The littlest of Pacific salmon is once again demonstrating big power in the ecosystem of the northern ocean.
Scientists have found evidence that appears to link pink salmon abundance to high death rates and low birth rates in critically endangered southern resident killer whales that inhabit the waters of Washington State, British Columbia, Canada; and Southeast Alaska.
A peer-reviewed study published earlier this month in Marine Ecology Progress Series concludes pink salmon abundance is the only likely explanation for even-year spikes in deaths, along with declines in births, that now characterize the small and struggling whale population shrinking toward extinction.
The connection is not what you might at first suspect: starving whales in years when pinks are scarce followed by healthier periods when pinks are abundant.
Instead, in this case, the situation is exactly the opposite. The whales suffer most when pinks are abundant and do better when they are scarce. The situation appears to have a lot to do with diet.
The fish-eating whales seldom prey on three- to five-pound pinks. The salmon on which they are largely dependent are Chinook – kings as Alaskans are prone to call them – five to 10 times bigger than pinks and sometimes more.
Despite any direct predator-prey relationship between pinks and the endangered killer whales, Seattle scientist Greg Ruggerone; Auke Bay, Alaska scientist Gus van Vliet; and colleagues from the University of Alaska Fairbanks and the Alaska Department of Fish and Game, found a hard to ignore link between the two species.
“From 1998−2017, 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 as the population decreased from 92 to only 76 whales,” their study reported. “Percent mortality was 3.1 times higher in even years during the recent 20-year period of population decline than during an earlier 22-year period (1976−1997) of population increase and relative high abundance, whereas mortality in recent odd years was 43 percent lower.”
“We have identified a previously unknown odd/even-year factor linked significantly to survival and low birth rate of southern resident killer whales (SRKW),” they concluded. “None of the purported factors of SRKW decline (Chinook salmon abundance, toxic contaminants, ship noise) can directly explain the biennial demographic pattern. In contrast, the extreme biennial pattern in abundance of pink salmon, coupled with the known or suspected effects they have on other species and ecosystem processes elsewhere in the North Pacific, is consistent with the demographic pattern observed in SRKWs.”
Lost in the crowd
The scientists suggested two different theories that might explain the linkage between pinks and whale survival. One is pretty simple; big schools of pink salmon simply get in the way of the whales.
“One hypothesis…is that highly abundant odd-year pink salmon interfere with the ability of whales to feed on co-migrating summer and early fall Chinook salmon in the Salish Sea during a two-month period when the (whale) are highly dependent on the already depleted Chinook salmon.”
Chinook salmon runs through the Pacific Northwest have struggled in recent years as ocean conditions have turned in favor of some Alaska salmon in particular – sockeye from the state’s Bristol Bay have been returning in record-breaking numbers – and pink salmon in general.
Pink salmon in the Salish Sea – a 6,500-square-mile body of water stretching north from southern Puget Sound in Washington State to Desolation Sound in British Columbia, Canada – are now about “50 times more abundant than the co-migrating Chinook salmon in this area,” the Ruggerone study notes. “Reduced foraging efficiency of the whales would lower their nutritional status, which would be expressed in the following even year because these large mammals have a strong physiological buffering capacity.
“Furthermore, the finding that lower nutritional status of southern resident killer whales is linked to higher rates of miscarriages and mortalities of newborns supports our observation of higher newborn mortality in even years.”
The southern resident killer whales largely spend the summer and fall in the Salish Sea feeding on salmon, and then range south as far as Monterey Bay, Calif. and as far north as the Panhandle in their winter and spring searches for food, according to the National Oceanic and Atmospheric Administration.
The dependence of the southern resident whales on Chinook salmon has long been known. A member of the Washington Fish and Wildlife Commission earlier this year called for stocking 50 million hatchery king salmon smolt in Puget Sound in hopes of providing more food for the hungry whales.
“These would be released in carefully selected areas where negative impacts to the genetic strains of wild Chinook salmon would be minimized and using genetic strains for hatchery production that have migration patterns that take them to the areas where the killer whales are so they can feed on them,” Don McIsaac, a former member of the Pacific Fisheries Commission, said on ESPN’s Outdoor Line in Seattle in August.
The whales are tangled in both the practicalities and politics of Pacific Northwest Chinook management. There are struggles to figure out how to get adult salmon upriver past dams and debate on how to pass young salmon back downstream without all being eaten by predators in reservoirs or churned into salmon burger in electricity-generating turbines.
There are struggles over the allocation of the small allowable catch of returning fish between wild predators, Indian tribes, anglers and commercial fishermen. And there are disagreements on whether hatcheries are a good thing for the fish or a bad thing for the fish.
Hatchery stocking has been reduced in recent years amid concerns one consequence of hatcheries is fewer wild Chinook, but the hatchery fish increasingly appear vital to the survival of the southern resident whales.
The whales could drive a change in hatchery policies, Karina Brown reported for The Courthouse News Service in September, writing that “in Puget Sound, about 80 percent of the salmon returning from the Pacific are hatchery fish.
“The government appears poised to increase hatchery Chinook by about 25 million per year. In a survey of members of the Southern Resident Killer Whale Task Force compiled Aug. 23, the top three recommendations out of 60 were to increase hatchery Chinook, accelerate habitat restoration for Chinook and require small boats to stop using echo sounders.”
