On a day the U.S. Fish and Wildlife Service reported a salmon population increase in Clear Creek off the Sacramento River near Redding, and touted an improved trout spawning route for endangered Lahontan cutthroat trout, it also reported juvenile Chinook salmon were dying in the Central Valley hatcheries.

Although the exact cause, and the greater effect in the rivers, have not been labeled, scientists believe the unusual mortality may be due to adult salmon feeding on anchovies over the past couple of years.

“In early 2020, staff at state and federal salmon hatcheries in California’s Central Valley observed newly hatched offspring of adult Chinook salmon that spawned in 2019. They were swimming in corkscrew patterns and dying at unusually high rates,” according to an article released by the Fish and Wildlife Service.

“They suspect the problem is linked to a deficiency of thiamine in the returning adult salmon that fed off the coast of central California in 2018 and 2019. This was a time of booming anchovy populations and scarcity of other typical salmon prey,” the article states. “Anchovies produce an enzyme called thiaminase that breaks down thiamine, which typically supports nerve, muscle, and heart function.”

Scientists said the situation could be a risk to Chinook stocks, “including endangered winter-run Chinook salmon and the fishery for fall-run Chinook salmon.”