Imagine a world where just six out of every 100 newborns make it to their teenage years, the rest unable to survive post-apocalyptic environmental conditions that have become too strange and dangerous for human life.
That’s the plight of California’s once-thriving Chinook salmon, a population that now sees 94 percent of its juveniles die within the few weeks they spend trying to reach the sea from the freshwater sources where they first hatched.
This tragic reality is almost entirely due to how their native waterways in the state’s Central Valley have been turned into a system of levees, channels, and large high-head dams that are tightly managed almost exclusively for human needs. In terms of how water is allocated, wildlife is essentially an afterthought.
But the Central Valley Salmon Ecology Group, a team of researchers that bridge academia and resource management facilitated by the Fisheries Collaborative Program (FCP) at the University of California, Santa Cruz, has come up with a playbook for how water managers can tweak the timing, temperature and volume of releases to dramatically increase the odds of juvenile salmon surviving the perilous journey to the open ocean.
The approach, called “facilitated migration,” is detailed in a paper published on July 3 by the Ecological Society of America’s journal Ecological Applications.
“We’re already playing God with these ecosystems,” said the paper’s lead author, Benjamin Burford, an assistant project scientist at the Institute of Marine Sciences (IMS) at the University of California, Santa Cruz. “Can we play God a little bit better for a couple weeks out of the year to help more fish survive the migration? Yes, we absolutely can.”
Like other members of his group, Burford has a dual affiliation with UC Santa Cruz and the Fisheries Ecology Division of the National Oceanic and Atmospheric Administration (NOAA). The group collaborates with the U.S. Fish and Wildlife Service, U.S. Geological Survey, California Department of Fish and Wildlife, UC Davis, and other partners whose shared mission is to produce applied research that can help state and federal water managers in the Central Valley make more informed decisions that serve both people and wildlife—a contentious issue, historically.
Conceptual and practical guidance
The paper’s authors present both a conceptual framework, which could apply to other species that migrate in highly modified environments, and practical steps spelled out in operational terms that water managers can understand and implement. The study shows that the approach can increase successful juvenile-salmon migrations by 40 to 400 percent.
Even before the paper was published, water managers used its approach last spring to design the most optimal pulse flows for juvenile salmon migrating in the Sacramento River, according to the paper’s senior author, Cyril Michel, an associate project scientist at IMS. This real-time fish-tracking website has a preliminary assessment of that effort.
Fundamental to the framework they developed is the breakdown of migration into three distinct phases and the measures water managers can implement:
Preparation — During this phase, juvenile salmon undergo biological changes for the transition from freshwater to saltwater habitats. At this stage, managers can cue these physiological adjustments by releasing reservoir water to adjust river temperature.
Initiation — This is when fish decide to start migrating, often in response to environmental cues like changes in flow. At this stage, managers can schedule pulse flows—abrupt surges in river flow—to synchronize and trigger migration at the best time. The magnitude of these pulse flows could range from about 11,000 to 13,000 cubic feet per second.
Passage — This is the migration journey itself, where success depends on flow conditions. At this stage, managers can give the young fish more of a fighting chance by releasing extra water into the river. This increases habitat volume, dilutes predator concentrations, and speeds migration by pushing fish through the system faster.
Flows should be held at high levels for about one to two weeks, the typical duration of the downstream journey, according to Burford. And once the migration ends, flows can be dropped back down to normal levels to preserve precious water resources for other uses. Doing this just a few times a year, tailored to different salmon populations and migration timings, is what would lead to the 40 to 400 percent increase in juvenile-salmon survival that the paper’s authors project.
They say this approach is remarkably straightforward from a management perspective, and despite its simplicity, the impact is potentially enormous. By facilitating juvenile-salmon migrations in this manner, the team estimates this could result, on average, in an extra 20,000 of them reaching the ocean.
“These are 20,000 fish born in the wild, whose genetic characteristics have helped them survive in this highly modified ecosystem,” said Burford, an adjunct faculty member in UC Santa Cruz’s Ecology and Evolutionary Biology Department. “Also, California is at the thermal limit of the temperature range for Chinook salmon — meaning these fish are the most heat-tolerant and, therefore, vital for climate adaptation.”
What’s so special about salmon?
The practice of intervening in animal migrations to protect a species is not new. For example, during unusually warm years, Indigenous tribes were known to cool salmon-bearing streams with fire smoke, or to physically pick these very same salmon out of the water and carry them to cooler upstream river reaches.
And now, even though alterations and engineering have completely transformed the state’s waterways, resource managers have an instruction manual for how to adjust conditions using the controls at their fingertips — a guide to taking a more purposeful, premeditated role in migratory species conservation.
“Almost every single drop of water that’s ever released from reservoirs for a wildlife intent is ultimately used by humans,” Burford said. He added that another reason to support the survival of Chinook salmon — designated a threatened species by the state — is that only 20 percent of their original freshwater habitat is accessible to them, and almost all of it altered dramatically to store and route water to humans.
Because of their anadromous (from freshwater, to sea, to freshwater) life cycle, Chinook and other species of salmon depend on the entire watershed from top to bottom, and therefore are an iconic example of ecosystem health and human stewardship — or failure. Healthy salmon runs indicate healthy watersheds that provide critical benefits such as groundwater recharge, natural water treatment, and riparian-forest irrigation.
“Healthy salmon populations and healthy watersheds go hand in hand; and this isn’t to say their only worth is an indicator species,” Burford said. “They are an iconic and compelling species — people love salmon.”
As with Indigenous communities before European settlement, salmon can sustain present-day cultural ties to rivers and prompt hands-on caretaking, which is essential for long-term conservation, and humanity’s persistence in California.
While he prefers not to frame the significance of salmon in terms of them as a food source, Burford points out that our symbiotic relationship is reason enough to help them survive and thrive. “We don’t even need to get in a boat to catch them,” he said. “They swim out of the ocean and literally deliver themselves to us.”
Other co-authors of the paper include Jeremy Notch, also jointly affiliated with UC Santa Cruz and NOAA, and William Poytress, at the U.S. Fish and Wildlife Service’s office in Red Bluff, Calif.
