A single dry winter decimated California’s salmon and trout populations

A single severely dry winter temporarily, but dramatically, altered the ranges of three fishes — Chinook salmon, coho salmon, and steelhead trout — in California’s northern waterways. 

In a new study, published this week in the journal Proceedings of the National Academy of Sciences, biologists found that the unusually dry winter of 2013-2014 caused some salmon and steelhead to temporarily disappear from individual tributaries and even entire watersheds along the northern California coast.

“California is at the southern end of the range for several species of salmon and trout, and because of a whole host of impacts, from colonization and engineered control of western rivers to climate change, these populations have been decimated,” said study lead author Stephanie Carlson, the A.S. Leopold Chair in Wildlife Biology at the University of California, Berkeley. “Our findings provide a glimpse into how an individual extreme event can trigger the widespread and sudden collapse of multiple populations and species and potentially result in longer term range shifts.”

During California’s historic multi-year drought of 2012-2016, the 2013-2014 winter was remarkable for having both very little rain and an extremely late start to the rainy season. By the time the first large rainstorms arrived in late January and early February 2014, many streams and rivers in Northern California were very low, and in some, the mouths had dried up completely, preventing salmon and steelhead from completing their annual voyages upriver to spawn.   

The study examined how the drought affected Chinook salmon, coho salmon and steelhead trout, all part of the genus known as “salmonids,” in 13 coastal watersheds ranging from Marin to Humboldt counties. While all three fish species were impacted, Chinook salmon were able to cope by shifting their breeding activities downstream. However, fish monitoring data from the summer of 2014 revealed that steelhead trout had been eliminated from a number of individual tributaries, and coho salmon disappeared entirely from three coastal watersheds.

“Because of the delayed rainfall, the timing of elevated river flows was mismatched with the arrival of the fish for breeding, and we saw different impacts for different species in different places,” Carlson said. “The most extreme cases were coho salmon that spawn in coastal rivers that have so-called intermittent estuaries, where a sandbar forms across the mouth of the estuary during the dry season. There were three systems in Mendocino where the sandbar never opened the whole year, and coho salmon were lost from the entire watershed.”

In the decade since that drought, all three species have fully recovered their original ranges. This is due to both lifecycle diversity within fish populations and, in the case of the Russian River, a conservation hatchery. Salmon and steelhead can vary in how many years they spend at sea before returning home to breed; because of this, some fish from the impacted rivers were still growing at sea during the 2013-2014 season and were able to return the following year to help repopulate those rivers.

“This complexity within populations is really important for buffering them against annual variation in climate, including against extreme events, as we saw here,” Carlson said. “This underscores the importance of diversity within populations and the need to prioritize recovering life history diversity in imperiled populations to restore resilience.”

Tracking the “missing cohort” of salmonids

The study began in the summer of 2014, when Carlson’s graduate students, Suzanne Rhoades and Cleo Woelfle-Hazard, were surveying salmon at two different field sites along the California coast. Rhoades made the puzzling observation that juvenile steelhead trout were missing from one of her study sites in the South Fork of the Eel River. At the same time, Woelfle-Hazard found that coho salmon were also missing from his study sites in the Salmon Creek Watershed in Sonoma County. 

“I thought, ‘Huh, that is really unusual,’” Carlson said.

Carlson began reaching out to colleagues to see if salmonid species were missing from other watersheds. She soon connected with Mariska Obedzinski, a California Sea Grant Extension Specialist and graduate student in environmental science, policy and management at UC Berkeley, whose team had observed similar troubling patterns in the Russian River system. 

“At that point, I started to understand that this was potentially a pretty widespread event,” Carlson said. “We were seeing juveniles missing, but also some observations of adults that had spawned at unusual times or in unusual places. We pretty quickly came to the realization that something happened that limited the ability of adults to access their breeding grounds.”

Through conference presentations and word of mouth, Carlson continued to connect with other scientists who had documented the mysterious missing cohort of salmonid species. The final paper combines datasets from Carlson’s group at UC Berkeley, the California Department of Fish and Wildlife (CDFW), California Sea Grant and the Mattole Salmon Group. Carlson said that the large-scale analysis was facilitated because all of the organizations had adopted the CDFW’s standardized protocols for collecting fish survey data.

“Partnerships like this between resource managers and academia are so important for salmon recovery,” said Obedzinski. “Without these statewide long-term monitoring efforts, we would never be able to understand the impacts of these hydroclimatic events on salmon populations.”

This type of detailed monitoring data will also be key to understanding how salmon and steelhead adapt to a warming world and what will allow them to persist in the face of increasingly variable climate conditions.

"It is particularly important to understand the behavioral and life history mechanisms that allow some salmonids to persist at the southern edge of their range, because these allow them to survive and adapt to conditions that will advance further north under climate change,” said study senior author Mary Power, a Professor of the Graduate School at UC Berkeley. “These traits could be keys to salmon survival over much of their range on a warmer Earth."

Additional co-authors of the study include Kasey Pregler of UC Berkeley; Sean Gallagher of CDFW; Nathan Queener of the Mattole Salmon Group; and Sally Thompson of the University of Western Australia. This work was supported by the National Science Foundation (CZP EAR-1331940, DGE 1752814 and 1106400), the U.S. Army Corps of Engineers, NOAA-Fisheries, CDFW (Q1996052), the Eel River Critical Zone Observatory and Solano County Water Authority.