As Greg Ruggerone remembers it, one day some time ago he and some colleagues were looking at a graph on the Center For Whale Research website. The graph showed the number of southern resident killer whales that died each year. Ruggerone studies salmon, not the southern residents, an endangered population that frequents the Salish Sea. But Ruggerone also knows a pattern when he sees one, and on this graph, he saw one: from the late 1990s through 2017, the southern residents’ deaths showed a pronounced two-year cycle. The whales did better in odd-numbered years and poorly in even-numbered years.
Ruggerone asked a pair of NOAA Fisheries biologists who actually did study killer whales if they had ever noticed this pattern. Neither had. He called Kenneth Balcolm, who had collected the killer whale data for decades. Balcomb had not noticed it, either. “We ended up publishing a paper in 2019 on this very unusual pattern, which should be important for the recovery of the species,” Ruggerone said.
After 1997—the year of a massive El Niño and the year in which the biennial mortality cycles started with the southern residents—the number of pink salmon spawners tripled in Puget Sound and doubled in the nearby Fraser River. They have stayed high ever since.
The paper was fairly short, but it was suggestive. In it, Ruggerone and his coauthors noted that southern resident mortality rates were 3.6 times higher and successful births 50 percent lower in even-numbered years. None of the factors most commonly attributed to the southern residents’ decline—Chinook salmon abundance, whale contaminant loads, ambient ship noise—seemed likely to explain the two-year cycle. Instead, Ruggerone suggested that the southern residents, which are known to specialize in Chinook salmon, might be suffering from the indirect effects of another Pacific salmon: the pink salmon. (They also noted that previous research had shown a one-year lag between a year of low Chinook abundance and increased killer whale mortality, suggesting that the adverse effect occurred in odd-years.)
Pink salmon are the smallest of the five major salmon species. Where pinks might weigh about 3 to 5 pounds each, Chinook salmon are much larger, averaging about 14 pounds. The biggest individuals might be 50 lbs.
What the pink salmon lacks in size, it more than makes up for in numbers. Most Pacific salmon in the region are declining; at present, Puget Sound Chinook salmon, steelhead, summer chum, and Lake Ozette sockeye salmon are listed as threatened under the Endangered Species Act. Not so the pink, which is increasing—or exploding might be a better word. Biologists estimate that pink salmon abundance reached about 800 million fish in 2021 and 2023. The species now comprises 80 percent of all adult salmon in the Pacific. Pink salmon are especially abundant in odd-numbered years, owing to their fixed two-year life cycle.
The success of pink salmon, however, comes at a cost to other salmon species. “Pinks are the climate change winners,” Ruggerone says. “There is a strong positive correlation between their abundance and the ocean heat index since the 1950s.” The increase has been especially notable since the late 1990s. After 1997—the year of a massive El Niño and the year in which the biennial mortality cycles started with the southern residents—the number of pink salmon spawners tripled in Puget Sound and doubled in the nearby Fraser River. They have stayed high ever since.
When Ruggerone and his colleagues published their 2019 paper, the thought of pink salmon somehow affecting the fortunes (or misfortunes) of the southern residents was mostly speculative. But this spring, he and his colleagues returned to publish a kind of follow-up to the 2019 paper. Forthcoming in the Canadian Journal of Fisheries and Aquatic Sciences, the new paper teases apart some of the chain of mechanisms by which pink salmon might affect a marine mammal that does not even feed on them.
Ruggerone and colleagues first focused on the Sultan River, a small river north of Seattle that is part of the Snohomish River watershed, which feeds Puget Sound. The Sultan supports runs of Chinook and pink salmon. To see how Chinook abundance and reproductive success might change with the presence of pink salmon on the spawning grounds, they used surveys along the length of the Sultan for salmon redds, or nests, as well as counts of Chinook salmon fry exiting in the river. To these numbers they added Chinook salmon spawners and total abundances in other Puget Sound watersheds provided by U.S. and Canadian agencies that collate those numbers.
They found that in years of peak pink salmon abundance—odd-numbered years—Chinook survival and abundance declined. Chinook spawners migrated farther up the Sultan River to avoid pink spawners that would otherwise disrupt Chinook redds. The number of fry produced per Chinook redd also dropped when pink salmon spawners were abundant.
The pattern was recapitulated with other Chinook runs in Puget Sound, such as in the Skagit River and elsewhere. “We didn’t anticipate finding such a strong biennial pattern in abundance in Puget Sound Chinook,” Ruggerone says. “It just stood out when I got the data and looked at it, and it fit into what we’re seeing with the killer whales.” This included the whales’ alternating-year pattern in body condition that other researchers had found.
The question becomes how many of these pink salmon patterns lurk in other food webs or with other species. Ruggerone suspects there are many more, even as there are about one hundred scientific papers on the subject. “Look at Bristol Bay sockeye,” he says. Bristol Bay was where he started research as a graduate student, and he has worked there for the past 45 years. The sockeye population in Bristol Bay is perhaps the most closely-studied and best-managed in the world. But until Ruggerone and colleagues published a paper in 2003 showing that sockeye at-sea growth rates, survival, and abundance went up and down with pink salmon abundance, no one had thought about the relationship between the two species.
Similarly, in 2009, in Canada’s Fraser River, another exceptionally-studied sockeye run crashed against expectations. “Scientists had predicted about 10.6 million sockeye would return,” Ruggerone says. “But only 1.4 million came back.” It was a big deal, leading to a $26 million federal inquiry. Then the population rebounded in 2010. For a 2010 Pacific Salmon Commission workshop, Ruggerone was asked by Randall Peterman of Simon Fraser University to test the competition hypothesis with 16 populations of sockeye in the Fraser system. He found, again, strong biennial patterns in sockeye survival and body size spanning a 40-year period: during odd years when pink salmon were abundant, Fraser River sockeye survival and body size declined.
“Sockeye and Chinook salmon populations often have diverse age structure that can cover up patterns that might relate to pink salmon,” Ruggerone says. To these rolls one can now add the southern resident killer whales. In this, the pink salmon becomes a sort of lurking ecological specter for a range of species. But you have to know where to look for the pattern, for the driver. “Unless you ask the right questions,” Ruggerone says, “you’re not going to see it.”