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

大西洋鲑鱼是加拿大东部人民的标志性物种，具有重要的经济、文化、人类生存和生态价值。近几十年来，返回加拿大河流的大西洋鲑鱼数量下降到前所未有的低点（历史水平的20%）。尽管我们做了大量的修复和保护工作，但由于海洋鱼类的高死亡率仍然是一个令人困惑的问题，我们未能阻止鱼类数量的下降。最近的研究提出了一种可能性，即春季融雪期间的间歇性河流酸化可能是这个问题的一个重要而被忽视的部分。这一自然过程因人类活动（如燃煤）产生的酸雨而加剧，已知会增加河流中铝的生物利用度，从而影响鲑鱼。在酸性条件下（pH值<6），这种天然丰富的金属被溶解成无机形式（Ali），即使在短期低水平接触也会损害鲑鱼的鳃。一个特别令人担忧的影响是在春季洄游（smolt）阶段发生的对海水适应的关键生理过程——smolt化的损害。最近欧洲和美国的研究提供的证据表明，在剂量依赖的情况下，小鲑鱼间歇性地暴露于淡水中的酸性/阿里会使早期海洋存活率降低20%至50%。尽管这个问题有可能发生，但尚未在加拿大大西洋地区进行调查。例如，拥有世界上最大鲑鱼群的米拉米奇河（NB），春天的pH值经常降至6以下。我们的项目旨在回答以下问题：在加拿大东部，间歇性的河流酸化是否显著地导致了鲑鱼的低回复率？在四年的时间里，我们将从新省和新州的河流中收集水样和洄游的小鲑鱼。我们将主要测量水样中的pH和Ali浓度，鱼鳃中的Al生物积累和熏蒸生物标志物，以及熏蒸盐耐受性。该项目旨在通过实施有效的缓解战略（如石灰化），就影响大西洋鲑鱼种群的可采取行动的淡水因素为管理决策提供信息。

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).