Study Shows How Record Abundance Of Pink Salmon In North Pacific Creating Adverse Impacts On Puget Sound Chinook, Killer Whales

Since the 1990s, the decline in numbers of southern resident killer whales in Puget Sound has followed a biennial pattern; births decline and deaths rise in even-numbered years. That biennial pattern matches the decline of Chinook salmon spawner abundance while abundance of pink salmon in the North Pacific and in Puget Sound rivers has risen, according to a study published this month.

Pink salmon abundance has reached record levels in the North Pacific Ocean and in Puget Sound, with a tripling of their abundance in odd-numbered years. That, researchers are concluding, has led to adverse effects on several marine species, especially Chinook salmon, the preferred food of southern resident killer whales, and the whales themselves.

Now researchers are directly connecting that biennial pattern of more pink salmon with fewer Chinook and fewer births and more deaths for SRKWs, which are listed as endangered under the federal Endangered Species Act.

For example, in the Sultan River, a tributary of the Skykomish River near Everett, Washington, up to 300,000 pink salmon spawn in odd-numbered years and almost zero spawn in even years, providing an ideal natural experiment, said Greg Ruggerone, lead researcher with Natural Resources Consultants, Inc. in Seattle.

“Over the past 25 years, we found that Chinook salmon, which spawn in the same areas and times as pink salmon, shift upriver in odd years to avoid abundant pink salmon in the lower river, leading to fewer fry per parent Chinook salmon,” he said. “We also found 34 percent fewer Chinook salmon returning to major Puget Sound spawning grounds in odd years.

“Interestingly, this pattern emerged in the late 1990s at about the same time that Southern Resident killer whales began to decline while also showing alternating-year patterns of mortality, births, and body condition that are consistent with pink salmon effects on Chinook salmon abundance. Pink salmon returning to Puget Sound rivers have tripled during this period. Importantly, if the SRKW population had not declined every other year, the population would have been growing not declining since the late 1990s.

“Our study highlights the need to consider the entire ecosystem when managing marine species,” Ruggerone concluded.

Seven Puget Sound populations of Chinook — about 75 percent of natural spawning Chinook – began to exhibit a biennial pattern of abundance when on spawning grounds in odd years beginning in about 1997 and that has continued through 2022. In contrast, from 1975 to 1996, spawner abundances were not significantly different in odd compared with even years, the study says.

Granted, abundances of Chinook populations throughout the Northwest have seen an overall decline over the years, but the decline in Puget Sound was 1.6 times faster during odd compared with even years from 1975 to 2021, the study says. And, the researchers found that biennial abundance patterns were not observed in Chinook salmon returning to the two Puget Sound watersheds that did not support pink salmon.

This coincides with higher numbers of odd-year pink salmon spawners in the same watersheds that tripled from 1975-1996 (0.75 ± 0.1 million fish) to 1997-2021 (2.23 ± 0.5 million fish), the study says.

“Chinook salmon abundance during odd years declined with increasing abundances of pink salmon spawning in these watersheds during the previous cycle, with pink salmon abundance explaining 28% of the annual variability in natural-spawning Chinook salmon entering Puget Sound (catch and spawners) during 1975 to 2021,” the study says.

According to the study, the population of SRKW began to decline in 1998, and from 1998 to 2021 the population declined by 46 individuals in even years (in response to a one-year delay in mortality after a poor feeding year) and increased by 11 individuals in odd years.

During the period 1975 to 1996, abundance of Chinook salmon returning to Puget Sound was 35 percent higher, with no biennial pattern than it was during the period 1997 to 2021, and there was no biennial pattern.

In contrast, odd-year pink salmon returning to Puget Sound during the period 1975 to 1996 was 65 percent less compared with 1997 to 2021.

Beginning around 1998, the study says, an unexpected biennial pattern of mortality and successful births of SRKW emerged. From 1998 to 2021, mortality of newborn and older SRKWs was nearly 3 times higher (65 versus 23 deaths) and successful births 44 percent lower (19 versus 34 calves) in even than in odd years as the population declined.

“This biennial pattern was not apparent during the earlier period (1976−1997) when the SRKW population was stable or increasing, the primary prey (Chinook salmon) was more abundant and larger, and pink salmon was less abundant,” the study says.

It goes on to say that SRKW body condition measurements in September of odd years revealed that the body condition of each age group of the L pod and subadult and reproductive females in the K pod were less than in even years.

In addition, the biennial body condition of the L pod, and K pod to a lesser extent, was linked to abundance of Puget Sound Chinook salmon more than to Fraser River and other Chinook populations. That, the study says, reflects “the unique foraging distribution of these pods and somewhat unique distribution of Puget Sound Chinook along the west coast of Vancouver Island.”

