Salmon and trout anglers across the Pacific Northwest are going to have fewer places to fish over the next 40 years, concludes a new study published this month.
Scientists at the Forest Service's Rocky Mountain Research Station in Boise found that in the summer and early fall, rivers in the Pacific Northwest have already warmed 1.8 degrees Fahrenheit since 1976. That's the same rise measured at the Bonneville Dam over the last 80 years and used in models by climate scientists.
The researchers studied 391 monitoring sites. The temperature pattern gives them confidence the warming trend is going to continue for the next 40 years, said Dan Isaak, the lead researcher. That means salmon and trout are going to have less habitat, and will be replaced by warm-water fish like smallmouth bass.
"This will be a big deal for people who fish for trout," said Isaak. "It might be that places their grandfather and father took them, there might not be trout anymore."
The good news is that the warming isn't happening as fast as scientists previously thought. That gives both the fish and managers time to adapt. But adjusting will be harder for some species than others.
The study — "Global warming of salmon and trout in the northwestern U.S.: Road to ruin or path through purgatory?" — was done after the 2015 season, when warm temperatures in the Columbia, Snake and Salmon rivers killed off nearly all of the sockeye salmon returning to Idaho's Sawtooth Valley. The research shows that sockeye, which migrate during the heat of the summer, are going to be exposed to lethal temperatures in rivers from 5 to 16 percent longer during their trip. For Redfish Lake sockeye, the exposure will be 30 times what coastal sockeye face.
Despite the warming, which has been hastened by human behavior, the scientists don't predict habitat loss by itself will send any of the fish species into extinction over the next 40 years.
"It's in the latter half of the century that we have more uncertainty," Isaak said.
The models show the warming trend slowing when the rise in greenhouse gases stops or is reversed. But the science is not as clear about when that will happen, or that it will affect the climate, Isaak said.
The study also showed that increases in river temperature correlate with those for air temperature, and with river flow declines in the late summer and fall. This means river managers can offset some of the problem by changing how much water is released from reservoirs, and when.
The U.S. Army Corps of Engineers already does this at Dworshak Dam on the Clearwater River in north-central Idaho. The Bureau of Reclamation does the same on the Klamath River in Oregon.
Fisheries biologists for the Corps and the states of Oregon and Washington also are identifying springs and other cool-water refuges along the salmon migration path. The fish can use those areas when temperatures in the main rivers turn lethal. Managers are looking for ways to oversee the cooler spots to protect the fish.
Trout anglers can expect the fish to gradually move upstream to similar, cold-water refuges. The lower ends of rivers like the Snake, the Boise, the Payette and even storied streams like Silver Creek could end up too warm for brown trout and other species.
In addition to Isaak, authors of the study include Charles H. Luce, Dona L. Horan, Gwynne L. Chandler, Sherry P. Wollrab and David E. Nagel.
The paper suggests several ways that managers could help offset warming and preserve cold-water river habitats. Those include minimizing water withdrawals from rivers, increasing shade though riparian vegetation, enhancing habitat diversity, and building on the number of deep pools that tap into cold groundwater. At dams, they say managers could also improve fish passage through areas that block access to cooler river sections.
“The data show that global trends are having local impacts on our western rivers,” said Isaak. "Some river reaches will gradually become too warm to provide traditional habitats and if the warming trend continues throughout the century, we could see some very different fish communities."