Coho Salmon's Survival Strategy: Different Paths to Success

A new NOAA-funded study using data from a California Sea Grant salmon-monitoring program has revealed that the migratory behavior of young endangered coho salmon is more diverse than scientists once thought. “Individual salmon make different choices about where to rear and that contributes to population stability,” says Hank Baker, a postdoctoral researcher with the University of California, Berkeley Freshwater Research Group and the lead author of the study. “From a restoration perspective, this means we need to start thinking about restoring habitats that are outside the traditional spawning areas — which can still be places where the fish spend a significant portion of time.”

Unexpected Diversity

The study, which was published in Ecology Letters, followed 3,270 tagged coho that belonged to  nine yearly cohorts. Many of the fish followed a previously known life history pattern: As new hatchlings they spent their first year in their upstream natal habitat, then quickly migrated downstream to the estuary and the ocean beyond before returning as adults to spawn in the same stream in which they had grown up.

But a surprisingly large number of young coho diverted from this traditional pattern, abandoning their natal environment earlier than expected and spending months in other parts of the stream closer to the estuary. These adventurous coho also often headed to sea earlier than salmon that stayed put in their natal habitat for their full first year.

This led to a “portfolio effect,” the researchers say: By exhibiting a variety of migration strategies, the local coho population was more stable, similar to how diversifying investments across markets reduces financial risk. When young coho head to different parts of the watershed at different times, “the population is essentially hedging its bets,” says Mariska Obedzinski, a California Sea Grant Extension Specialist, who participated in the study. “We found that the number that made it to adulthood and returned to spawn each year was more consistent than if all the young had stayed in their natal habitat.”

A creek transformed

The study took place in Willow Creek, a tributary to the Russian River which flows from its headwaters north of the town of Ukiah to the Pacific Ocean near the town of Jenner. Sitting in one of California’s prime wine-producing areas, the Russian River watershed provides drinking water to over 600,000 residents across Sonoma, Mendocino and Marin counties and attracts hundreds of thousands of annual visitors. It is also a core habitat for the Central California Coast coho salmon. The fish, which can grow over two feet long, occur all around the Pacific Rim but the unique populations native to Central California have declined to about 1% of their historic population levels, largely due to the destruction of their freshwater breeding and rearing habitats. 

 Since 2005, the National Oceanic and Atmospheric Administration (NOAA) and other organizations have worked on restoring Willow Creek, which had been straightened and widened for agriculture, and its surrounding hillsides have been intensively logged in the 20th century, causing the stream to become choked with sediment. “The creek went from having a really rich history of coho salmon to coho salmon going pretty much extinct in it,” says Joe Pecharich, a habitat specialist with the NOAA Restoration Center in Santa Rosa, which funded the present study. For the past 20 years, NOAA has worked with a coalition of state and local partners to rehabilitate Willow Creek habitat and improve fish passage. “Since then we've seen fish coming back pretty much every single year,” says Pecharich. “It’s super exciting to see these signs of recovery.”

Two tales of migration

Each year, the restoration team releases thousands of juvenile coho from a conservation hatchery into the creek, some of which are tagged for monitoring. Biologists kept noticing juvenile coho showing up in the lower parts of the creek — sometimes months before their traditional outmigration to sea was expected to bring them there. 

“Upstream, the creek begins as a classic cold-water coho stream, flowing through second-growth redwood forest over gravel and cobble but at its midpoint, it transforms dramatically," says Baker, the study's lead author. "Here, the creek spreads into a complex network of braided channels that wind through a wide, flat valley with dense alder forest."

“Adaptive strategy” 

Previously, scientists had thought that juvenile coho leave their natal upstream areas early when they are forced out by stronger rivals. But Baker found that competition had little influence on the number of fish leaving prematurely in any given year. What’s more, the precocious emigrants often grew faster in their new habitats and were overrepresented among the fish that later returned as adults to spawn. This indicates that rather than being underdogs forced into inferior habitats, these coho made a calculated gamble by leaving their natal stream early — a high-risk, high-reward tactic that often proved successful. “Our paper shows that it's quite an adaptive strategy,” says Baker.

Intraspecific diversity — in which certain members of the population diverge from typical behavior — is common in other animals too. For example, some caribou undertake long-distance migrations, while others are more sedentary; finches of the same species have been observed to specialize in eating seeds of different sizes. 

Climate threat

“Salmon recovery efforts need to take such intraspecific diversity into account”, says Obedzinski. “With coho, we get into this mindset that the fish spend the majority of their freshwater life in the natal habitat rather than rearing in different parts of the watershed,” she notes. “We need to think more about the big picture: Fish that start out in one stream might actually need to use habitat in other parts of the watershed to successfully complete their life cycle.”

The study also revealed that leaving early only seemed to work when stream flows were adequate. During the two most severe drought years studied, few fish departed early. As droughts become more frequent due to climate change, this could mean that the diversity in migration timing could diminish — adding yet another threat to the population stability of this highly endangered fish.