PORTLAND, Ore. (KOIN) – Contrary to what the scientists predicted, trout populations in a Southern Oregon watershed showed no signs of decline one year after a wildfire burned a majority of the stream system, a study conducted by Oregon State University found.

Despite a rise in water temperatures caused by the fire and the subsequent loss of riparian zone trees that shaded the stream in the summer, researchers found no noticeable loss of life for steelhead, cutthroat and rainbow trout in the stream system. Instead, fish populations reportedly increased in studied areas.

The study’s lead researcher Dana Warren, of OSU’s colleges of Forestry and Agricultural Sciences Department, said that the information shows the ability of these cold-water fish to endure higher-water temperatures associated with climate change and its repercussions, including more frequent and severe wildfires.

“It’s crucial that we improve our understanding of the factors that influence how fish respond to postfire changes in stream temperature,” Warren said. “The loss of streamside cover during a fire can lead to substantial increases in stream temperature, but the effects of changes to stream thermal regimes on salmonid fishes can be complicated. The fish in this system proved to be quite resilient to these increased temperatures — at least within the range that we saw here.” 

While the fire reportedly had no noticeable effects on trout mortality in the studied watershed, researchers said that this doesn’t mean wildfires pose no threat to trout populations.

“Acute mortality is important but isn’t the only impact,” Warren said. “There may be sublethal effects, like a weakened ability to grow or reproduce. Given the short-term nature of our observations, more research is needed on the mechanisms that drive fish responses to warmer water temperatures, and long-term monitoring is also needed.” 

Timber destroyed by the Archie Creek Fire in Southern Oregon in September 2020 (courtesy Seneca Jones Timber Co.)

The studied area included 5,000 acres in the already heavily studied Hinkle Creek Paired Watershed and second-growth timber land in Douglas County owned by Roseburg Forest Products, study co-author Kevin Bladon said. This area was heavily impacted by the Archie Creek Fire, which burned 131,542 acres in September of 2020.

“The fire burned an area for which we have all this historical data on stream flow, water temperature, sediment, nutrients and fish,” Bladon said. “Going back there and measuring the same parameters provides some really robust insight into the effects of wildfire.”

Native salmonids were chosen for the study, OSU said, because of their environmental, cultural and economic significance to North America.

 “These are ecologically, culturally and economically important species distributed across western North America,” Warren said. “Recent studies have speculated about the potential effects of climate change on trout and salmon as summer stream temperatures gradually rise above 16 to 20 degrees Celsius. Abrupt disturbances like fire can produce rapid and substantial increases in stream temperatures that provide insights not only into how these increasingly common disturbance events affect native salmonids, but more broadly how salmonids may respond to other aspects of climate change.”

While trout have been documented surviving elevated stream temperatures in the past, past studies have focused on regions that are warmer and more fire-prone than the Western Cascades.

“Although temperatures increased beyond what’s considered the salmonids’ optimal threshold in the Cascades, there were no classically warm-[water] species present, so competition from them was not an issue,” Warren said. “A combination of other factors could also have contributed to the persistence of salmonids: high abundance of cooler microhabitats created by groundwater discharge; physiological recovery at night when temperatures were cooler; and an increase in food availability. More investigation is needed to know for sure.”