Salmon are very much a cold-water fish, so they can get quite stressed when swimming up increasingly warm rivers to spawn. A new study shows that by installing "cooling stations" in those rivers, we could help the threatened fish make the trip.
While global warming is causing river temperatures to rise, spawning salmon still get some respite at what are known as "thermal refuges." These are places where cool water from underground springs or spring-dominated tributaries flows into the main river, bringing the water temperature down.
Stopping to rest at these locations does help the salmon, but they still have to eventually continue their journey upstream.
What's more, thanks to the effects of climate change, natural thermal refuges are becoming less common and making less of a difference. With that problem in mind, hydrology PhD candidate Kathryn Smith and colleagues from Canada's Dalhousie University set about creating human-made thermal refuges. Two such refuges – an active one and a passive one – were tested in rivers in the province of Nova Scotia this summer.
The active refuge utilized a pump to deliver cool (9 ºC/48 ºF) groundwater from a municipal well into a warm (30 ºC/86 ºF) stretch of river, creating a plume of cooler water which extended at least 60 meters (197 ft) downstream.
In the passive refuge, a covered trench was used to divert water from a meander back into the main river. During the time the redirected water was shielded from the sun's rays, its temperature dropped accordingly.
A combination of thermal probes, drone thermal mapping and time-lapse underwater cameras showed that even when these refuges cooled the water by only a few degrees, spawning Atlantic salmon readily congregated in the areas. What's more, greater numbers of the fish gathered at the stations during a heat wave, indicating that the refuges did indeed make a big difference.
Smith is now planning on scaling up the technology for more extensive testing. She is presenting her research this week at the Geological Society of America’s GSA Connects 2023 meeting in Pittsburgh.
Source: Geological Society of America