Study shows how coho salmon survive during droughts

California Sea Grant/Courtesy

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In a study published April 15, scientists from UC Berkeley and California Sea Grant — a program that funds marine research — discovered that drought “refuges” are contributing to the preservation of coho salmon, an endangered species.

The study, which was published in the journal Global Change Biology, examined the survival of juvenile coho salmon in Northern California’s Russian River watershed over a period of seven years, during which California experienced an extreme drought. The findings provide insight into potential techniques for better maintenance of populations of endangered marine species in the face of habitat fragmentation.

“The over-summer survival of this species, coho salmon, declined significantly during the drought period,” said Ted Grantham, assistant cooperative extension specialist and environmental science, policy and management adjunct professor at UC Berkeley. However, we also found that some streams were very resilient to drought.”

Grantham added that this shows that some streams could continue to support fish populations in nondrought years at similar levels, which he said was “somewhat surprising.”

The study began in 2011 as an effort to learn more about factors affecting coho salmon survival during the summer. The team tagged about 20,000 fish in the watershed with microchips so the growth and survival of individual fish could be tracked throughout the course of the study.

“We were in general just trying to understand the relationship between survival and these environmental factors, but it just so happened that during that period we had a drought,” said Mariska Obedzinski, California Sea Grant extension specialist. “It provided a really good opportunity to understand the effects of these different environmental barrier variables, both in drought years and nondrought years.”

During the summer, the surface level of the water in the watershed often falls, creating fragmented pools of water and preventing streamflow. The main determinant of juvenile coho salmon survival was whether or not these pools of water dried up.

The amount of time the pools were disconnected from the main stream also contributed to the salmon survival rates. The longer the pools remain fragmented, the survival of the juvenile coho salmon becomes less likely.

Now that the researchers know that there are “refuges,” or pools, in which the salmon can survive at higher rates, a potential next step is to determine how to identify these refuges on a larger scale, according to Obedzinski. This could eventually lead to more focused protection efforts in marine habitats.

Expanding the scope of the study in future research requires use of remote sensing technology, according to Ross Vander Vorste, an assistant professor of biology at the University of Wisconsin-La Crosse who was a postdoctoral researcher at UC Berkeley while working on this study.

“We only focused on this Russian River watershed, but these salmon, they stretch up across from the coast of California,” Vander Vorste said. “If we want to identify these drought refuges, we need to figure out a way to automate identifying them.”

Emma Rooholfada is the lead research and ideas reporter. Contact her at [email protected] and follow her on Twitter at @erooholfada_dc.