Adult Chinook salmon that arrive at Astoria, Ore., in early spring for the long migration up the Columbia River are twice as likely to succumb to the jaws of a California sea lion compared with those that arrive later, a new University of Washington and NOAA Fisheries study found.
Scientists tracked 18 populations of spring and spring-summer Chinook, and determined that in years when sea lions are abundant at the Columbia River mouth, an additional 21.1 percent of early arriving Chinook—1 out of 5—were likely consumed by these predators compared to prior years. For those that arrived later in spring, an additional 10.1 percent—1 out of 10—were eaten by California sea lions.
"Estimating population-specific predation effects on Chinook salmon via data integration," was published in the Journal of Applied Ecology on Oct. 18. It concluded that early migrating Chinook are more at risk because they stay in the lower Columbia River between Astoria and Bonneville Dam for a longer period of time, and are there when California sea lions are more abundant, before many of the marine mammals depart for summer breeding grounds in California.
The loss of greater numbers of early migrating Chinook before they're able to spawn has serious implications for recovering both endangered upper Columbia River spring Chinook and threatened Snake River spring-summer Chinook, study lead author Mark Sorel told NW Fishletter. "Selective pressure to return later could interfere with migration to high-elevation spawning grounds, and lead to increased exposure to warm water," the study stated.
The study also brings up important issues related to recovery in the face of climate change. A doctoral student at the UW School of Aquatic and Fishery Sciences, Sorel noted that if sea lions are taking out more early arriving Chinook, and climate change is impeding the journey for the late arriving Chinook due to warmer water and lower flows, the migration window is smaller.
In addition, by taking out twice as many early arriving Chinook, California sea lions are reducing the number of fish that make it to spawning areas with the genetics to migrate early. That means sea lions are contributing to a loss of genetic diversity that could otherwise help Chinook survive climate change. He noted anadromous fish can migrate early either due to genetics or through adaptation.
Sorel said without that diversity, it becomes much harder to delist threatened or endangered salmon or steelhead. He said recovery plans and delisting criteria often include a requirement to have different populations within an evolutionarily significant unit, or ESU, at a low risk of extinction.
The study integrated two available datasets to estimate the population-specific and the year-specific survival rates in light of seasonal sea lion abundance. One dataset involved 1,096 hatchery fish and 343 natural-origin fish captured at Astoria and released between March 20 and June 14 in the years 2010 through 2015.
The second dataset involved detection dates of 3,393 natural-origin adult Chinook at Bonneville Dam fish ladders that had been PIT-tagged as juveniles. Co-authors of the study were Richard Zabel and A. Michelle Wargo Rub from NOAA's Northwest Fisheries Science Center, Devin Johnson of NOAA's Alaska Fisheries Science Center, and Sarah Converse, a UW associate professor and leader of the U.S. Geological Survey Washington Cooperative Fish and Wildlife Research Unit.
Through modeling, the scientists devised a mortality rate for Chinook returning to 18 different tributaries to the Snake and upper Columbia rivers. They then compared survival rates for each population from 2010 through 2012—which were considered baseline abundance years for California sea lions—with survival rates from 2013 through 2015, which brought a notable increase in California sea lion abundance.
"Earlier migrating populations had lower survival than later migrating populations during all years and experienced higher mortality corresponding with increased sea lion abundance in 2013-2015," the study stated. On average the nine earliest migrating populations experienced an additional 21.1 percent mortality and the nine populations that migrated later experienced an additional 10.1 percent mortality between 2013 and 2015, the study said.
Those nine populations of early migrating Chinook include the East Fork Salmon, Lemhi, Marsh, Upper Grande Ronde, Catherine, Minam and Tucannon river populations from the Snake River spring-summer Chinook ESU; and the Methow and Entiat river populations from the upper Columbia River spring Chinook ESU. Sorel said that for the early migrating populations, survival averaged 78 percent in 2010 to 2012. That average dropped to 57 percent for early migrants from 2013 to 2015.
The study noted considerable variation in migration timing, even in stream reaches close to each other. "For example, the Upper Grande Ronde and Catherine Creek populations migrated substantially earlier than the Lostine River population. All three of these populations spawn within the Grande Ronde Basin," the study said.
Rivers with late migrants include the Upper Salmon, Pahsimeroi, Big, East Fork South Fork Salmon, Secesh, Imnaha and Lostine populations from the Snake River spring-summer Chinook ESU; and the Wenatchee River population from the upper Columbia River spring Chinook ESU.
The study also noted successful conservation of many marine mammals, such as California sea lions, has resulted in new management challenges, creating a conflict with recovery of threatened salmon. "Moreover, recovery of certain marine mammals may impede the recovery of other marine mammals that rely on the same prey. Pinniped predation on salmon stocks that are the major food source for certain fish-eating populations of killer whales Orcinus orca has been hypothesized to contribute to declines in these killer whale populations," it noted.
Sorel said he sees the study as the first step in a larger examination of sea lions' impact on each population of ESA-listed salmon and steelhead over their entire life cycle.
"Any time you have a population that's at low abundance, and additional mortality of adults close in time to when they're going to be reproducing, that's definitely concerning," he said.
The study did not look at impacts of Steller sea lions, which have become more of a problem for fish runs in recent years. It also did not involve other ESA-listed salmon or steelhead.
"We focused on the spring period," Sorel noted. "But California sea lions come back in the autumn." That's another time period when California sea lions could be consuming significant portions of other listed species in the Columbia Basin, such as steelhead or fall Chinook, he said.
In August, National Marine Fisheries Service approved a new permit for three states and four tribes in the Northwest to capture and euthanize both California and Steller sea lions throughout the lower Columbia and Willamette rivers, and agency officials have recently launched that effort.
The study noted much ongoing uncertainty about impacts of sea lions on salmon, or the effectiveness of removing sea lions from the system.
"Identifying the management actions that will produce optimal trade-offs between conflicting values will require decision-making frameworks that explicitly wrestle with competing objectives and account for uncertainty. These frameworks will be most effective over time if they are coupled with concurrent data collection on both predator and prey—including abundance, demographic rates, and mortality—and information on species interactions," it concluded.