Warmer stream temperatures in Oregon’s scorched watershed have not led to declines in trout numbers

Trout numbers in a southern Oregon stream system have not declined a year after a wildfire burned nearly the entire watershed, including riparian trees that helped maintain optimal stream temperatures for the cold-water fish.

An Oregon State University study sheds light on the ability of steelhead, cutthroat and rainbow trout to tolerate the higher water temperatures expected to accompany climate change and its effects, including increased frequency, scale and severity of wildfires.

“It is critical that we improve our understanding of the factors that influence how a fish respond to post-fire changes in flow temperature” said study leader Dana Warren, a research associate in OSU’s Colleges of Forestry and Agricultural Sciences. “Loss of riparian cover during a fire can lead to significant increases in streamflow temperature, but the effect of changes in the thermal regime of the stream on salmonids can be complex. The fish in this system have proven to be quite resistant to these elevated temperatures – at least in the range we’ve seen here.”

The scientists emphasize that their findings, which showed an increase in the number of fish in their study areas during the summer, do not indicate that wildfires do not pose a threat to the trout population. The study found no immediate effects during peak water temperatures in summer, which regularly reached 24 degrees Celsius, but the researchers, for example, did not assess long-term or less lethal effects water temperature is increasing.

“Acute mortality is important, but not the only one,” Warren said. “There may be sublethal effects, such as reduced ability to grow or reproduce. Given the short-term nature of our observations, further research into the mechanisms that govern fish response to higher water temperatureand long-term monitoring is also needed.’

A study published in Ecosphere, included the 5,000-acre Hinkle Creek Paired Watershed Study Area, a secondary forestland in Douglas County owned by Roseburg Forest Products. As of September 2020, the Archie Creek Fire has burned 131,542 acres in the county, including the Hinkle Creek watershed.

“The fire burned down in the square for which we have all this historical information on stream flow, water temperature, sediment, nutrients and fish,” said study co-author Kevin Bladon, a hydrologist in the College of Forestry. “Going back there and measuring the same parameters gives a really accurate picture of the effects of a wildfire. “

The Hinkle Creek Study, founded in the early 2000s, was organized to learn how the Oregon Forest Practices Act and current logging systems protect forest streams during logging by looking at the impacts of logging on all watersheds.

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

For this study, Bladon, Warren, and colleagues in Oregon looked at local salmonids, trout and steelhead/rainbow trout (ocean steelhead, like salmon).

“These are ecologically, culturally and economically important species distributed throughout western North America,” Warren said. “Recent research suggests a possible impact of climate change on trout and salmon, with summer stream temperatures gradually rising above 16-20 degrees Celsius. Sudden disturbances such as fire can lead to rapid and significant increases in stream temperatures, providing insight into not only how these increasingly frequent disturbances affect native salmonids, but more broadly how salmonids may respond to other climate change.”

The Archie Creek Fire has engulfed the entire Hinkle Creek catchment, including the riparian zone — three-quarters of the watershed burned moderately to severely — causing stream temperatures to regularly rise above 22 degrees Celsius in the summer of 2021, about 7 degrees warmer than than before the fire.

“And there were two long periods, 10 days and six days, where the stream temperature never got below 16 degrees,” Bladon said.

Contrary to what the scientists expected, fish numbers did not decline during the summer of 2021—in fact, they increased in areas where fish numbers were tracked at both the beginning and end of summer.

The researchers note that the survival of trout in a stream system with elevated temperatures after a fire is not unprecedented. But most of the studies with such findings have been done in regions that are generally warmer and have a higher fire frequency than the western Cascades.

“Even though the temperatures have risen above what’s considered optimal for salmon in the Cascades, the classic warm-warm species weren’t there, so competition from them wasn’t an issue,” Warren said. “A combination of other factors may also have contributed to salmonid survival: the abundance of cool microhabitats created by groundwater discharge; physiological recovery at night when the temperature was lower; and increasing food availability. More research is needed to know for sure.”

Alison Swartz of OSU’s College of Forestry and David Roone of the Colleges of Forestry and Agricultural Sciences also collaborated on the study.