The Skagit River coho run of 2015 ranked as the worst on record, and the few salmon that did make it back to freshwater spawning grounds were scrawny and undersized.

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The Skagit River coho run of 2015 ranked as the worst on record, and the few salmon that did make it back to freshwater spawning grounds were scrawny and undersized. While an average adult coho spawning in the Skagit weighs some 6 to 8 pounds, the vast majority of fish that returned last fall weighed only 3 to 4 pounds.

“I personally have never seen them any smaller, and I have been doing this for 26 years,” said Brett Barkdull, a district biologist based in Skagit County.

The dismal run was less than 12 percent of the Skagit’s average of the past decade, and was part of a broader implosion of coho salmon returns to Puget Sound and rivers in Washington and Oregon.

Scientists say that “the blob” — a vast expanse of abnormally warm coastal waters — was the most likely suspect in the poor show of fish because it reduced the food supplies for coho that primarily forage in the coastal waters.

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The blob is expect to undermine some of this year’s runs, as well.

“We know the (coastal ocean) conditions have been very unfavorable,” said Mara Zimmerman, a research scientist with the state Department of Fish and Wildlife. “We are expecting very poor coho returns in 2016.”

Some salmon species, such as Northwest pinks, venture farther offshore and therefore seem be less vulnerable to the blob. But the blob appears to have affected other salmon in addition to coho.

In Alaska, the Bristol Bay sockeye, a mainstay of commercial fisheries, were abundant but smaller than they’ve been in more than 20 years, according to Laurie Weitkamp, a NOAA Fisheries researcher.

And to the frustration of Washington fishermen, Canada’s Fraser River sockeye appeared to have altered their migration path to avoid U.S. harvest grounds where the temperatures had warmed.

The Columbia River chinook had banner returns in 2015. But this year, there is concern about the fall run of chinook, which like the coho, spend much ocean time in coastal waters.

The blob emerged during climatic pattern shifts as a high-pressure system blocked storms that could mix up layers of the ocean. It gathered strength in early 2014 and spread unusually warm water from Mexico to Alaska.

In salmon-friendly ocean years, cold-water upwelling brings nutrients to the surface. But during the blob’s reign, temperatures rose up to 7 degrees Fahrenheit higher than normal. That changed the mix of species offshore, weakening the food chain that supports shrimplike amphipods, krill, herring and other creatures that young salmon feed on.

The warm water also has been linked to the die-off of starfish, high numbers of beached, emaciated Steller sea-lion pups and toxic algae blooms.

When NOAA’s Weitkamp participated in a sea survey last June, many of the young coho were undersized, and a higher number than normal had empty bellies.

The blob has faded this year, but it is expected to contribute to slightly higher than normal ocean temperatures. Some of the coastal waters also may be influenced by an El Niño climate pattern that in years past has driven a deep layer of warm water to the north.

“We’re still seeing the blob. It’s cooler, but the food chain is still screwed up,” said Bill Peterson, a NOAA Fisheries oceanographer, who was on a survey cruise earlier this month.

All this is expected to create less than ideal conditions this spring for young salmon arriving at sea from freshwater rearing grounds.

“The forecast for temperatures don’t look good all the way to Alaska,” Weitkamp said.

The fate of the Skagit River coho is viewed by biologists as a stark example of the blob’s impact.

In spring 2014, state fishery biologists monitoring the downstream migration of Skagit River coho found their survey traps full of young fish. That raised hopes that the salmon would head out to sea to fatten for a year, then return in good numbers as adults in the fall of 2015.

While average Skagit coho returns over the past decade tally more than 50,000 fish, preliminary estimates have put the 2015 returns at about 5,600 fish — the lowest since record-keeping began back in the 1960s.

And such a small run of undersized fish will result in far fewer offspring, and thus smaller returns in the future.

“The effects of the climatic conditions will be felt over multiple years,” said Zimmerman, the state research scientist.