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NW Fishletter #383, July 2, 2018
 Scientists Find New Link Showing Impact of Pink Salmon On Other Species
Scientists say pink salmon are causing a rarely detected "trophic cascade," which serves as one more piece of evidence that other salmon species--including sockeye, Chinook, coho and chum--are greatly impacted during their time in the Pacific Ocean by their smaller, but more abundant, cousin.
A new study, published this month in Fisheries Oceanography, describes why scientists believe the peaks and ebbs of annual pink salmon runs are causing the abundance of the Pacific Ocean's tiny plants (phytoplankton) and animals (zooplankton) to rise and fall dramatically each year as well.
And when one species has such a major impact on these basic building blocks of the ocean's ecosystem, the study says, it impacts everything else. This top-down influence of a predator on its prey--zooplankton, and on its prey's food source--phytoplankton, is known as a trophic, or food-chain, cascade.
The findings have important implications for the ocean's food web, and are already being studied by the Alaska Department of Fish and Game as it considers whether to limit the production of hatchery pink salmon released in Prince William Sound in order to reduce their impact on other species, according to Gregory Ruggerone, a co-author of the study.
"This paper is, in my mind, really important because it provides much more solid evidence for how pink salmon actually do impact the North Pacific ecosystem," Ruggerone told NW Fishletter. A research scientist at Natural Resources Consultants. Ruggerone is also a member of the Independent Scientific Review Panel, and has extensively studied salmon abundance and the impacts of increasing numbers of pink salmon.
In his most recent abundance study, he found that pink salmon are now more abundant than at any other time since monitoring began in 1925. They dominate the Pacific's salmon species in both numbers and biomass, and now make up nearly 70 percent of the total hatchery and wild salmon, and almost half of their biomass. Unlike some other salmon species that suffer from warming ocean temperatures in southern areas, pink salmon have thrived in warmer ocean conditions, Ruggerone added.
Pink salmon spend only one year in the ocean, and run sizes trend quite high in odd-numbered years and significantly lower in even-numbered years. For example, from 2000 through 2012, numbers of eastern Kamchatka pink salmon, from far eastern Russia, averaged eight times higher in odd years, at about 122 million fish, compared with even years, with 15 million fish.
Plankton. Courtesy: Marine Biological Association
The study, "Pink Salmon induce a trophic cascade in plankton populations in the southern Bering Sea and around the Aleutian Islands," was also led by British Columbia oceanographer Sonia Batten and University of Washington oceanographer Ivonne Ortiz. The scientists used data collected by sampling machines, called continuous plankton recorders, towed by commercial ships each summer from 2000 through 2014 to determine the abundance of plankton across thousands of kilometers in the south Bering Sea and North Pacific.
When analyzed, the data showed zooplankton, a principle prey of pink salmon, dropped significantly in years when pink salmon populations were high, while phytoplankton, which are consumed by zooplankton, rose. The opposite was true in even numbered years.
The correlation was strengthened in 2013, when a usually high population of pinks in one part of their study area crashed, and abundance of zooplankton boomed. "Correlations don't necessarily mean cause-and-effect, but in this case, we think they do," Ruggerone said, largely because the study provided three lines of evidence to support their tropic cascade hypothesis.
Furthermore, the scientists concluded, the variability in plankton abundances caused by pink salmon during the 15-year study period was greater than variability caused by physical oceanography.
In its discussion section, the article notes growing evidence that indicates foraging pink salmon affect the feeding and reproduction of sea birds, and the growth and survival of sockeye, Chinook, coho and chum salmon. Ruggerone said that these prior studies have shown that pink salmon have a "major, major impact on sockeye salmon growth, survival and abundance, and age at maturation." Similar studies are exploring whether the same impacts are influencing the size and abundance of Chinook salmon throughout Alaska.
He said the work has caught the eye of the Alaska Department of Fish and Game, and influential sport fishing groups that have asked the state to reject a request by hatchery proponents to release some 20 million additional pink salmon in Prince William Sound--already the world's largest producer of hatchery pinks.
Ruggerone said Alaska releases approximately 900 million hatchery pink salmon every year, including about 650 million in the sound. Statewide, about 50 million adult hatchery pinks return each year to Alaskan waters. And even though pink salmon originating in hatcheries comprise only about 15 percent of the Pacific Ocean's 650 million adult pink salmon in odd years, it's still a lot of fish.
"On average, 82 million hatchery pink salmon return from the North Pacific each year," he said, adding, "That number is quite a bit greater than the total wild chum salmon production, and about the same as wild sockeye salmon production in the North Pacific."
Ruggerone said his main hope is that their study will help salmon managers make sound decisions as they begin to understand that the tremendous releases of hatchery salmon not only impact their wild counterparts, but also other species, including salmon that are now endangered, threatened or declining in numbers. -K.C. Mehaffey
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