A trio of North America’s top salmon scientists has underlined their belief a 2020 fishery collapse in the Pacific Ocean was sparked by a deadly combination of warm water and over-abundance of fish.
But not all species of salmon suffered equally.
“Harvests of each species of Pacific salmon declined 35 percent, on average, in 2020 when compared with the previous 10-year average,” Greg Ruggerone, James Irvine and Brendan Conners wrote in a technical report prepared for the North Pacific Anadromous Fish Commission. “(But) harvests of Chinook salmon in 2020 were the lowest on record since 1925, declining 54 percent compared with the previous ten years.”
Overall, sockeye salmon harvests declined only 10 percent in 2020, the Pacific Northwest-based scientists said, but they added that the percentage decline was distorted by a huge harvest in Alaska’s Bristol Bay.
“…Bristol Bay, in the southeastern Bering Sea, remained robust and offset the
exceptionally low harvests of sockeye salmon in the Gulf of Alaska, British Columbia, Washington, and the Columbia River,” they wrote.
Fight for survival
Sockeye heading into the Gulf face more competition from pink salmon than do sockeye from Bristol Bay, and pinks are now the dominant species in the ocean.
“Overall, pink salmon represented approximately 74 percent of total salmon abundance in 2018/2019,” the researchers reported to the Salmon Commission, an international entity. “Most pink salmon are of natural origin, but abundance of hatchery pink salmon during 2005 to 2015 was greater than abundance of wild chum salmon and approximately equal to abundance of wild sockeye salmon.”
Pinks pushed the ocean’s all-salmon abundance off the charts in 2015. In a peer-reviewed study published in Marine and Coastal Fisheries: Dynamics, Management, and Ecosystem Science in 2018, Ruggerone and Irvine reported that 90 years of salmon-harvest data showed salmon more abundant then than at any time in history.
The study did not mention “global warming,” but did observe that “since the 1977 ocean regime shift (to warmer waters), oceanographic conditions have been most favorable for pink salmon, chum salmon, and sockeye salmon in northern regions where many large salmon populations occur in relatively intact habitats.
“In contrast, many southern natural-origin populations along both (the U.S. and Asia) continents have been more adversely influenced by habitat degradation, ocean conditions, interactions with hatchery salmon, and overharvest in mixed-stock fisheries, resulting in abundance declines. In particular, we note that abundances of natural-origin Chinook salmon, coho salmon, and steelhead in the eastern North Pacific Ocean, including Alaska, and cherry salmon in Asia have declined over time.”
Still, the trend of overall abundance continued in general through the end of the 2010s.
Numbers peaked “in 2018 when approximately 950 million pink, chum, and sockeye salmon returned from the ocean,” the new technical assessment notes. “In 2019, salmon abundance remained exceptionally high ( approximately 854 million salmon). Together the
2018/2019 period was the highest two-year period of salmon abundance on record since 1925, nearly 20 percent greater than the previous two-year high in 2009/2010, and more than 3.2 times higher than average abundance during relatively low salmon production years from 1960 to 1975” when the North Pacific was dominated by cold water.
Unfortunately, this level of production – driven in part by hatchery fish – proved unsustainable. The scientists said it appears productivity hit a “tipping point” preceding the big crash of 2020.
“Preliminary commercial catch statistics for all salmon species indicate Pacific salmon harvests, which provide an index of abundance, declined more in 2020 than in any other period on record since 1930,” the technical report said. “Commercial salmon harvests declined by approximately 187 million fish compared with average harvest during the previous 10 years.
“Although the COVID-19 pandemic may have reduced commercial harvests to some extent in some regions, most fishery reports and preliminary escapement estimates indicate low abundance rather than harvest impacts from the COVID-19 pandemic led to unusually low harvests of Pacific salmon in 2020.
“We hypothesize that a tipping point was reached in the North Pacific Ocean, leading to the substantial decline of all five species of Pacific salmon in 2020. We infer that the tipping point was caused by the combined effects of unusually frequent marine heatwaves since 2014 and exceptional back-to-back year abundances of pink salmon in 2018/2019.”
Nature at work
The hypothesis mirrors what has happened to some wildlife populations managed to achieve maximum numbers.
“Consider a population near or at ecological carrying capacity (K). Intraspecific competition would be intense, per capita forage availability low, and nutritional condition poor. Under such circumstances, even a winter of moderate severity would be capable of causing high overwinter mortality,” researchers from Idaho State University, the California Department of Fish and Wildlife, the University of Nevada-Reno and the University of Wyoming observed in paper prepared for the Eastern Sierra Center for Applied Population Ecology.
They cited a predator-control study of Dall sheep in Alaska that found that the removal of wolves and coyotes from a small area in the Wrangell St. Elias National Park and Preserve significantly increased the number of sheep there.
When there followed a severe winter with a lot of snow, warm weather and crusting that made it hard for sheep to get food, however, the big population in the area where predators had been removed crashed while the smaller population in an adjacent control area remained stable.
