Drought threatens Russian River salmon

Drought is posing another threat to endangered native salmon in the Russian River by narrowing the window of time for young coho salmon to travel through to the ocean, which limits their feeding opportunities and threatens their chances of surviving to adulthood, new research indicates.

When their native streams run low and warm during drought, the young smolts or year-old salmon get a signal to leave earlier than usual for their migration to the ocean. They may also face obstacles along the way due to lower flows of water. Those two factors shrink their window of migration by over three weeks, according to a study in the journal Ecosphere led by Brian Kastl, a doctoral candidate at UC Berkeley.

“The reason that’s so important is because salmon risk reaching the ocean at the wrong time, when food is scarce,” said Kastl, whose field research was funded by National Geographic. “A long migration window is really critical for the future of the California salmon, because it increases that likelihood that at least a portion of that population reaches the ocean when unpredictable food — zoo plankton, shrimp and krill — is abundant.”

The salmon have evolved to migrate to the ocean between March and June. It’s a time when there can be a lot of upwelling, when seasonal winds push warmer water from the ocean surface and bring cold water and nutrients from the depths, creating a base for the food chain. But upwelling is unpredictable, and if the entire juvenile coho salmon population arrives when those conditions aren’t in place, not as many would survive, Kastl said.

Kastl and his colleagues studied over 25,000 1-yearold coho salmon from six streams that feed into the Russian River and out to the ocean between 2008 and 2020. The fish had microchips about the size of a grain of rice that pinged transponders at the confluence of each stream and the Russian River, giving an indication of when they migrated. The scientists looked at water flow rates and temperature of streams to see how those factors influenced the salmon’s migration.

While other research has looked at the effect of drought on salmon in the summer months, less is known about them in this vulnerable stage of life, Kastl said. Coho salmon are found in rivers on the West Coast as far south as Santa Cruz, but many California populations are barely holding on.

Russian River coho salmon were once a major source of food for local indigenous tribes and the base of a commercial fishery, with as many as 15,000 fish in 1952, according to Sea Grant California. They almost disappeared until a captive breeding program began in 2001, bringing the number of adults that return from the ocean each year into the hundreds. Kastl said the lessons can apply to other salmon populations that are increasingly impacted by drought.

Saving water at home and more efficient water use in farming, so that less groundwater pumping is needed, help the salmon especially during spring and summer, Kastl said.

“Upstream water conservation can increase downstream flows, ultimately protecting salmon migration,” he said. “And it reminds us that our actions often affect others downstream, even when those other folks — or those species — might be out of sight.”