SULTAN — Against the backdrop of turning autumn leaves, the water of the Sultan River wriggled with life as salmon heaved themselves to spawning grounds, their scales catching the late September sunlight.

Even with the river chock full of fish, the only Chinook seen from a vantage point on the Sultan was on the banks, impaled by a stick and starting to decay. The salmon in the river, with the males’ humped backs slicing through the surface, appeared to all be pinks.

The lopsided numbers have caught scientists’ attention across Puget Sound because the pinks’ domination of the Sultan is not a coincidence.

Pink salmon, the smallest of the Pacific salmon species, have a distinct two-year life cycle. And while it’s not yet fully understood by researchers, the fish are 25 times more numerous across Puget Sound in odd years, such as 2025, than even years.

Since 1997, when a major El Niño event and correlated warmer ocean temperatures occurred, odd-year pink populations have exploded, with nearly 800 million adults counted in 2021 and 2023.

The one species now accounts for almost 80% of all Pacific salmon, and the boom has had ripple effects across the food web.

Pinks are a factor in decreasing populations of threatened Chinook salmon, consequently also harming endangered Southern Resident orcas, which feed primarily on Chinook, a study published in April suggests.

World-renowned salmon researcher Greg Ruggerone, who led the study, has spent decades analyzing how the drastic increase in pinks has affected other species.

Using the stark difference in pink numbers during odd and even years as natural experimental conditions, Ruggerone drew connections between the intertwined biennial patterns present among the three species.

His hope, he said, is that fish management becomes more holistic, underscoring that changes in one species affect others in fragile ecosystems.

“Agencies need to manage the growing abundance of pink salmon reaching the spawning grounds to reduce the impact of this interaction between pink salmon and Chinook salmon,” he said.

“Chinook are more main-stem spawners”

Ruggerone worked with the Snohomish County Public Utility District for the study, using the PUD’s fish counts on the Sultan River in conjunction with public data collected from the Pacific Salmon Commission, the Pacific Fishery Management Council and the Center for Whale Research.

On Tuesday, Snohomish PUD Manager of Natural Resources Andrew McDonnell, a co-author of the study, made his way through the winding trails of Osprey Park in Sultan, pointing out the newly created side channel the utility district constructed last year for salmon habitat.

The channel, which adds to previous side channels the PUD built a decade ago, was filled with spawning pinks.

“Chinook are more main-stem spawners — bigger fish, bigger substrate,” he said as he made his way through a mix of birch and cedar to the Sultan.“But the pinks will spawn really wherever they can. I’ve seen them try to spawn on bedrock before.”

During odd years, when pinks swarm the water, outnumbered Chinook are pushed further up the Sultan in the search to find less crowded areas to spawn, McDonnell said.

The higher reaches of the Sultan are narrower and steeper, which means the water has more force. The higher velocity runs the risk of scouring away redds — nests female salmon create by constructing a divot in the rocky bottom to lay and then protect eggs under.

If a Chinook decides to spawn in the lower reaches, it’s likely a redd will get disturbed by the masses of pink salmon vying for space in the river, McDonnell said. In the Sultan, there are approximately 300 times more pink redds than Chinook redds during odd years, counts show.

“Once that [Chinook] female dies and then her nest is just left there by itself, the pinks can come in and spawn on top of that and dislodge those embryos,” McDonnell said. “When it’s a pink year, we know that there’s generally less of those [Chinook] eggs in the gravel that turn into fish the next spring.”

Chinook typically have a four-year life-cycle, meaning if they are born in an odd year, they’ll also return to the river to spawn in an odd year.

Since 1997, the number of Chinook salmon reaching the spawning grounds has been 39% less in odd years compared with even years, the study states.

“Everybody’s focused on climate change, which they should be, but physical factors can’t explain these biennial patterns,” Ruggerone said, looking out over the Sultan River full of pinks. “There are no other species that we know of that might directly cause these effects.”

“That’s just a tremendous amount of biomass”

Climate change, commercial fishing, fish hatcheries and countless other factors have thrown wrenches in food webs across the globe.

Ruggerone’s study lies on the foundation of the interconnectedness of food webs — if one species’ population shifts in a dramatic way, other species are likely to react.