One of the researchers involved in that study was a member of the Boone and Crockett Club, an avid hunting group. It observed on its website that “while predator control may result in an increase in prey populations, wildlife managers and predator control advocates have to consider not only the actual effects of predation on ungulate populations, but also habitat conditions, especially in relation to carrying capacity, and weather patterns, to avoid unintended consequences.”
Alaska, with its wildly varying weather patterns, has been something of a poster child for the dangers of trying to manage big game at absolute maximum carrying capacities. Historically, this has exacerbated winter kills by creating situations that crowd too many animals onto two little range when snows pile unusually deep.
A growing number of fisheries biologists are starting to believe the same sort of thing can happen in the ocean although the complicated ocean ecosystem makes it hard to prove, a fact that has been pointed out by Bill Templin, the Alaska Department of Fish and Game’s chief “salmon fishery scientist” and a defender of commercial-fishermen controlled hatcheries engaged in massive free-range production of pink salmon in the Pacific.
The hatcheries, primarily in Prince William Sound and on Kodiak Island, have been hugely successful. They this year rebounded nicely from the 2020 crash with Fish and Game crediting the Sound hatcheries for a harvest of about 51 million pinks and Kodiak hatcheries for another 14 million.
Fish and Game has yet to summarize the statewide, pink-salmon hatchery catch for the season, but it is expected to comprise more than 40 percent of the 161 million pinks that accounted for about 69 percent of the entire, statewide salmon harvest, reported by the agency.
The big harvest – about double that of the 34 million salmon bust of 2020 – was the good news. The bad news was that all those pinks – a small, mild-tasting fish – accounted for but 28 percent of the value of the statewide salmon catch.
Whatever happened to the ocean range the year before was apparently over and repairing itself by the time they hit their main feeding grounds. A Fish and Game summary of the Sound season this year cited strong returns of wild pinks as well as hatchery pinks although it is never quite clear in that area what fish are truly wild and what are hatchery strays.
Still, the agency called the returns to the Sound’s streams and rivers “encouraging given that wild fish were from the parent year in 2019 when spawners returned to dewatered streams amid a record-setting drought.”
That year was the hottest in Alaska history, and the news was full of stories about spawning salmon being cooked alive in overheated streams. “The water is so hot in Alaska it’s killing large numbers of salmon,” CNN reported at the time.
The long-term consequences, at least for pinks, appear to have been minimal.
“Alaska and Russia pink salmon abundances appear to have rebounded to the exceptionally high values in 2019,” Ruggerone reported to the Commission. “Sockeye salmon returning to Bristol Bay, Alaska, set a new record high in 2021 (66 million fish), apparently in response to favorable early marine growth in the Bering Sea and relatively few maturing pink salmon in 2020. In contrast, commercial harvests of (longer-lived) Chinook, coho, chum, and sockeye salmon remain somewhat low in many regions of Alaska and Russia.”
That ‘tipping point’
They said their hypothesis that heat and an over-abundance of pinks pushed salmon to a “tipping point” in 2020 is buttressed by a 2020 peer-reviewed study in the Canadian Journal of Fisheries and Aquatic Sciences that “found that a 1.5-degree Centigrade increase in sea surface temperature was associated with a 23 percent increase in sockeye productivity in the Bering Sea, a 9 percent productivity increase in the Gulf of Alaska, but with a 12 percent decline in productivity in the southern region (British Columbia and Southeast Alaska).”
That study, however, focused on water temperature changes from 1976 to 2009 when the northern Gulf of Alaska was still considerably cooler than in the mid-2010s. It is probable the northward movement of warm water in the 2010s years was responsible for the record increases in Bristol Bay sockeye at the expense of sockeye in the Gulf along with those in Southeast and British Columbia.
As for the explosion of pinks, which appear to enjoy a warm-water advantage, the Canadian study examining ocean warming, “also reported that a 119 million increase in pink salmon abundance was historically associated with a 9 percent decline in sockeye productivity in the Bering Sea and the Gulf of Alaska, and a 21 percent decline in British Columbia.
“This finding is consistent with the (theory of a) trophic cascade caused by abundant pink salmon and other studies indicating adverse effects of pink salmon on the growth, age-at-maturation, survival, and abundance of sockeye salmon, Chinook salmon, coho salmon, chum salmon, marine fishes, seabirds, and potentially southern resident killer whales.
“Support for the adverse interaction between pink salmon and other marine species is strengthened by biennial patterns in marine species that are consistent with the biennial pattern in pink salmon in most years; these biennial patterns cannot be explained by physical oceanography. The adverse effect of numerous pink salmon on vital rates of other salmon species is far-reaching because salmon are known to migrate thousands of kilometers at sea.”
Actually proving this to be the case is, however, hard as Templin has noted.
And thus, the scientists told the Salmon Commission that “further evaluation of 2021 and 2022 harvests and returns of Chinook, coho,
chum, pink, and sockeye salmon are needed to evaluate the validity of our tipping point hypothesis.”