Warmer stream temperatures in Oregon’s scorched watershed haven’t led to declines in trout

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Newswise – CORVALLIS, Oregon – Trout numbers in a southern Oregon stream system have shown no decline a year after a fire scorched nearly the entire watershed, including trees in the riparian zone that had helped maintain optimal stream temperatures for cold water fish.

Oregon State University research sheds light on the ability of rainbow trout, cutthroat and rainbow trout to withstand the higher water temperatures expected to accompany change climate and its manifestations, including the increased frequency, extent and severity of forest fires.

“It is crucial that we improve our understanding of the factors that influence how fish respond to changes in stream temperature after the fire,” said study leader Dana Warren, a researcher at the Colleges of Sciences. forest and agricultural sectors of the OSU. “Loss of riparian cover during a fire can lead to substantial increases in stream temperature, but the effects of changes in stream thermal regimes on salmonids can be complicated. Fish in this system have proven to be quite resilient to these high temperatures – at least within the range we’ve seen here.

The scientists point out that their findings, which showed an increase in fish numbers in their study areas over the summer, do not indicate that wildfires are not a threat to trout populations. In the study, there did not appear to be any immediate impact during summer water temperature peaks – which regularly reached 24 degrees Celsius – but the researchers did not, for example, assess long-term or less than lethal. consequences of the increase in water temperature.

“Acute mortality is significant but is not the only impact,” Warren said. “There may be sub-lethal effects, such as an impaired ability to grow or reproduce. Given the short-term nature of our observations, further research is needed on the mechanisms that drive fish responses to warmer water temperatures, and long-term monitoring is also needed.

The research, published in Ecosphere, was for the 5,000-acre Hinkle Creek Twin Watershed Study Area, second-growth forest 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 stream flow, water temperature, sediment, nutrient and fish data,” the co-author said. study, Kevin Bladon, hydrologist at the College of Forestry. “Going back there and measuring the same parameters provides really solid insight into the effects of wildfires.”

Established in the early 2000s, the Hinkle Creek Study was set up to see how Oregon’s Forest Practices Law and modern logging systems protect forest streams during harvesting operations. harvesting, examining the effects of logging on entire 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 their collaborators from Oregon State looked at native salmonids, cutthroat trout and rainbow trout (rainbow trout are ocean-going, like salmon) .

“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 increase above 16-20 degrees Celsius. Sudden disturbances such as fire can produce rapid and substantial increases in stream temperatures that provide information not only on how these increasingly common disturbance events affect native salmonids, but more broadly on how whose 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 at moderate to high severity – resulting in summer stream temperatures 2021 which regularly exceeded 22 degrees Celsius, about 7 degrees warmer than pre-fire conditions.

“And there were two extended periods, 10 days and six days, where stream temperatures never went below 16 degrees,” Bladon said.

Contrary to what scientists had predicted, throughout the summer of 2021 fish abundance did not decline – in fact, it actually increased in areas where fish numbers were tracked early. and at the end of summer.

The persistence of trout in a stream system with high temperatures following a fire is not unprecedented, the researchers note. But most of the studies with these types of results come from regions that are generally warmer and with higher fire frequency than the Western Cascades.

“Although temperatures increased beyond what is considered the optimum threshold for salmonids in the Cascades, there were no classically warm-warm species present, so competition from them was not a problem,” Warren said. “A combination of other factors may also have contributed to the persistence of salmonids: a large abundance of cooler microhabitats created by groundwater discharge; physiological recovery at night when temperatures were cooler; and an increase in food availability. Further investigation is needed to be sure.

The National Council for Air and Stream Improvement, Oregon Forest Industries Council, Roseburg Forest Products, and the OSU Fish and Wildlife Habitat in Managed Forests program supported this research.

Allison Swartz of the OSU College of Forestry and David Roon of the Colleges of Forestry and Agricultural Sciences also collaborated on the study.

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