How Seals May Be Impacting Salmon Conservation

Researchers examine the link between seal diets and the declining salmon populations.

Story by Olivia Hobson

December 11, 2020

Harbor seals have been likened to the dogs of the sea. Their outward appearance is lovable, with fat bodies and big, round eyes. Videos of them cuddling and playing frequent the internet, and the pups are even more adorable than the adults. They even bark.

It’s hard to look at these cute marine mammals and think of them as a pest. But Zoe Lewis, graduate research assistant at Western Washington University, is investigating how skyrocketing sea lion and harbor seal populations might be having a negative impact on food chain processes in the Salish Sea.

She’s standing next to the Whatcom Creek Waterway in downtown Bellingham. It’s a cool fall day, and the calm water lapping at the shore reflects the cloud cover in the sky above. She’s talking about her research, but Lewis is keeping her eye peeled for a familiar marine creature: a harbor seal.

Specifically, a harbor seal and its lunch. The salmon runs that go up Whatcom Creek make for excellent dining, and the seals have learned that over time, Lewis said.

“Where we’re standing right now, there is a huge coho and chum salmon run that goes up Whatcom Creek, and they sit right here and just go for it as soon as the run starts,” Lewis said.

Lewis has her research assistantship through the Washington Sea Grant and is contracted with the Makah Tribe on the Olympic Peninsula. Her work is investigating harbor seal diets in the Salish Sea, specifically focusing on how much salmon the seals are eating during the winter and spring season. By determining how much salmon harbor seals are eating, when they are eating it and where, salmon conservation and recovery efforts can be better targeted, Lewis said.

Lewis joins a large group of marine biologists and ecologists studying the impact of the harbor seal diets on local salmon population and survival, with studies spanning years and the big question being: What are seals eating, and why does it matter?

Biology and science education professor Alejandro Acevedo-Gutierrez is Lewis’s advisor at Western’s Marine Mammal Ecology Lab. Acevedo-Gutierrez has been researching harbor seals at the lab since 2008. Over the summer of 2020, he co-authored a paper about the consumption differences between male and female harbor seals.

Acevedo-Gutierrez made sure to point to the root cause of declining salmon numbers. Salmon face challenges, and their populations are struggling to recover, because of human activity.

“My personal opinion is that we just fished the hell out of them, and now it’s really hard for Chinook to recover,” Acevedo-Gutierrez said. “Yes, seals certainly are a part of salmon predation… but everything eats salmon.”

While harbor seal and sea lion populations have been on the rise, salmon have not fared so well. Chinook salmon, an invaluable cultural and economic resource in the Pacific Northwest that feed everything from humans to the Southern Resident orcas, continue to face monumental challenges in population survival and recovery, according to the collaborative conservation platform Puget Sound Info.

After decades of urban development around the Puget Sound, Chinook salmon populations have been negatively impacted by habitat loss and dammed rivers, which stops them from reaching their spawning grounds, according to Puget Sound Info. Climate change and warming waters also increase salmon mortality at multiple life stages.

Chinook are listed as threatened under the Endangered Species Act and are currently two-thirds below their recorded abundance from the early 1900s, according to Puget Sound Info. In 1985, wild Chinook salmon abundance in Puget Sound rivers was slightly over 200,000. In 2010, it numbered at about 50,000.

Seal salmon consumption is on the rise, inspiring proposed seal management strategies. A 2017 study found that between 1970 and 2015, seal predation on salmon had increased from 68 to 625 metric tons, double the amount of salmon consumed by orcas and six times more salmon than combined recreational and commercial catches in 2015.

Seals are an opportunistic predator, Lewis said, which means that they will go to where the food is and stay there, not unlike the raccoons foraging through garbage outside college campus dining halls. If there’s a productive salmon run that regularly feeds them, it’s in their best interest to stay there, Lewis said.

Since the Marine Mammal Protection Act of 1972, harbor seals and sea lions in the Salish Sea, two salmon predators, have thrived and their populations have grown dramatically according to a 2003 study. The researchers assessed that from 1978 to 1999, harbor seal populations in Washington State grew from 7,000 to more than 19,000. The study found that harbor seal populations could be reduced by 20% and still be above the optimal population size for filling their ecological role. Since that study was published, seal populations have continued to rise, Lewis said.

