SEATTLE — During a summer that brought record-breaking onshore heat in the Northwest, young salmon emerging from the Columbia River encountered cool coastal waters and an abundance of food.
Those prime ocean conditions are expected to result, during the next few years, in increased numbers of coho and Chinook salmon returning to spawn in Northwest rivers and streams.
The conditions were created by a series of strong upwellings in the spring, summer and into September, that brought cold nutrient-rich waters up from deeper layers of the ocean, according to an “ocean indicators” summary published this month by NOAA Fisheries based on surveys by federal and Oregon State University scientists.
The coastal surveys have been conducted for 24 years, and the 2021 overall ocean ecosystem conditions ranked second best, after 2008, for juvenile salmon and other marine species, according to the summary.
The survey results indicated a dramatic upsurge in the number of Neocalanus, a subarctic species of copepods — tiny crustaceans less than a third of an inch in size — that are an important food source for larval fishes salmon prey upon.
“The [net] samples were just pink with them. So that was really exciting,” said Jennifer Fisher, a National Oceanic and Atmospheric Administration biologist who assisted in monthly surveys aboard the Elakha, an Oregon State University research vessel, as well as other research cruises.
This was a dramatic contrast with some bleak years in the past decade, when weaker seasonal upwellings and warmer water offered more meager fare for the young salmon.
Climate change appears to be adding to the intensity of the periodic marine heat waves. And the surveys have helped scientists develop a better understanding of ocean conditions during a critical point in the development of young Northwest salmon: the transition period when they enter saltwater after earlier stages when they hatch and rear in fresh water.
During research cruises, scientists conduct a range of sampling of plankton collected throughout the upper 100 meters, or 328 feet, of water. They also monitor temperature, oxygen levels, salinity and other variables to an ocean depth of 3,500 meters, or more than 11,000 feet. Two surveys this year, performed under contract, focused on sampling the abundance of salmon.
In the ocean, salmon forage and grow to adult size. Those that survive their time in saltwater return to spawn in rivers and streams. Once in freshwater, they face new challenges, including drought and heat waves that, at time, can raise some water temperatures to lethal levels for the fish. Sockeye salmon have been the most vulnerable as they return to spawn in summer months as water temperatures climb.
In the Columbia River Basin, 13 wild runs of salmon and steelhead are listed for protection under the Endangered Species Act, and billions of dollars have been spent to try to restore runs, in part, by improving freshwater habitat. There have also been decades of investments to produce hatchery fish.
The surveys of the Northwest coast have documented how — even in years where freshwater conditions favor the salmon — juveniles may struggle they first emerge into saltwater.
The researchers found salmon survival rates plummeted in 2015 through 2017, when a marine heat wave known as the blob took hold. During these years, there were weak upwellings when Neocalanus and other cold-water fare were largely absent and replaced by warmer-water copepods with less nutritional value. This resulted in skinnier salmon and fewer surviving to return to spawn, according to Fisher.
“The assumption is that when these juvenile fish hit the ocean, that’s their most sensitive life history stage … that’s where most of their mortality occurs,” Fisher said.
The cold-water upwellings that favor young salmon survival result from seasonal winds blowing from the northwest. In 2021, as a big high-pressure area built in the Pacific, ocean waters did warm. But the coastal upwellings helped to keep that water mass further offshore, and provided a cooler haven and more bountiful fare for the young salmon.
Satellite images from 2021 showed a strip of cold water near the Pacific Northwest coast and a larger mass of warm water farther offshore, according to Fisher.
The 2021 survey results build upon a broader upswing in ocean conditions stretching back several years, according to Brian Burke, a federal biologist working at the Northwest Fisheries Science Center, where he heads up an ocean ecology team.
Those earlier improvements in coastal ocean conditions likely contributed to a jump in the number of coho salmon that returned to the Columbia River this year, according to Burke.
Most of these coho spawn in the lower river, and a final tally is not yet available, according to Marisa Litz, an Olympia-based research biologist with the Washington Department of Fish and Wildlife.
A smaller portion head further upstream and are counted as they make their way past Bonneville Dam east of Portland. As of Dec. 22, their numbers topped 243,000 — more than double the 10-year average, according to counts posted by the Fish Passage Center.
So far, signs for the 2022 coastal ocean conditions also are positive. The pool of offshore warm water has shrunk and is now more distant from the West Coast. Meanwhile, the past months of near-shore cold-water upwellings are expected to provide another boost to coho returns.
“Looking to next year, there is a lot to be optimistic about,” Litz said.
But in the decades ahead, even as ocean temperatures cycle between cooler and warmer periods, climate change is forecast to keep pushing the upper range of temperatures higher, according to Nathan Mantua, a Santa Cruz-based NOAA Fisheries scientist who leads a landscape and seascape ecology team.
This is expected to make the task of restoring Northwest wild runs more difficult.
“Basically, the heat content of the ocean, it just keeps going up,” Mantua said.