California’s rarest salmon face a narrow window for survival

A study published in Science Advances finds that Sacramento River winter-run Chinook salmon, down to a single surviving population of a few thousand fish, live or die based on river temperatures during their first 15 days. Survival odds fell about 73% for every 1.8-degree Fahrenheit rise in average temperature during that window. Shasta Dam blocks the salmon from their original cold-water habitat, forcing them to spawn in the Sacramento River, where cold water released from the reservoir competes with demand from farms and cities.

Researchers used otoliths, tiny ear structures that record temperature history, to determine that only a small fraction of spawning events actually produce surviving juveniles, even in seasons that look successful by traditional counts. The findings suggest managers have been tracking the wrong number for years. Scientists caution against concentrating limited cold water on select nests, warning it could narrow the population's genetic diversity, and instead recommend restoring access to historical cold-water habitat.

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