Droughts are Shifting Migration Patterns in Already Endangered Russian River Coho Salmon

Lower flows and warming water temperatures are creating a potentially deadly timing mismatch for young salmon headed to sea
Clare Leschin-Hoar


MEDIA CONTACT: Kate Murphy, k6murphy@ucsd.edu


Pacific salmon are known to have the remarkable ability to leave the streams where they hatch and find their way to the ocean when food is most plentiful. That migration timing plays a key role in the ability of young fish to survive to adulthood and return to spawn. But in a new paper published in the scientific journal Ecosphere, the authors describe how this natural timeline is being altered by droughts and is putting further pressure on California’s already endangered Russian River coho salmon. 

Researchers from UC Berkeley and California Sea Grant found that in drought years, decreased stream flow and warmer water temperatures are shrinking and hastening the migration window of young fish (smolts) by over three weeks, and in some cases blocking passage to the sea entirely. These changes are potentially creating a deadly mismatch between when young coho salmon arrive at sea and when their food is available.

The authors of the study, “Migration in Drought: Receding Streams Contract the Seaward Migration Window of Endangered Salmon”, used microchips the size of a rice grain to track coho salmon smolt migration over 13 years, between 2008 and 2020, to determine how worsening drought conditions are altering the window of migration.

“This study sheds light on an often-overlooked, critical stage of coho salmon lives,” says lead author Brian Kastl, a PhD candidate at UC Berkeley and National Geographic Explorer. Kastl is also a former California Sea Grant graduate research fellow. “Changes in flow and temperature influence the timing of their seaward migration and hadn’t been fully recognized as a contributing factor to coho salmon population declines.” 

A young coho salmon swims in a tributary to the Russian River. Photo courtesy of Brian Kastl.
A young coho salmon swims in a tributary to the Russian River. Courtesy of Brian Kastl.

Co-author Mariska Obedzinski, an extension specialist with California Sea Grant says there has been a lot of focus on the impacts of drought on juvenile salmon during the summer dry season, but the study highlights the need to pay more attention to streamflow during the spring as well.

“It’s critical to keep water flows high enough to maintain connectivity and even small increases in flow could help keep the migration window open for fish,” says Obedzinski. 

While this study looked specifically at Russian River coho salmon, the findings also apply to many other salmon-bearing streams. As the changing climate continues to worsen droughts and increase temperatures, Kastl says the actions communities can take to reduce drought impacts on salmon by limiting water withdrawals from streams and groundwater wells will become increasingly important.

“We now know that increasing streamflow between March and June expands the salmon’s window of opportunity by over three weeks,” he says. “That’s 31% and it’s a lot. By improving access to the ocean, we have the opportunity to improve the chances that adults will return. Russian River coho salmon was once a principal food source for Native Americans and supported a commercial fishery. We now have another opportunity to move the needle back towards reviving this inspiring species.”

About California Sea Grant

NOAA’s California Sea Grant College Program funds marine research, education and outreach throughout California. Headquartered at Scripps Institution of Oceanography at the University of California San Diego, California Sea Grant is one of 34 Sea Grant programs in the National Oceanic and Atmospheric Administration (NOAA), U.S. Department of Commerce.