Impacts of pink salmon on North Pacific ecosystems

By

PRESS RELEASE

From diatoms to killer whales: impacts of pink salmon on North Pacific ecosystems

Gregory T. Ruggerone1*, Alan M. Springer2, Gus B. van Vliet3, Brendan Connors4, James R. Irvine5, Leon D. Shaul6, Matthew R. Sloat7, and William I. Atlas7

1Natural Resources Consultants, Seattle, WA (GRuggerone@nrccorp.com)

2College of Fisheries and Ocean Sciences, University of Alaska Fairbanks

3Auke Bay, AK

4Fisheries and Oceans Canada, Institute of Ocean Sciences, Sidney, BC, Canada

5Fisheries and Oceans Canada, Pacific Biological Station, Nanaimo, BC, Canada.

6Douglas, AK

7Wild Salmon Center, Portland, OR

Pink salmon abundance in the North Pacific Ocean is surging in response to climate change and ocean heating, and their record high numbers in recent years are exacerbating a long pattern of adverse effects on other species of Pacific salmon, steelhead trout, forage fishes, multiple species of seabirds, humpback whales, and critically endangered southern resident killer whales, according to a Feature Article published in Marine Ecology Progress Series (available for free: https://www.int-res.com/abstracts/meps/v719/p1-40/):

From diatoms to killer whales: impacts of pink salmon on North Pacific ecosystems 

Pink salmon abundance reached record highs during 2005-2021, comprising 70% of all Pacific salmon. In 2021, pink salmon attained its highest level on record (800 million adult salmon, or nearly 80% of all Pacific salmon) since detailed record-keeping began in 1925 (Figure 1). In 2023, the salmon industry reported that commercial harvests of pink salmon will likely exceed the record harvests in 2021. While pink salmon abundance is surging in the North Pacific Ocean, the abundance of other salmon species is declining in many regions, and the authors of this new study provide strong evidence that competition for prey is a key factor in their decline.

Simon Fraser University Professor Emeritus Randall Peterman said, “This is a very important review paper. It is a “tour de force” that synthesizes the overwhelming evidence of the large and multiple types of effects of pink salmon on the North Pacific ecosystem. I expect that this paper will become a widely cited classic that will influence many scientists and conservation groups, as well as fisheries managers and hatchery managers.”

Pink salmon fry emerge from gravels in natal rivers in spring, then spend only one winter at sea while migrating about 3,500 miles before returning to spawn. They are about 25 times more numerous in odd- than even-numbered calendar years in many regions, a unique pattern that has allowed scientists to look for those biennial signals in the growth, survival, productivity, and abundance of other marine species that compete for food with them. The alternating-year, up-down pattern of abundance stems from pink salmon’s unique fixed two-year life history. No factor other than pink salmon has been linked to biennial patterns in numerous marine species, as described by the authors. This innovative approach has helped unravel some key mysteries of open ocean ecology, while also raising new questions about factors affecting additional species of cultural and economic value.

The investigation was led by Dr. Greg Ruggerone and Research Professor Alan Springer, who were joined by six other scientists. The paper, which reviews previously published studies and includes additional new analyses, reveals startling facts about the importance of species interactions at sea and how climate change and ocean heating contribute to those interactions. Below are some key conclusions stemming from the evidence presented in the publication:

