Is episodic river acidification affecting wild Atlantic salmon populations in the Maritimes?

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The Atlantic salmon is an iconic species for the people of Eastern Canada, with important economic, cultural, human subsistence and ecological values. In recent decades, the number of Atlantic Salmon returning to Canadian rivers has declined to unprecedented lows (20% of their historic levels). Despite great restoration and conservation efforts, we have failed to prevent the declines as high fish mortality at sea remains a puzzling issue. Recent studies have raised the possibility that episodic river acidification during spring snowmelt may be an important and overlooked part of the problem. This natural process, which has been exacerbated by acid precipitation from human activities (e.g. coal burning), is known to affect salmonids by increasing aluminum bioavailability in rivers. Under acidic conditions (pH <6), this naturally abundant metal is solubilized into inorganic forms (Ali) that can damage salmonids gills, even in short-term, low-level exposures. A particularly concerning effect is the impairment of smoltification, a critical physiological process for seawater adaptation, occurring during the spring outmigration (smolt) phase. Recent European and American studies have provided evidence that episodic exposure of smolts to acidity/Ali in freshwater can reduce early ocean survival by 20 to 50%, in a dose-dependent matter. This issue has not yet been investigated in Atlantic Canada, despite its potential occurrence. For example, the Miramichi river (NB), which has the largest salmon runs in the world, has a pH that often drops below 6 in the spring. Our project aims at answering the following question: Does episodic river acidification significantly contribute to the low return rates of salmons in Eastern Canada? For four years, we will collect water samples and migrating smolts from various NB and NS rivers. We will notably measure pH and Ali concentrations in water samples, Al bioaccumulation and smoltification biomarkers in fish gills, as well as smolt salinity tolerance.The project is designed to inform management decisions on actionable freshwater factors affecting Atlantic salmon populations, by implementing effective mitigation strategies if applicable (e.g. liming).