Many salmon species face a multitude of often devastating threats, including overfishing, habitat degradation (e.g., as a result of mining, timber cutting, agriculture, and urbanization), obstruction of migratory routes (e.g., by dams and hydroelectric plants), and interbreeding and other ecological interactions with hatchery-raised salmon (IUCN 2009).
In addition to these threats, salmon face additional challenges caused by rapid climate change (Battin et al. 2007; Crozier et al. 2008; IUCN 2009). For example, because the developmental rate of salmon is directly related to water temperature, it is possible that increasing temperatures could cause the more rapidly developing juveniles to enter the ocean before their planktonic food source has reached sufficiently high levels to sustain them (IUCN 2009). Increased water temperatures could cause other problems as well. Areas of particularly warm freshwater can present a thermal barrier to migrating salmon that then requires additional energy to navigate around. Such barriers can also delay or even prevent spawning. As air temperatures warm, much of the snow that feeds the river systems is expected to melt earlier. In many cases snow is predicted to be replaced by rain. This will lead to a reduction in the summer flows of many rivers, coupled with an increase in freshwater inputs during the winter. A reduction in summer flow levels will serve to increase water temperatures further and is likely to reduce the overall habitat available to salmon. Increased winter flows are likely to scour the river beds, disturbing nests and causing physical damage to both salmon eggs and juveniles. Coupled with an increase in freshwater inputs is an increase in the sedimentation of river and stream beds. Such sedimentation is likely to reduce the amount of gravel substrate available for spawning and to smother both eggs and juveniles (IUCN 2009).
Because salmon inhabit diverse habitats, and a single species may live in both freshwater and marine habitats during different parts of its life history, the impacts of climate change may be very complex and highly dependent on both the particular species and local geography and ecology. Predicting the specific effects of climate change on salmon in their marine environment is especially difficult as a consequence of our limited knowledge of the marine habits of salmon, combined with uncertainties about how marine habitats will be affected by climate change.
Some salmon populations at higher latitudes may actually benefit from warmer temperatures through increased productivity. It is possible that a warmer climate could make new spawning habitats available, as has been observed in parts of Alaska. Rapid climate change is likely to lead to unexpected consequences and shifts in ecosystems and fisheries, and humans will need to be prepared to adapt to these new conditions (IUCN 2009). Some evolutionary response on the part of salmon to changing environmental conditions is to be expected, but these environmental changes might produce conflicting selection pressures in different life stages, which will interact with plastic (i.e., nongenetic) changes in complex ways that are very challenging to predict (Crozier et al. 2008).
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