Washington state Gov. Jay Inslee has proposed the state spend more than $1 billion over the next several years to try to help save the whales. Most of the money would go toward habitat restoration, but there’s almost $100 million for hatcheries in the proposal, the Chinook Observer reported.
“An adult male orca needs about 325 pounds of Chinook salmon every day,” its story added. “The problem is that the number of Chinook available in the inland seas from southeast Alaska to southern Puget Sound and along the west coast to the Columbia River has…been in decline.”
While it is possible pink salmon are preventing the orcas from finding the Chinooks they need, the latest study says, there is also a possibility the bounty of pinks is in some way helping keep the whales from disappearing.
“An alternative hypothesis is that pink salmon enhance the ability of whales to feed, resulting in lower mortality in odd years of high pink salmon
abundance,” the scientists wrote.
The high death rate and low birth rate for even years when few pinks are present could be the norm for the southern population with the situation improving with a “hypothesized positive effect of odd-year pink salmon since 1998,” the study said.
“As noted earlier, mortality of southern resident killer whales in odd years since 1998 has been 43 percent lower than in odd years during 1976−1997. However, at present we do not have a mechanistic explanation for how pink salmon would enhance southern resident killer whale foraging on Chinook salmon.”
The study suggests that given the seemingly obvious link between pinks and the whales further investigations should soon be undertaken.
“….A field study that quantifies foraging efficiency of southern resident killer whales during even versus odd years could be designed to test the two, pink-salmon hypotheses identified here,” it says. Modeling of southern resident killer whale demographics in relation to Chinook salmon and pink salmon abundance would also be informative.”
Such studies, the scientists said, could help shed light on both the why of the death rate among older and younger whales, and the biennial drop in births.
The swings in pink numbers in the Salish Sea since 1976 are extreme with returns of only 100,000 to 400,000 in even years followed by odd year returns in the range of 16 to 19.6 million.
“Pink salmon have been shown to be very influential in ecosystems elsewhere in the North Pacific, where they initiate pelagic trophic cascades
and impact the growth and productivity of various other consumers, including several other species of salmon, other fishes, and seabirds in odd years of high abundance,” the study’s authors wrote.
Alaska’s chief commercial fisheries scientist has challenged that conclusion. Alaska Department of Fish and Game scientist Bill Templin in October dismissed a variety of studies dealing with trophic cascades. There are so many complicating interactions between the hundreds of species of North Pacific predator and prey interacting out of sight beneath the waves that it is impossible to draw conclusions as to why various bird or fish populations collapse, he said.
His comments came while the state Board of Fisheries was considering a proposal to freeze hatchery production of pink salmon in Alaska. The Board eventually voted against imposing a limit.
Alaska is a world leader in hatchery salmon production in the Pacific. It released almost 1.6 billion young fish last year, according to the North Pacific Anadromous Fish Commission, and that combined with about 375 million from hatcheries in the Lower 48 helped the U.S. claim the title of number one in the hatchery business.
In the definitive paper on North Pacific salmon populations, Ruggerone and Canadian scientist James Irvine in April reported that pink, chum and sockeye salmon populations in the Pacific are at never-before highs with pinks leading the way.
The scientists linked the increase to a warmer ocean, writing that a “1977 regime shift benefited each species. During 1990–2015, pink salmon dominated adult abundance (67 percent of total) and biomass (48 percent), followed by chum salmon (20 percent, 35 percent) and sockeye Salmon (13 percent, 17 percent). Alaska produced approximately 39 percent of all pink salmon, 22 percent of chum salmon, and 69 percent of sockeye salmon, while Japan and Russia produced most of the remainder.”
The peer-reviewed study was published by the American Fisheries Society and added to the debate about just how far Alaska’s hatchery-driven, salmon-ranching program should go. The state long ago banned salmon farms and now tries to compete with the pen-raised fish that dominate the market by ranching salmon.
“Although production of natural‐origin salmon is currently high due to generally favorable ocean conditions in northern regions,” Ruggerone and Irvine wrote. “Salmon abundance in large areas of Alaska (Prince William Sound and Southeast Alaska), Russia (Sakhalin and Kuril islands), Japan, and South Korea are dominated by hatchery salmon. During 1990–2015, hatchery salmon represented approximately 40 percent of the total biomass of adult and immature salmon in the ocean.”
A 2017 study of the Exxon Valdez Oil Spill (EVOS) that went looking for long-term environmental damage stumbled on evidence the Sound hatcheries were affecting runs of sockeye salmon to the Copper River just south of the Sound.
The peer-review study by a team of state, federal and univerity scientists who published at PLOS One “found no evidence supporting a negative EVOS impact on sockeye salmon or pink salmon productivity (and) weak evidence of a slightly positive EVOS signal on Copper River Chinook salmon productivity,” but “all sockeye salmon stocks examined exhibited a downward trend in productivity with increasing PWS hatchery pink salmon returns.
“While there was considerable variation in sockeye salmon productivity across the low- and mid-range of hatchery returns (0–30 million), productivity was particularly impacted at higher levels of hatchery returns.”
The latest study adds to the growing evidence that big swings in salmon abundance, whether natural or human driven, can have significant effects on more than just the salmon species enjoying a boom.