Engineers Create ‘Air Conditioning’ for Salmon With Chilled Patches of River Water

Wild Atlantic salmon can struggle with heat as they swim upstream to spawn—but artificial “thermal refuges” may help them cool off

Salmon swimming up a waterfall
Atlantic salmon spend most of their lives in the cool waters of the ocean. When they venture upstream in freshwater rivers to spawn, however, they encounter challenging warmer waters. Paul Abrahams / 500px / Getty Images

Just as humans flock to air conditioned spaces on hot summer days, wild Atlantic salmon congregate in chilly patches of river water as they make their way upstream to spawn. These so-called “thermal refuges” are becoming increasingly important to the salmon’s survival as river temperatures rise amid human-caused climate change.

Natural thermal refuges tend to occur near groundwater springs that release cool water into the stream. But over this summer of record-breaking heat, researchers in Nova Scotia, Canada, tried to help the fish by creating additional cold areas in the region’s Wrights River.

Hundreds of fish from various species took refuge in the chilly plumes the team artificially built, suggesting this might be a viable approach for keeping cold-water populations healthy as the planet continues to warm.

“We need to think about how we can adapt to that warming world and still preserve the Atlantic salmon [and other] cold-water biodiversity in our rivers,” says Kathryn Smith, a civil engineer at Dalhousie University who led the project, to Time’s Alejandro de la Garza.

Researchers presented the results of their proof-of-concept experiment—which have not yet been published in a peer-reviewed journal—at the Geological Society of America’s GSA Connects 2023 meeting earlier this month.

Atlantic salmon spend most of their lives in the cool waters of the ocean. They are born in rivers, though, and when they reach sexual maturity, they head back upstream to their birthplaces to spawn. But the fish can struggle when river waters get too hot.

Their ideal water temperature is between 43 and 72 degrees Fahrenheit—and, though salmon can survive in warmer waters for short periods of time, excess heat can cause them to become stressed and eventually die, per Time. Indeed, wild Atlantic salmon populations have been declining—and biologists think warming waters are partially to blame. (The fish also face threats from dams and culverts, pollution, degraded habitats, disease and more.) Artificial thermal refuges may help the fish be more resilient against global warming.

Side-by-side image showing river area and thermal view of the same spot
After creating cold areas in the Wrights River, the researchers studied them using thermal probes, drone thermal mapping and underwater cameras. Kathryn Smith

Past research has focused on counting and preserving natural thermal refuges—but as far as the researchers know, no one has tried to build new ones. The team came up with two different ways of creating thermal refuges in the Wrights River: one active and one passive.

With the active approach, they pumped 48 degree Fahrenheit water from a municipal well into the river, which reached up to 86 degrees, for two weeks in July and one week in August. They used thermal probes, drone thermal mapping and underwater cameras to monitor the area.

The team found that the infusion of well water lowered the surrounding water temperature by as much as 36 degrees Fahrenheit compared to its surroundings, reports Science News’ Nikk Ogasa. This cold water plume continued nearly 200 feet downstream from the spot where they were pumping in the water, according to a statement from the Geological Society of America.

The passive approach, meanwhile, entailed digging an underground trench and rerouting some of the river’s water into it. After shedding some heat into the soil and getting a brief reprieve from the sun’s rays, the water flowed back into the river. There, it lowered the temperature by just a few degrees for a few feet. However, even small thermal changes can be helpful to the fish, which can detect even miniscule variations measuring a fraction of a degree, Smith says in the statement.

When they checked the underwater footage, the researchers saw fish were indeed using both of the human-made thermal refuges. Atlantic salmon, brown trout and other species hung out in the artificially created chilly plumes. Many of them were juveniles, but during a July heat wave that caused water temperatures to spike, adults also congregated in the thermal refuges.

Though these early results are promising, researchers plan to conduct further studies to figure out how—and whether—to implement these strategies more broadly. But so far, “the concept is solid,” says John Ackerman, an independent engineer and geologist who was not involved in the study, to Science News.

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