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Study Looks At How Pink Salmon Biennial Abundance Years May Be Connected To Orca Births, Deaths
Posted on Friday, February 01, 2019 (PST)

Over the past 20 years, endangered Southern Resident Killer Whales (Orcas) in the Salish Sea have had more births and fewer deaths in odd years than in even years, according to a recent study.


That biennial pattern coincides with an “exceptional abundance” of pink salmon in odd years, leading scientists to wonder if pink salmon abundance in the Sea during years when Orca birth rates are low and their death rate is higher are connected.


From 1998 to 2017, mortality of newborn and older whales was 3.6 times higher (61 versus 17 whales) and successful births 50 percent lower (16 versus 32 whales) in even years than in odd years. During that 20-year period, the population of the Southern Resident Killer Whales declined from 92 to only 76 whales.


During the recent 20 year period of population decline, mortality was 3.1 times higher in even years than it was during the earlier 22 year period (1976-1997) of population increase, whereas mortality in recent odd years was 43 percent lower, according to the study’s lead author, Dr. Greg Ruggerone, research scientist at Natural Resources Consultants in Seattle.


According to the State of Washington’s Southern Resident Killer Whale Task Force, the leading reasons for the whales’ decline has been the low abundance of its key prey species (chinook salmon), toxic contaminants and vessel noise. (See CBB, November 20, 2018, “Orca Recovery Task Force Recommendations Include Considering Removal Of Lower Snake Dams,”


But those factors alone do not explain the biennial pattern, Ruggerone said.


“Our leading hypothesis is that the exceptional abundance of pink salmon returning to the Salish Sea in odd years (avg. 17.8 million versus 0.4 million in even years) interferes with foraging of the killer whales that target chinook salmon during spring through early fall,” Ruggerone said. “We present evidence that the causal mechanism is indirectly linked to pink salmon (O. gorbuscha), which exhibit a unique and extreme biennial pattern of abundance and interact strongly with other species in marine ecosystems in the North Pacific.”


The biennial pattern began almost exactly when the Southern Resident population began its decline and when pink salmon escaping fisheries in the Salish Sea more than doubled, he added.


Ruggerone said that diet studies show that Southern Resident Killer Whales almost never eat pink salmon, but the abundance of pink salmon in odd years could interfere with both chinook salmon and the whales:


-- Pink salmon return to the Salish Sea from approximately mid-July to early September, which is about the same time as summer-run chinook and the early portion of fall run chinook salmon. 

-- Pink salmon migrate along the same migration route as chinook salmon (e.g. west side of San Juan Island and into Boundary Pass as they return to the Fraser River), which is the core foraging area of the Southern Residents.

-- Pink salmon migrate along surface waters whereas chinook are deeper.


“Although SRKW can distinguish between pink salmon and the larger chinook salmon via echolocation, we suspect that the high density of pink salmon (about 50 pinks to one chinook) reduces foraging efficiency, especially in years when chinook abundance is relatively low,” Ruggerone said.


During the period of Southern Resident decline, the number of pink salmon escaping fisheries in this region more than doubled (because fishing effort was reduced to conserve chinook and sockeye salmon), he said.


“Whale mortality occurred primarily in even years because other studies show a one year lag between low food abundance and mortality--this lag is expected in such a large mammal,” he said.


Previous studies by Ruggerone and others have documented the large impact of pink salmon on the North Pacific ecosystem, including zooplankton, phytoplankton, other species of salmon, and seabirds. (See CBB, April 13, 2018, “Carrying Capacity: High Numbers Of Pink, Chum Salmon In North Pacific May Be Hurting Chinook,”


“Pink salmon returning from the North Pacific have never been more abundant than they were during 2005-2015 (since detailed records began in 1925),” Ruggerone said, adding that pink salmon account for nearly 70 percent of all adult Pacific salmon.


The scientists do have an alternative hypothesis: the high mortality in even years since 1998 is the normal response to low chinook salmon abundance during this recent period of decline, and reduced mortality in odd years is because pink salmon somehow enhance SRKW foraging in odd years. 


“We do not have a mechanism that explains this hypothesis, but we offer it so that it may be considered and evaluated in future analyses,” Ruggerone said.


The study, “Unprecedented biennial pattern of birth and mortality in an endangered apex predator, the southern resident killer whale, in the eastern North Pacific Ocean,” was published online Jan. 3, 2019 in the Marine Ecology Progress Series (


Ruggerone’s co-authors are Dr. Alan Springer, professor emeritus, University of Alaska, Leon Shaul, fisheries biologist III, Alaska Department of Fish and Game, and Gus van Vliet, retired scientist.


Neither hypothesis conflicts with current thought about Southern Resident decline, which is due to a lack of chinook salmon. However, Ruggerone said, the biennial pattern adds complexity. Still, this complexity also offers keys to better understanding the decline of Southern Residents and so provides additional ideas for enhancing their recovery.


Two types of studies are needed to test the opposing hypotheses, he said. The first is a field study to evaluate the foraging efficiency of Southern Residents during odd versus even years. The second study is to model Southern Resident mortality and births in relation to chinook salmon, pink salmon, and other salmon abundances.


“We have begun the modeling effort but progress is slow because we have no funding for this effort,” Ruggerone said. “We have begun developing actions needed to improve whale foraging, depending on the outcome of these studies.”


Also see:


-- CBB, September 28, 2018, “Orca Task Force Recommendations Include Focus On Salmon Runs; Non-Native Game Fish To ‘Predatory,’”


-- CBB, Sept. 14, 2018, “NOAA Fisheries Studying Nighttime Behavior Of Endangered Killer Whales As Part Of Action Plan”


--CBB, May 11, 2018, “Puget Sound Boaters Asked To Observe ‘No-Go’ Zone To Protect Foraging Orcas,”


--CBB, March 16, 218, “Washington Governor Signs Executive Order To Protect Orcas, Chinook Salmon”


-- CBB, Jan. 15, 2016, “Study: Chinook Salmon Make Up 80 Percent Of Diet For ESA-Listed Killer Whales In Pacific Northwest”


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