Warmer stream temperatures in burned-over Oregon watershed didn’t result in fewer trout

Trout numbers in a southern Oregon stream system showed no decline a year after a fire burned nearly the entire watershed, including trees in riparian zones that helped maintain optimal stream temperatures for the cold-water fish.

The Oregon State University research sheds light on the ability of steelhead, cutthroat trout and rainbow trout to tolerate the higher water temperatures expected to accompany climate change and its manifestations, including increased frequency, size and severity of wildfires.

“It is critical that we improve our understanding of the factors that influence how fish respond to changes in stream temperature after a fire,” said study leader Dana Warren, a researcher at the OSU colleges for forestry and agricultural sciences. “The loss of bank cover during a fire can lead to significant increases in stream temperature, but the effects of changes in stream thermal regimes on salmonids can be complicated. The fish in this system have been shown to be quite resilient to these elevated temperatures – at least within the range we saw here.”

The scientists emphasize that their findings, which showed an increase in fish numbers in their study areas over the summer, do not indicate that wildfires pose no threat to trout populations. In the study, there appeared to be no immediate impact during peak summer water temperatures — which regularly rose to 24 degrees Celsius — but the researchers, for example, did not assess any long-term or less-than-deadly consequences of the rise in water temperature.

“Acute mortality is important, but is not the only impact,” Warren said. “There may be sub-lethal effects, such as a weakened ability to grow or reproduce. Given the short-term nature of our observations, more research is needed into the mechanisms driving fish to warmer water temperatures, and long-term monitoring is also needed .”

The research, published in Ecosphere, the 5,000-acre Hinkle Creek Paired Watershed Study area was the second growing timber land in Douglas County owned by Roseburg Forest Products. In September 2020, the Archie Creek Fire consumed 131,542 acres in the county, including the Hinkle Creek watershed.

“The fire burned an area for which we have all this historical data on stream flow, water temperature, sediment, nutrients and fish,” said study co-author Kevin Bladon, a hydrologist in the College of Forestry. “If you go back there and measure the same parameters, you get a really robust understanding of the effects of wildfires.”

Founded in the early 2000s, the Hinkle Creek study was designed to look at how the Oregon Forest Practices Act and modern logging systems protect forest streams during harvesting operations, looking at the effects of logging across entire watersheds.

Scientists collected data on water quality, water quantity, fish, amphibians and aquatic invertebrates five years before harvest and four years after.

For this study, Bladon, Warren, and Oregon State collaborators looked at native salmonids, cutthroat trout, and steelhead/rainbow trout (steelheads are seagoing, like salmon).

“These are ecologically, culturally and economically important species that are distributed across western North America,” Warren said. Recent studies have speculated on the potential impacts of climate change on trout and salmon as summer stream temperatures gradually rise above 16 to 20 degrees Celsius. Increasingly common disturbances affect native salmonids, but more broadly how salmonids may respond to other aspects of climate change.”

The Archie Creek Fire consumed the entire Hinkle Creek watershed, including the riparian area – three quarters of the watershed burned moderately or severely – resulting in summer 2021 flow temperatures regularly exceeding 22 degrees Celsius, about 7 degrees warmer than before. the fire. .

“And there were two longer periods, 10 days and six days, where the flow temperatures never dropped below 16 degrees,” Bladon said.

Contrary to what the scientists expected, fish abundance did not decrease in the summer of 2021 — in fact, they actually increased in the areas where fish numbers were monitored both early and late in the summer.

The persistence of trout in a stream system with elevated temperatures after a fire is not unprecedented, the researchers note. But most studies with such findings come from regions that are generally warmer and with a greater firing rate than the western Cascades.

“Although temperatures rose above what is considered the optimal threshold of the Cascades salmonids, no classically warm-warm species were present, so competition from them was not an issue,” Warren said. “A combination of other factors could also have contributed to the survival of salmonids: a large amount of cooler microhabitats created by groundwater runoff; physiological recovery at night when temperatures were cooler, and an increase in food availability. More research is needed to be sure.”

Allison Swartz of the OSU College of Forestry and David Roon of the colleges of Forestry and Agricultural Sciences also contributed to the study.