As seal populations increase and salmon populations struggle to recover, more attention has been focused on seal diets. For Acevedo-Gutierrez’s paper, his team collected scat samples from seal haul outs, or the land where seals leave the water. Once the scat was collected, it was sent to a lab, where geneticists were able to recreate the seals’ theoretical dinner plate.

By analyzing seal and prey DNA samples within the scat, graduate students at Western, were able to determine the differences in how much salmon male and female seals were eating. Comparing prey DNA from within the scat to a library of pre-collected prey DNA, like a painter comparing paint samples at a home renovation store, the scientists were able to match the traces of DNA in the scat to the DNA in the library, figuring out what prey each seal was eating by sex.

The results of the study suggested that during the winter and spring, females hunt and eat more specific prey types while males will eat what is available when it’s available. Females tend to do deep dives and feed on fish that live on the ocean floor, which is a specialized hunting behavior. Males, on the other hand, will stay closer to the surface and eat the fish species there, like salmon.

When it comes to salmon conservation and restoration, this is important for better management, Acevedo-Gutierrez said. A group of male seals poses a different level of threat to salmon than a group of females.

“Looking at males and females and their specialization matters if you want to examine the impacts on salmon at a regional or local level,” Acevedo-Gutierrez said.

These studies, Acevedo-Gutierrez said, rely on context to have an impact on conservation efforts. Seals eat many things, but what matters is the impact the seal diets are having on salmon recovery.

“Salmon may be representing a little bit of what seals eat overall, but that might be enough to have an impact on salmon,” Acevedo-Gutierrez said. “We need to see it from the point of view of the salmon.”

Adrianne Akmajian, marine ecologist for the Makah Tribe, is advising Lewis in her research and sending her scat samples from Neah Bay. For the past 10 years, Akmajian has been working for the Makah Tribe in Neah Bay and the outer coast, studying the impact of harbor seals and sea lions preying on salmon returning from the outer coast to their salmon runs on the inner coast.

Having a stronger understanding of natural salmon survival rates and the impact of a doubled sea lion population on tribal fisheries will result in better fisheries management and negotiations, Akmajian said.

“Papers don’t include data from the outer coast because there isn’t much data from the outer coast,” Akmajian said, highlighting the importance of the Tribe’s ongoing research. “A lot of the importance to the Makah Tribe has to do with better understanding what’s going on in the ecosystem now.”

Acevedo-Gutierrez, Akmajian, and Lewis’ research is ongoing, and the impact of the results are still being determined, Akmajian said, but their findings will influence future seal management strategies.

There are some solutions for harbor seal and sea lion management that are based on diversion, like using sonic pulses to keep seals from preying on salmon, Acevedo-Gutierrez said. Others involve reducing the number of seals available to prey on salmon.

Harvesting seals and reducing their population slightly is an idea that has floated around, Akmajian said. Tribes, which have historic rights to marine mammals, could investigate reopening negotiations for more seal and sea lion management if research shows it would benefit salmon recovery. But so far, those discussions are still far out at sea. Besides, Acevedo-Gutierrez suggests that seal harvesting would have to be done properly to be effective.

“One conversation has been about harvesting, or killing, the seals. Well, it is reasonable to assume that other seals can come pick up the slack,” Acevedo-Gutierrez said. Targeting seals that are unable to shift their foraging behavior, or “problem animals”, would make more sense than indiscriminately starting to remove seals, Acevedo-Gutierrez said.

Killing harbor seals pulls at the emotional heart strings, Lewis said. Seals are cute, protected marine mammals- killing them seems drastic. But over the course of her research, Lewis has had to confront questions about the impact of equal protection for all marine mammals, which is stipulated in the Marine Mammal Protection Act. While it has done tremendous things for whales, it has also resulted in a large population of seals and sea lions that are now raising these salmon recovery questions, Lewis said.

Finding management strategies that consider the impact of human activity- both when it comes to environmental degradation as well as conservation efforts- is important in the next step of ecological management, Lewis said.

“Let’s look at this ecosystem and see how we can interact with the ecosystem as humans being a part of it, not just taking from it,” Lewis said. “I think something we tend to ignore as scientists is our roles in the ecosystem. It’s not necessarily the best thing to remove ourselves from the equation, because the fact of the matter is that we’re a large part of the equation. Just by protecting something, is that really the right thing to do?”