This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Sacramento River winter-run Chinook salmon are California's most endangered salmon, with a single surviving population and an average of a few thousand returning fish in recent decades. Shasta Dam blocks their original habitat in cold mountain rivers draining Mount Shasta, leaving them to spawn in the low-lying Sacramento River, heated by the summer sun.
The research, published this week in Science Advances, found that cool river temperatures in the first 15 days after their eggs were fertilized were crucial for their survival. In warm years with less water, cold water barely covered this critical thermal window, sharply limiting survival in what may become an increasingly common reality for salmon as the climate warms. "Salmon may spawn throughout the summer season, but our results suggest that only a small fraction of those spawning events ultimately produced the juveniles that survived," said Kohma Arai, who authored the research as a postdoctoral scholar at UC Davis with other scientists from the university, NOAA Fisheries, UC Santa Cruz, UCLA and the Norwegian Institute for Nature Research.
"This helps us understand the connection between what individuals experience during development and how populations respond to environmental change, which is becoming increasingly important as climate change continues to reshape aquatic ecosystems," he said. Water managers try to release cold water from Shasta Reservoir to cool the river enough for salmon eggs incubating in river gravel to survive, while also delivering water to farms and cities vital to California's economy. The balance grows more difficult in low-water years.
Water agencies, water users such as irrigators, and federal and state fish agencies prioritized the new research to address science questions key to salmon survival in a changing climate. Fisheries biologists long measured the annual success of winter-run Chinook salmon by the number of adult salmon returning upriver to spawn, presuming many were successful. However, laboratory studies suggested eggs may be sensitive to river temperatures.
The new research probing that connection shows that in some years the true success of the species may be much more limited. "We learned that even when surveys show many salmon spawning over an extended season, only those eggs that experience the right temperatures are likely to survive," said Rachel Johnson, a research scientist at NOAA's Southwest Fisheries Science Center and University of California Davis who helped lead the research. "In some years, that number can be shockingly small.
It's this number of successful spawners that we need to track to assess the true risk of extinction and goals toward recovery." This survival bottleneck can dramatically reduce population size and erode diversity that helps salmon adapt to a changing climate, she said. She cautioned against using the results to target limited cold water to the key 15-day window for only part of the spawning season, favoring a reduced number of redds, or nests. "Selecting the winners at this life stage can have unintended consequences by narrowing the chance that salmon encounter favorable conditions at later life stages—it's like putting all your eggs in one basket." Instead, the findings argue for strategies such as reintroduction to historical habitat that give fish more options to survive, benefiting the environment, tribal communities and economy in the long run.
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