Saving N.S. salmon by building a ‘cold-water Mecca for migrating fish’

Wild Atlantic salmon swimming upriver to their original birthplace to spawn can struggle with rising water temperatures brought on by climate change.

The loss or fragmentation of cold-water habitat has led to drops in salmon populations, and the fish are considered endangered or at risk within many Nova Scotia watersheds.

“The climate’s changing and cold-water species like salmon and trout really are impacted by those warming waters,” said Amy Weston, managing director of the Nova Scotia Salmon Association.

“If we can create better conditions, there’s more resilience in the whole ecosystem and they will be able to weather those difficult hot-water times which we are seeing more frequently.”

‘Thermal stress’

To that end, armed with an undergraduate degree in civil engineering, Kathryn Smith has been trying to create cold water rest stops to provide some relief for stressed-out salmon.

“Atlantic salmon tend to thrive in temperatures between 15 C and 20 C,” Smith, a Ph.D. candidate at Dalhousie University’s Coastal Hydrology Lab, said in a recent interview.

“That’s kind of considered their optimal range when we’re considering the summer water temperatures. And then when it starts to exceed that 20 C, they start to get this accumulation of thermal stress.”

‘Could definitely die’

The warmer the water, the less oxygen there is in there for salmon to extract with their gills, Weston said.

“They are stressed in terms of being able to really get enough oxygen into them. They could definitely die if the conditions are warm, and they cannot get to respite.”

When water temperatures hit 25 C, salmon start to seek out cold water patches in rivers caused by natural groundwater springs and cold tributaries called thermal refuges. But those are becoming increasingly rare.

Two summers back, Smith helped members of the Nova Scotia Salmon Association install a passive system on the Killag River, the major tributary of Sheet Harbour’s West River, aimed at creating a thermal refuge.

“That consisted of essentially digging an underground trench, filling it with some rock, and then capping it with some soil,” Smith said.

“And then within that trench we re-directed a portion of the river water through it and then, hopefully, it would cool it down, and then re-inject it back into the river.”

‘We were catching salmon’

The soil on top of the trench acts as a thermal buffer between direct solar radiation and air temperature, and the water in the trench, Smith said, noting that method cooled the river water by a few degrees.

Salmon can detect temperature differentials of up to one tenth of one degree C, she said.

“They’re definitely smart enough to know to seek out these cold-water patches.”

Researchers could see salmon taking advantage of the cooler rest area.

“It was quite interesting,” Smith said. “It was the first time I actually did any fish angling in my life was in that river system and we were catching salmon, which was not as common as it used to be a few years ago. So, they were quite excited about that.”

‘Better shape’

Predators could key-in on the cooler areas looking for a meal, Weston said.

“They’re potentially a little more exposed to predation, but they might rather take their chances there than in the hot water somewhere else … If they’re in better shape in a place, then they’re going to be better able to escape predation.”

Weston compared it to a pedestrian in a hot city “stepping into the air conditioning to have a break from the heat. Then you’re a bit revived and you can carry on.”

This past summer, Smith was involved in building an active system to cool another Nova Scotia river.

“We were lucky that the Municipality of Antigonish allowed us to use their back-up water supply located on Wright’s River,” she said.

“That’s one of the three main tributaries that feed into the harbour there.”

‘Pretty cool’

Over two different weeks in July and August, the researchers pumped 9 C well water at about 32 cubic metres per hour into the river. Within a few metres of where it was going in, the water downstream stayed between 9 C and 12 C.

“So, quite cool compared to the 30 C that the river exhibited during that July study,” Smith said. “We had quite a thermal difference there and we were able to actually see aquatic species utilizing that thermal refuge, which was pretty cool.”

Researchers monitored the results with thermal probes, thermal imaging drones, and underwater cameras.

Introducing the well water created a cooler patch that could still be detected about 40 metres downstream, she said.

“We were seeing white lake chub in there as well brown trout as well as early life stages of Atlantic salmon,” Smith said.

“So, that’s a good sign that they’re within the system. There were known populations to begin with prior to the study and so it was interesting seeing them actually utilize the thermal refuge.”

‘Telling their friends’

She watched the site from a nearby rock.

“Each day … we would see more and more fish occupying the thermal refuge, which was exciting,” Smith said. “I was screaming, ‘Oh they’re all finding out and they’re telling their friends.’”

Next summer, Smith is planning to help set up a couple of passive cooling systems at undisclosed locations on the LaHave River, which once teemed with wild salmon.

“We’re thinking of implementing them in even more river systems within Atlantic Canada and even beyond,” she said.

‘Fine tune’

While the active system used in Antigonish was successful, it can be difficult to find well water with chemistry that won’t harm fish.

“So, we’ve kind of geared more to see if we can hone in and fine tune some of the designs with that passive system and then implement them elsewhere.”

For Weston, “there’s something beautiful about the idea of drawing (water) through the ground to take that heat away, as opposed to drawing very cold water up from the depths and using it.”

Adding a couple of thermal refuges to a warming river could create “a cold-water Mecca for migrating fish,” Weston said.

‘Short list’

Building one of the passive systems costs between $40,000 and $80,000, Weston said, noting that includes volunteer help from interested anglers.

“We’ve just finished up a pretty involved watershed restoration planning process for eight southern uplands rivers,” Weston said. “Those are the rivers flowing to the Atlantic from Guysborough to Digby.”

The Nova Scotia Salmon Association is now studying those rivers “to make a bit of a short list for candidate sites that have the potential to do more of this cold-water work,” Weston said, noting the St. Mary’s River, West River in Sheet Harbour, LaHave River and, possibly, the Medway River could make that list.

‘Really crucial’

The Margaree River in Cape Breton and South River in Antigonish also have potential when it comes to adding underwater cooling stations, Weston said, noting Fisheries and Oceans Canada helped fund the work.

“That kind of federal investment in our aquatic ecosystems is really crucial if we’re going to get to do this kind of innovation,” Weston said.

Smith presented the results of her study this fall at the Geological Society of America’s meeting in Pittsburg.

“Even the folks there, within the USA, were very excited about the idea and could see the vision of potentially implementing this elsewhere,” Smith said.