  • Pink salmon can initiate a “trophic cascade” in the North Pacific Ocean and Bering Sea. When abundant in odd-years, pink salmon reduce the numbers of their zooplankton prey, leading to lower grazing pressure on phytoplankton, the principal food of zooplankton, which allows phytoplankton to increase in abundance. When pink salmon are less abundant in even years, zooplankton abundance, and thus grazing pressure on phytoplankton, are higher and phytoplankton abundance declines. Zooplankton is the primary conduit of energy flowing from phytoplankton to fishes, squid, seabirds and other marine animals.
  • During their second and final growing season at sea, pink salmon consume nutritious squid and small fishes, as well as zooplankton, often the same prey consumed by Chinook, coho, and sockeye salmon, steelhead trout, smaller forage fishes, and seabirds. In odd years when pink salmon are abundant, consumption of prey by other marine species is often reduced. For example, prey consumption of Chinook salmon declined 56% in odd years when pink salmon were 40 times more abundant than in even years, according to a 10-year study by Dr. Nancy Davis, formerly of the University of Washington. Prey consumption by abundant pink salmon declined only 23%.
  • Pink salmon often affect the growth, age, survival and abundance of sockeye, Chinook, coho, and chum salmon, and steelhead trout, as shown by biennial patterns spanning many decades (see figures, below).
  • Climate change and ocean heating have contributed to record high abundances of pink salmon (Figure 2), leading to increased competition for prey with numerous other species. For example, more prey is required by fishes in warmer water, especially larger salmon such as Chinook salmon. The potential effect of pink salmon on the growth and abundance of Chinook salmon in Alaska and British Columbia, as shown in Figure 3, highlights the opposite relationships between them over the past 70 years.
  • Recent genetic identification studies have shown that numerous Chinook salmon from the Pacific Northwest migrate north into the Gulf of Alaska and Bering Sea where they can interact with abundant pink salmon. Growth of those north-migrating Chinook salmon declines with increasing abundances of pink salmon. Growth relationships with pink salmon were stronger than growth relationships with oceanographic variables, such as sea surface temperature. Thus, Chinook salmon from the Columbia River Basin and nearby regions are influenced by competition with abundant pink salmon in addition to unfavorable habitat in freshwater and coastal marine habitats.
  • Forage fishes are critical prey for many marine fishes, salmon, seabirds, and marine mammals. Growth and/or survival of forage fishes (for example, herring, sand lance, Atka mackerel, and Pacific Ocean perch) exhibit biennial patterns that reflect competition with pink salmon.
  • Seabird diets, egg laying dates, productivity, and survival are linked to pink salmon, as revealed by conspicuous biennial patterns in those measures (e.g., Figure 5). Impacts to 11 bird species have been identified, including shearwaters that nest near Australia and migrate over 7,000 km to the North Pacific each year to feed during the austral winter.
  • The birth rate of humpback whales in Southeast Alaska was 33% lower in odd years (avg. 7.5%) compared with even years (11.3%) during 1981-2013, possibly in response to pink salmon effects on their prey (zooplankton and forage fishes). This pattern was previously unknown.
  • Mortality, births, and body condition of the critically-endangered Southern Resident Killer Whale (SRKW) show biennial patterns, apparently in response to highly abundant pink salmon that may reduce whale foraging efficiency on their primary prey, Chinook salmon (Figure 6). From 1998−2020, mortality of newborn and older SRKW was 3.1 times higher (65 versus 21 deaths) and successful births 42% lower (19 versus 33 calves) in even than in odd years as the population decreased from 92 to 74 animals. Body condition of the L pod, one of three groups of SRKW, was reduced in September of odd years when pink salmon were abundant. Mortality was higher in even years after apparently reduced foraging in odd years. SRKW do not consume pink salmon. If births and mortality during even years had been similar to those during odd years, then the SRKW population would have substantially increased rather than decreased during the past 20 years.
  • Pink salmon are expanding into the Arctic Ocean and have invaded European countries as a result of intentional hatchery releases in western Russia (White Sea) and climate warming in northern regions. In 2023, considerable effort began in northern Europe, especially Norway, to catch and remove non-native pink salmon in order to protect Atlantic salmon and sea trout.
  • The biomass of hatchery-produced salmon is approximately 40% of the total biomass of mature and immature salmon. The authors conclude that the surge in pink salmon abundance in response to climate change (ocean heating) and industrial scale production of hatchery pink and chum salmon in Alaska, Russia, and Japan has exceeded the capacity of the ocean to support both wild and hatchery Pacific salmon, especially highly desirable Chinook and coho salmon.


