Chinook salmon spawning season improves

The picture has brightened slightly for endangered winter-run Chinook salmon after two disastrous spawning seasons.

The number of juveniles migrating downstream this fall is roughly twice what it was last year, thanks to better temperature conditions in the Sacramento River.

“We’re declaring success on maintaining temperatures,” said Maria Rea, a regional fisheries administrator for the National Oceanic and Atmospheric Administration.

The improvement is crucial for the winter-run, which has suffered devastating spawning losses in California’s prolonged drought. Lethally high river temperatures meant most of the eggs and salmon fry below Shasta Dam perished in 2014 and 2015.

Since the salmon have a three-year life cycle, another spawning failure this year would spell calamity for the once abundant fish.

Knowing that, NOAA demanded extra measures this year to maintain the amount of cold water behind Shasta Dam that could be released into the river to keep the eggs and newly emerged fry cool.

That resulted in the U.S. Bureau of Reclamation, which operates the dam, reducing spring and summer releases to irrigation districts, prompting complaints from growers.

“These decisions are difficult and there are water-supply trade-offs,” Rea acknowledged.

Her agency also set a lower temperature standard for water in the spawning sections of the river, which Rea said the reclamation bureau largely succeeded in meeting.

The efforts slashed temperature-related mortality of eggs and fry to a fraction of what it was the past two years, she said Thursday.

Still, this spawning season is far from ideal.

The spawning Chinook adults were born in 2013 and 2014, during stressful drought conditions, so there are fewer of them. Biologists also said poor ocean conditions last year mean the returning salmon are smaller, which in turn reduces the number of eggs each female lays.

Fishery managers have so far counted about 480,000 juveniles migrating down the Sacramento to the sea, where they will spend their adulthood.

That is far more than the 236,220 counted in the same period in 2015 or the 322,600 in 2014. But it’s way below the 840,439 migrating juveniles tracked as of early November 2013 – or the 1.1 million counted in 2010.

bettina.boxall@latimes.com

Twitter: @boxall

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