.

“Before jumping to lethal removals, we need a much better understanding of what and where management will be most effective,” Clark said. “Surveys like these help us build that understanding and allow us to take a science-based approach to reducing those impacts.”

This science-based approach also aligns with 2019 recommendations from the Governor’s Southern Resident Orca Task Force, which instructed managers to research whether pinniped predation is a limiting factor for Chinook in Puget Sound and the Washington coast. In addition to the Stillaguamish study, WDFW and partners continue to more broadly study pinniped impacts and diets in Puget Sound and coastal waters.



The Washington Department of Fish and Wildlife

The Washington Department of Fish and Wildlife is dedicated to preserving, protecting and perpetuating the state’s fish and wildlife resources.