Pinks are better suited to dealing with climate change because they’re smaller fish that need less food than larger Chinooks, Ruggerone said. Pinks are also better adapted to warming waters.

But in addition to biologically granted advantages, hatcheries release massive numbers of pinks every year. While most of these hatcheries are in Alaska, the hatchery fish still interact with fish from Puget Sound, which typically migrate north during their years in the ocean.

After reading Ruggerone’s study, Dr. Deborah Giles, an orca scientist for the SeaDoc Society, a flagship program for the University of California Davis, said her biggest takeaway from the research paper is the last sentence.

“Managers should consider curtailing industrial-scale production of hatchery pink salmon,” of which 1.38 billion are released annually and compete for limited prey in the ocean, the sentence states.

“We’re talking about here 1.38 billion, that’s billion with a B, released every year,” she said. “That’s just a tremendous amount of biomass that is competing for the same prey base as the other species of salmon, most importantly to me is the Chinook salmon.”

Giles cares about Chinook because they’re the primary prey of the Southern Resident orcas’ diet.

Since the boom of pinks in 1997, data shows Chinook populations suffer in odd years, when they’re directly competing for spawning sites and hatched juveniles then have to compete for food.

Because Southern Resident orcas rely on Chinook, they suffer when there are fewer of those fish. But orcas are much bigger than salmon and have much longer life spans, so there’s an ecological lag with the effect pinks have on orcas.

“It takes a while for a big animal like an orca to die because you’re not getting enough food,” Ruggerone said.

While examining Southern Resident data, he noticed a pattern emerge in 1998, the year after pink salmon populations exploded — an even year.

From 1998 to 2021, the mortality rate of newborn and older Southern Resident orcas was nearly three times higher in even years compared to odd years, and successful births were 44% lower in even years compared to odd years.

This sequence didn’t appear before 1998, when the orca population was stable or increasing, Chinook were more abundant and pinks were less overwhelming, the study said.

“I think that the patterns are pretty clearly laid out in this paper,” Giles said. “We’re not having the recruitment into the population in the form of babies being born and living. And furthermore, we’re losing males and females that are within breeding age because they’re nutritionally deprived.”

The theory is that high densities of pink salmon in odd years make it difficult for the orcas to use echolocation to find Chinooks, which, on average, swim deeper than pinks but sometimes overlap and get lost in the masses.

The biennial pattern is compounded by annual stresses like Puget Sound Chinook getting caught in troll fisheries in Alaska and having to deal with increasingly hostile ocean water conditions. But hatchery pinks don’t help the situation, Giles said.

The pink salmon flooding the Sultan River are wild fish, not from hatcheries, but Ruggerone’s study shows that if trends continue, Chinook and Southern Residents face an increasingly uphill battle to recovery.

“It’s definitely something we’re trying to gather more data for specific systems and understand what that means for exactly Chinook returns specifically, and whether or not we see that effect on more of a local scale,” said Matthew Bogaard, a chum, pink, and sockeye salmon species specialist for the Washington Department of Fish and Wildlife. “That paper does present one perspective, and there might be other perspectives that might disagree with that takeaway.”

But for Giles, the takeaway from Ruggerone’s study is clear.

“If the goal is to recover federally listed endangered species like the [S]outhern [R]esident killer whales and the [C]hinook salmon they rely on, then we need to be doing fisheries management significantly differently than we have been,” she said in a text. “Because clearly what we have been doing [is] not working.”

Eliza Aronson: 425-339-3434; eliza.aronson@heraldnet.com; X: @ElizaAronson.

Eliza’s stories are supported by the Herald’s Environmental and Climate Reporting Fund.

Gallery

Pink salmon navigate their way up the Sultan River to spawn on Sept. 30, 2025 in Sultan, Washington. (Olivia Vanni / The Herald)

Greg Ruggerone, a salmon researcher, poses for a portrait along the Sultan River on Sept. 30, 2025 in Sultan, Washington. (Olivia Vanni / The Herald)

A salmon egg is visible through the water on the Sultan River on Sept. 30, 2025 in Sultan, Washington. (Olivia Vanni / The Herald)

A pair of Pink salmon’s humps are visible as they navigate their way up a side channel of the Sultan River on Sept. 30, 2025 in Sultan, Washington. (Olivia Vanni / The Herald)