Additional key findings:

  • Survival of 47 populations of sockeye salmon, ranging from the Fraser River in British Columbia to Bristol Bay Alaska, declined 9% to 21% in relation to a 119 million increase in pink salmon numbers since 1977. Strong biennial patterns in growth, survival, abundance, and age were also identified, further linking the decline of sockeye to pink salmon.
  • Survival of Chinook salmon released from 13 hatcheries in the Salish Sea during 1984-1997 declined 59%, on average, when released in even years when abundant juvenile pink salmon were present. Juvenile Chinook salmon growth in the Salish Sea was also lower in even years, indicating food limitation when large numbers of juvenile pink salmon were present.
  • From the mid-1970s to the recent five year period 2017-2021, commercial harvests of Chinook salmon in Alaska and British Columbia declined from 2,000,000 to 420,000 fish per year as average pink salmon abundance increased from approximately 160 million to 520 million fish per year (Figure 3). During that time, the average size of Chinook salmon in Alaska declined from 20.5 lbs to 12.5 lbs.
  • From 1955 to 1981, the catch of Chinook salmon in the Japanese high seas fishery in the Bering Sea and North Pacific was 39% lower in odd years (avg. 254,000 fish) compared with even years (avg. 417,000 fish) (Figure 4). During that 27-year period, Chinook salmon catch declined with increasing catch (abundance) of pink salmon.
  • Coho salmon returning to Southeast Alaska show a strong biennial pattern in size: smaller coho when pink salmon are abundant and larger coho when pink salmon are less abundant. Coho size declined from approximately 7.9 lbs to 5.7 lbs as pink salmon numbers increased.
  • New findings indicate that the survival and abundance of steelhead from the Columbia River, Fraser River, and Vancouver Island are reduced by competition with pink salmon at sea, where they share common prey. For example, returning (B-run) steelhead counted at Bonneville Dam in the lower Columbia River were 38% less abundant in odd- versus even-years during 1984-2021.
  • The manuscript is further supported with supplemental text and a database of findings that are available on the MEPS website.

Pink salmon returning to Prince William Sound, Alaska, hatcheries contributed to record-setting abundances in recent years and to impacts on other marine species.

Figure 1. Pink salmon abundance in North America and Asia has reached record-high levels in response to climate change and the heating of the North Pacific Ocean (see Figure 2) and hatchery production. Record high pink salmon abundance occurred in 2021, and initial reports indicate 2023 will be even higher. Pink salmon represent approximately 70% of all salmon species in the North Pacific, and their abundance is rapidly growing in the North Atlantic Ocean, raising concerns for Atlantic salmon and sea trout.

Figure 2. Pink salmon abundance has steadily grown with increasing ocean heat content. Humans have exacerbated competition at sea between pink salmon and other marine species though increased production from hatcheries in Alaska and Russia, and from ocean heating due to climate change.

Figure 3. Trends in Chinook salmon harvests in Alaska and British Columbia (upper graph) and average size of Chinook salmon in Alaska (lower graph) are opposite average pink salmon abundance during the four years prior to harvest, i.e., the period of species overlap. Since 1980, abundance and size of Chinook salmon have declined and pink salmon abundance has increased. While many factors contributed to those trends, the strong inverse relationship between Chinook and pink salmon led the researchers to further test for interactions at sea.

Figure 4. From 1955 to 1981, the Japanese high seas salmon fishery caught an average of 417,000 Chinook salmon per year in even-numbered calendar years when few pink salmon were present, but only 254,000 Chinook salmon in odd-numbered years when pink salmon were abundant, a 39% decline. The biennial pattern, and diet overlap between the species, indicates that pink salmon affected the abundance of Chinook salmon on the high seas.

Figure 5. The average nesting season of tufted puffins, an abundant species of seabird nesting at Buldir Island in the western Aleutian Islands, begins later in odd-numbered calendar years (relative to the long-term average date) than in even-numbered years (negative values). The pattern corresponds with the odd-year (high), even-year (low) biennial pattern in pink salmon abundance, when in odd years pink salmon reduce the ability of puffins to obtain prey.

Figure 6. Critically endangered southern residence killer whales (SRKW) have shown a strong biennial pattern in deaths and births consistent with the hypothesis that abundant pink salmon interfere with foraging on Chinook salmon in odd-numbered calendar years, leading to higher death rates and fewer successful births in the following even year. From 1998−2020, mortality of newborn and older SRKW was 3.1 times higher (65 versus 21 deaths) and successful births 42% lower (19 versus 33 calves) in even than in odd years as the population decreased from 92 to 74 animals. Growth of L pod members is reduced in odd years, possibly due to pink salmon interference. If births and mortality during even years had been similar to those during odd years, then the SRKW population would have substantially increased rather than decreased during the past 20 years.