Study examines potential seal predation of Stillaguamish salmon

As salmon runs returning to rivers across Washington have dwindled, seals and other pinnipeds that may prey on them have gained the interest of salmon recovery managers. 

To determine whether problematic predation exists locally and threatens treaty rights, the Stillaguamish Tribe of Indians is studying harbor seal diet and behavior in the Stillaguamish River delta and Port Susan Bay. 

Stillaguamish Tribe fisheries biologists Anya Voloshin, left, and Robert Roose hold captured harbor seals in place before tagging them for a study of predation on local salmon.

“Before developing a management strategy, we want to figure out what the actual impact is; Whether seals that haul out in Port Susan Bay and are resident to the area are targeting salmon in marine or freshwater environments,” said Jennifer Sevigny, the tribe’s wildlife program manager. 

The tribe launched the study in March 2022 in partnership with the Washington Department of Fish and Wildlife (WDFW), Naval Station Everett and Lummi Nation. The project is supported with Tribal Wildlife Grant funding from the U.S. Fish and Wildlife Service.

During spring 2022, project partners attached GPS tags to 24 harbor seals to track their movements. 

“We want to understand their spatial distribution and haul-out behaviors,” Sevigny said. 

The tribe has received past reports of seals in the Stillaguamish River. The study now underway will shed light on when, how often and how far seals travel upriver, as well as whether the timing overlaps with salmon migrations. 

Observations so far suggest there are about 100 resident seals in the area year-round. They are mostly females and juveniles. Additional seals are present seasonally, but it’s not yet clear what proportion of Stillaguamish salmon makes up their diet. 

“During the breeding and molting seasons, seals tend to eat less and spend more time hauled out, so more animals does not necessarily mean more consumption of salmon,” Sevigny said. 

When the seals tagged in 2022 molted, meaning they shed their fur, the GPS tags were also left behind. The project team recovered each of them. 

Biologists prepare to attach a GPS tag to a harbor seal using an epoxy in April 2022.

Preliminary data from the tags does not show movement up the Stillaguamish River, but revealed a network of eight popular haul-out sites in Port Susan Bay, Sevigny said. 

A second round of tagging is getting underway this spring, again with the help of WDFW, Naval and Lummi biologists. The goal is to attach GPS tags to another 20 seals and recover the tags—and the data they carry—after the seals molt.

Meanwhile, Sevigny and Stillaguamish wildlife biologist Amanda Summers trek each month into the mudflats of Port Susan Bay at low tide to search for other valuable evidence seals leave behind: their poop. 

“Being out there on foot is amazing because we can see where the seals are actually hauling out and we can collect fresh scat samples for diet analysis,” Sevigny said. 

While the GPS tags reveal seal movements for only a few months, the ongoing scat collection will help the study team analyze what the seals eat throughout the entire year, Summers said. 

In a lab, bones are filtered from the samples to show the types of fish the seals ate. The size of the bones can also be used to determine the life stages of the fish consumed. As of March, bones collected so far indicate predation primarily of perch, sculpin, herring and flounder.

The tribe continues to look for clues about how much salmon the seals are eating. After two years of scat collection, the team plans to analyze the samples for DNA markers of different prey species to gain additional information. 

While seal and other pinniped populations have grown significantly since the Marine Mammal Protection Act became law in 1972, many salmon populations have seen the reverse trend and become listed as threatened under the Endangered Species Act. Puget Sound chinook were listed in 1999 and runs have continued to decline since. 

A heart-shaped salmon bone called an otolith was found in a seal scat sample, along with other small fish bones.

The state of the Stillaguamish River run is particularly alarming. A dismal 984 of the fish were forecast to return in 2022, said Kadi Bizyayeva, Stillaguamish fisheries director. Over the past decade, an average of 1,222 chinook have returned each year.

In an effort to recover the fish population, the tribe has invested in salmon habitat restoration throughout the watershed. But for that work to pay off, the fish must be able to reach their spawning grounds. 

“We’re doing everything possible to slow the rate of decline and reverse it toward recovery for our salmon stocks, but there’s still much to do to rebalance our ecosystem,” Bizyayeva said. “Understanding pinniped and salmon interactions is a key part of this work.”

Above: The study team searches for harbor seals to capture and tag in Port Susan Bay in March 2022. Story: Kimberly Cauvel. Photos: Stillaguamish Tribe