SHERBROOKE — Critically low water levels in the St. Mary’s River are endangering Atlantic salmon populations and other cold-water species, prompting concern from conservationists and local community members who strive to preserve the river’s ecological health.
“This year, water levels are the lowest I believe I have ever seen,” said Scott Beaver, president of the St. Mary’s River Association (SMRA). Combined with the extreme summer heat, there is a dramatic increase in the water’s temperature, presenting real challenges to species that rely on cold, oxygen-rich water, including fish and rare and endangered mussels.
For salmon, water temperatures more than 20 degrees Celsius are dangerous.
“The fish start to burn energy faster than they can replenish it,” said Beaver. “There is more chance for disease, and it’s easier for predators to find them.”
Mortality rates go up significantly, he added.
According to Deirdre Green, program director with the Atlantic Salmon Federation (ASF), when river temperatures rise, salmon respond by seeking out thermal refuges – cooler spring-fed pockets or deeper pools and lakes – and may even back down into estuaries until conditions improve.
“These stressors can impact their ability to migrate, feed, and even survive and spawn in the fall,” Green said.
For other cold-water species, including brook trout, she noted angling pressure during warm spells can be equally harmful.
“Fighting and handling fish in high-temperature, low-oxygen waters increases mortality risk,” she said. “We’re grateful to those who choose to give the fish a break when it’s hot and dry – even when regulations allow otherwise.”
Green said the ASF has been tracking these patterns closely in its blog and encourages anglers to follow warm-water protocols “to voluntarily restrict fishing during heatwaves and to make choices rooted in conservation ethics.”
Aside from fish, other aquatic life is also being affected. Hunter Stevens, aquatic research manager for the Canadian Parks and Wilderness Society, said the river is being stressed in several ways.
“Superficially, [the circumstances] reduce the habitat available for living things as there is purely less water to live in and impedes the movement of different species,” he said. Sedentary species like mussels may simply dry up and perish.
This is particularly concerning in the St. Mary’s River, “which is home to healthy populations of globally and nationally rare species like [the] Eastern pearl mussel, as well as some of the only remaining populations of endangered mussels like the brook floater,” Stevens said.
In terms of the rising temperature, the way different flora and fauna react to heat stress varies, he explained.
“Salmonids like brook trout and our precious Atlantic salmon are particularly vulnerable to it,” said Stevens. These fish prefer cold, oxygen-rich water, and the warmer water gets, the less oxygen it contains, leaving fish reliant on deeper pools or outflows from deeper and colder lakes to weather the dry spell.
He noted this situation highlights the critical importance of keeping older growth forest intact throughout the watershed, as the shade provided by trees will help to cool the water and provide refuge for fish species.
As oxygen levels drop, harmful algae blooms and bacteria can take over.
“This creates the necessary conditions for bacteria that don’t use oxygen to survive (called anaerobic bacteria) to multiply,” Stevens explained.
These bacteria create hydrogen sulfide – a toxic compound with a foul odour sometimes found around stagnant ponds or in piles of washed-up algae.
The production of these toxic compounds and near-zero oxygen conditions “create a pretty inhospitable environment,” he said. “Life in the river is surely stressed.”
Still, said Stevens, “It’s not all bad.” River life has evolved over thousands of years to cope with dry spells and periods of stress, ranging from physiological adaptations like entering hibernation-like states to behavioural adaptations like actively seeking cold refuge.
“Life is resilient,” said Stevens. “Resilient, but not invincible.”
Restoration efforts offer hope
Conservation organizations like the SMRA and the Nova Scotia Salmon Association work to restore and enhance salmon habitat across the province.
“On the St. Mary’s, this has included extensive work to restore cold-water tributaries and reconnect spawning and rearing habitat – critical actions as we face increasingly unpredictable climate patterns,” said Green.
Projects across the province are also helping to moderate water temperatures and improve resilience during heatwaves by increasing habitat complexity and shading, boosting the flow of groundwater into river systems, stabilizing banks and re-establishing riparian buffers.
“These efforts are not only helping wild Atlantic salmon but also benefiting biodiversity and long-term water quality for surrounding communities,” she said.
“As climate change intensifies, these kinds of grassroots, science-informed restoration projects are among our best tools to ensure rivers remain viable for cold-water species in the decades ahead.”
Beaver said recent habitat restoration efforts on the St. Mary’s – some dating back more than a decade – were designed with these scenarios in mind.
“Our goal is to deepen and narrow up the river while taking advantage of cold-water springs and brooks that enter the river system in various locations,” said Beaver. This can create a thermocline – layers of cold and warm water – improving the habitat with sites of refuge for fish.
Beaver noted that the St. Mary’s system has an advantage because it runs through several key lakes. “Because of the lakes, fish that manage to make their way to them have a little oasis to hold up in during the drought,” he said. “Fish that don’t may not be so lucky.”
In the long term, Beaver is optimistic about federal conservation support. The watershed is under consideration for designation as an ecologically significant area under the federal Fisheries Act.
“After the conservation and protection objectives have been ironed out and the river actually is designated, there will be more resources available for our river,” he said, “to help mitigate problems negatively affecting fish and fish habitat.
“Due to our climate changing we will probably see these conditions more frequently,” Beaver added.
