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Warm river habitats are important for cold-water fish

Warm river habitats are more important than expected for the survival of cold-water fish, such as salmon and trout, according to a new study from Oregon State University. This discovery has important implications for fish conservation strategies. 

As the climate gets warmer, experts are working to identify habitats that remain suitable for cold-water fish during the summer. Yet this implicitly devalues areas that are seasonally warm, even if they are suitable for fish most of the year, explained study lead author Professor Jonny Armstrong. He said that this is a potentially severe blind spot for climate change adaptation.

“Coldwater fish like trout and salmon are the polar bears of river ecosystems – iconic species that are among the most vulnerable to climate change,” said Professor Armstrong. 

“A huge challenge for conservation is to figure out how to help these fish survive a warmer future. The conclusion is that we should not waste money on warm habitats and instead focus on saving the coldest places, such as high mountain streams, which are already the most pristine parts of basins. Most people agree we should give up on places that are warm in summer, but forget that these places are actually optimal for much of the year.”

The researchers found that warm river habitats are suitable for cold-water fish during the spring and fall, when the rivers are not at peak summer temperatures. 

These warm habitats can provide fish with the energy they require to travel to cooler parts of the river during the summer and to reproduce.

“The synergy between cold water and warm water is really important,” said Professor Armstrong. “We’re not saying cold water is not important. We’re saying that warm portions of basins are also important because they grow fish during the shoulder seasons. Conserving this habitat is critical for unlocking the full potential of rivers to support fisheries.”

“In a warmer future, many fish will need fish to take a summer vacation and move to cold places to survive the hottest months of the year. Their ability to do that could often depend on how much energy they can get in the spring and how well they can feed in the fall to bounce back. The places that are stressfully warm in summer are just right in spring and fall, and there is growing evidence that they can fuel fisheries.”

The study was focused on data from previous research that used remote sensing technology to obtain river water temperature data across 14 river basins throughout the year.

The OSU team incorporated these temperatures into a “bioenergetics model,” which predicts fish growth potential based on equations derived from lab studies. The analysis revealed new clues about how growth opportunities shift across river basins throughout the year. In particular, the results showed the potential for a large fraction of total fish growth to accumulate during the spring and fall in places that are too hot during summer.

Next, the experts created a simulation model to investigate how these warm river habitats could contribute to fisheries. Simulations based on the behaviors of rainbow trout over the course of a year demonstrated that the majority of fish moved into cooler waters in the summer,  where they experienced insufficient growth rates. 

However, in the months before and after summer, the fish mainly resided in seasonally warm downstream habitats that fueled their growth.

“In conservation, we often judge streams by their summer conditions; this is when we traditionally do field work, and this is the season we focus on when planning for climate change,” said Armstrong. 

“We place value on places that hold fish during summer and devalue those that don’t. Our simulations showed why this can be a problem – the portions of rivers that contribute most to growth may not be the places where fish are found during summer, so they get written off.”

According to Professor Armstrong, the simulations reveal the synergy between seasonally warm and perennially cool habitats and that fish that lived in these two types of habitats grew much more than fish that were restricted to either habitat alone.

“We think of things in this binary way – it’s either warm-water habitat or its cold-water habitat. And we have definitions for fish – it’s either a warm-water fish or a cold-water fish. But the places we think of as warm are, in fact, cold way more than they are warm.”

Professor Armstrong pointed to the Willamette, a tributary of the Columbia River that runs nearly 200 miles from Eugene to Portland, Oregon.

“When it’s warm enough for humans to swim, it’s bad for cold-water fish. But there’s only like six weeks of the year where it is comfortable to go swimming in Oregon.”

“That speaks to the fact that we write off places because they get too hot through the lens of August. They’re actually pretty nice for most of the year if you’re a cold-water fish. And fish don’t necessarily have to live there in August, just like you don’t have to go swimming in the Willamette in December.”

The study is published in the journal Nature Climate Change.

By Chrissy Sexton, Staff Writer

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