The advice was released in response to a North Atlantic Salmon Conservation Organization (NASCO) request which included a question on the impacts of climate change on Atlantic salmon stock dynamics. While much work was carried out by the Working Group on North Atlantic Salmon (WGNAS) to respond to the NASCO request, a specific workshop was convened in order to obtain a wide range of expertise in addressing the question of climate change impacts. The workshop addressed these issues by explaining the background of climate change and identifying environmental and biological drivers in freshwater, estuaries and marine waters, presenting projections on these drivers and reviewing literature on climate change effects on the salmon.
The workshop identified that climate change has the potential to affect the distribution, productivity, migration patterns, generic variation, and other biological characteristics of the species within the range of populations. Projections from modelling suggest that climate change will result in future atmospheric and aquatic conditions that have not yet been observed historically, meaning the response of Atlantic salmon to these new conditions is highly uncertain.
With regards to the impact of climate change on the distribution of Atlantic salmon, it is likely that the species' marine and freshwater habitats will extend further north under continuing trends in global warming. In the southern part of the range some suitable freshwater habitat may be lost, but this is unlikely to result in the loss of entire regional stock components.
The most frequently expressed climate change response is to changes in temperatures. Increased stream temperatures might result in increased freshwater growth in juveniles as well as greater productivity in many areas throughout the range; this in turn could change smolt age and run timing, which might not be beneficial to survival. In the absence of cooler pockets of water (thermal refugia), freshwater habitats could become limiting for some populations in areas where stream temperatures exceed lethal limits.
The impact on the oceanic and freshwater conditions through changing weather and teleconnection patterns will, in turn, influence growth and predation pressure. This means that salmon will have to compete more with other native and introduced fish species for prey as well as being more vulnerable to being preyed upon themselves. Increased prevalence and virulence of parasites and pathogens may reduce survival.
Climate change might also alter migration routes and distribution of salmon at sea, with unknown consequences for survival. Environmental and genetic adaptation can facilitate adjustment to changing environmental conditions, as long as the rate of environmental change does not exceed the salmon's capacity for genetic adaptation. However, if the effects of climate change are too rapid or severe, plasticity in physical traits may be inadequate to allow populations to persist and genetic adaptation to occur.
While recent findings have advanced understanding of the potential consequences of climate change on Atlantic salmon, substantial uncertainties remain. Many climate change projections are at seasonal or annual scales, but it is at the finer scales that the most significant damaging consequences for survival of salmon can occur. The current resolution of earth system models cannot reproduce realistic small-scale features, which are important for coastal regions and rivers. Local impacts depend on local variations in weather patterns and the frequency of episodic events, and even though local projections can be made through downscaling, uncertainty is introduced at every stage of the process.
The greatest uncertainty is the inability to predict sudden large phase-shifts in either climate, marine, or terrestrial systems. With current carbon dioxide concentrations exceeding any levels previously recorded, the consequences for Atlantic salmon remain unknown.
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