“In the Discussion, we note that the body condition of the L pod, which had the strongest biennial growth patterns, had the strongest correlation with Puget Sound Chinook abundance—not Fraser Chinook abundance,” Ruggerone said. “L pod was the biggest pod and the pod that has shown the greatest decline since these biennial patterns began.”

About 40 percent of the SRKW’s annual diet consists of chum salmon, the study says. That population of salmon has also seen a biennial decline in recent years.

There are three pods of Southern Resident killer whales, with a total population of just 74 individuals, according to the Whale Museum in Friday Harbor, San Juan Island, WA.

L Pod is the largest and is composed of 34 individuals. J Pod is composed of 25 individuals. Granny (J-2) died in late 2016 at the age of 105. K Pod is composed of 15 individuals.

The Museum said that spring-run Chinook salmon are the preferred and primary food for Southern Resident killer whales. They, like the SRKWs are listed under the federal Endangered Species Act. Lower numbers of Chinook numbers and their smaller body sizes means that the fish-eating whales must travel further and work harder to find sufficient food.

The study, “Long-term biennial patterns in Puget Sound Chinook salmon and Southern Resident killer whales: the role of pink salmon and implications for ecosystem management,” was published Feb. 13, 2025, in the Canadian Journal of Fisheries and Aquatic Sciences, https://cdnsciencepub.com/doi/10.1139/cjfas-2024-0262 and for open access http://dx.doi.org/10.1139/cjfas-2024-0262

Authors are Ruggerone; Larry Lowe, Keith Binkley and Andrew McConnell, Snohomish County PUD, fisheries.

For background, see:

— CBB, October 18, 2024, What’s Happening In North Pacific Between Hatchery, Wild Salmon? Study Stresses More Research To Reduce Unintended Interactions, https://columbiabasinbulletin.org/whats-happening-in-north-pacific-between-hatchery-wild-salmon-study-stresses-more-research-to-reduce-unintended-interactions/

— CBB, September 13, 2024, ANOTHER SIGNAL TOO MANY HATCHERY PINK SALMON IN NORTH PACIFIC? STUDY SHOWS PINK SALMON ABUNDANCE IMPACTING SIZE OF SNAKE RIVER STEELHEAD, HTTPS://COLUMBIABASINBULLETIN.ORG/ANOTHER-SIGNAL-TOO-MANY-HATCHERY-PINK-SALMON-IN-NORTH-PACIFIC-STUDY-SHOWS-PINK-SALMON-ABUNDANCE-IMPACTING-SIZE-OF-SNAKE-RIVER-STEELHEAD/

— CBB, September 28, 2023, STUDY: THE MASSIVE SURGE OF HATCHERY PINK SALMON IN NORTH PACIFIC HARMING ABUNDANCE OF OTHER SALMON SPECIES, WHALES, BIRDS, HTTPS://COLUMBIABASINBULLETIN.ORG/STUDY-THE-MASSIVE-SURGE-OF-HATCHERY-PINK-SALMON-IN-NORTH-PACIFIC-HARMING-ABUNDANCE-OF-OTHER-SALMON-SPECIES-WHALES-BIRDS/

— CBB, February 1, 2019, STUDY LOOKS AT HOW PINK SALMON BIENNIAL ABUNDANCE YEARS MAY BE CONNECTED TO ORCA BIRTHS, DEATHS, https://columbiabasinbulletin.org/study-looks-at-how-pink-salmon-biennial-abundance-years-may-be-connected-to-orca-births-deaths/

— CBB, October 12, 2018, CANADA STUDY LINKS STEELHEAD LIFE CYCLE TO ENVIRONMENT FACTORS, PINK SALMON ABUNDANCE, https://columbiabasinbulletin.org/canada-study-links-steelhead-life-cycle-to-environment-factors-pink-salmon-abundance/

— CBB, July 1, 2018, RESEARCH SHOWS LARGE NUMBERS OF PINK SALMON IMPACTING FOOD AVAILABILITY IN NORTHERN SEAS, https://columbiabasinbulletin.org/research-shows-large-numbers-of-pink-salmon-impacting-food-availability-in-northern-seas/

— CBB, July 21, 2017, STUDY: ALASKA PINK SALMON ADAPT TO CLIMATE CHANGE WITH EARLY MIGRATION TIMING, https://columbiabasinbulletin.org/study-alaska-pink-salmon-adapt-to-climate-change-with-early-migration-timing/

–CBB, April 3, 2015, HOW MANY SALMON CAN NORTH PACIFIC SUPPORT? STUDY LOOKS AT COMPETITION BETWEEN SOCKEYE, PINKS https://columbiabasinbulletin.org/how-many-salmon-can-north-pacific-support-study-looks-at-competition-between-sockeye-pinks/