California’s native salmon have been harmed by more than a century of mining, dam building, floodplain reclamation, fishing pressure, hatchery practices, and introduced predators. These stressors have undermined the resilience of California’s native salmon to the accelerating effects of climate change, new research shows.
Researchers from NOAA Fisheries’ Northwest and Southwest Fisheries Science Centers chronicled the loss of habitat and genetic diversity that once helped salmon withstand a changing climate.
“Now, that salmon diversity is mostly lost, and dams confine salmon to the hottest part of the watershed, where nonnative predators and a lack of rearing habitat reduce their survival. The result of all these stressors is that today’s salmon track climate more tightly than they used to in a warming landscape that routinely experiences drought.”
They said restoring degraded habitat and access to habitat above dams could help revive that diversity. By reopening and improving different areas, restoration can reclaim lost niches and adaptations that help spread the risk. In addition, other recovery actions such as increasing streamflow can make the restoration more effective.
The result today is that even a year or two of poor watershed or ocean conditions reduce survival over a much larger share of the population. Fishing opportunities now drop sharply in the years when those missing fish would have matured and returned to rivers to spawn.
Salmon returning early in the year also could no longer reach high-elevation rivers cool enough for them to survive the summer. These winter and spring-run salmon declined precipitously.
Scientists found proof of these changes in receipts from San Francisco fish markets near the turn of the 19th century, among other sources. The receipts held in the Northwest Fisheries Science Center’s library documented the sales of millions of pounds of native salmon. They included winter-run and spring-run Chinook, which are now threatened and endangered species under the Endangered Species Act.
As the ages of salmon have narrowed, salmon fishing has become more sensitive to climate swings, where a single year of poor conditions can sharply curtail fisheries. The scientists were able to document the fishery’s relationship with precipitation going back into the mid-19th century. They used tree ring records to reconstruct annual precipitation in years before instrumental climate records began.