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Salmon are anadromous fish, meaning that they swim upstream to spawn after spending most of their adult lives at sea. After hatching, young salmon remain in the stream for a few weeks to several years, depending on the stock (this is a crucial distinction: there are only 5 species and within each species there are a wide variety of life cycles), then swim downstream to the ocean. Most species make this trip in spring or early summer, taking advantage of the high streamflows that accompany peak spring melting (in snowfed rivers) and arrive at roughly the time of the spring blooms of plankton that are an important food source.
In the ocean the salmon grow to adulthood and live for several months to six years before returning to their original streams that now become their spawning grounds. Most die in the streams after spawning. By swimming upstream and dying, the fish have carried nutrients derived from the ocean to the inland waters. Those nutrients are now recognized as important inputs to stream and riverbank ecosystems.
Northwest salmon stocks have been highly stressed for decades by intense fishing pressure. They have also been stressed by threats to their stream habitats that include urbanization, sedimentation and pollution of streams, wetland draining, and dam building. Construction of the Grand Coulee and Hell's Canyon dams eradicated all salmon stocks above these points on the Columbia and Snake Rivers, respectively. Fish ladders on other dams are only partly effective at allowing fish to pass. Dams also degrade salmon habitat by changing free-running rivers into chains of lakes, warming in-stream temperatures, reducing dissolved oxygen, and altering the amounts and types of sediment carried by the streams.
Non-climatic stresses (e.g., loss of habitat) on Northwest salmon presently overwhelm climatic stresses. However, salmon abundance has shown a clear correlation with 20th century climate variations in that fewer salmon are found during warm/dry phases of PDO and more during the cool/wet phases. It is likely that a relationship would continue with future changes as well. Climate models cannot yet project changes to the most important oceanic conditions for salmon. But the likely effects of projected climate changes on their freshwater habitat, such as warmer water and reduced summer streamflow, are all highly likely to be unfavorable. Climate change is likely to impede efforts to restore already depleted stocks and to stress presently healthy stocks. The potential environmental, social, and economic impacts of climate change on salmon and some strategies that could be employed to cope with those impacts are explored in the discussion that follows.
Salmon are sensitive to various climate-related conditions, both inshore and offshore, at various times of their life cycle. Eggs are vulnerable to stream scouring from floods. Migrating juveniles must make the physiological transition from fresh to salt water and require food immediately on reaching the ocean. There are two theories related to the survival of salmon once they reach the ocean and the affects of either of these could be influenced by projected climate changes. One theory is that salmon survival is dependent on the timing of their arrival at the ocean relative to the onset of summer northerly winds and the resulting spring phytoplankton (tiny marine floating plants) bloom: either too early or too late, and their survival is in danger from insufficient food. The other is that their survival is predominantly controlled by the balance between predator and baitfish populations at the time of their arrival at the ocean, that balance determines if the young salmon have a lot of predators after them or not.
Although the relative importance of the climate or non-climate factors, and inshore and offshore conditions is still highly uncertain, salmon stocks throughout the North Pacific show a strong association with the Pacific Decadal Oscillation (PDO -- described earlier). Salmon in the Northwest are more abundant in the cool PDO phase, less abundant in the warm phase, while Alaska salmon show the opposite pattern.
When the PDO shifted from cold to warm in 1977, catches in the Northwest dropped sharply while Alaskan catches soared. How this observed climate effect actually affects the salmon is poorly known, and probably includes some effects of both freshwater and marine changes. It is speculated that coastal waters off Washington and Oregon during the warm phase are warmer and less well-mixed and consequently poorer in nutrients and provide less food for the fish. Although ENSO and PDO have similar effects on ocean and terrestrial environments in the Northwest, the influence of PDO on salmon stocks is much stronger than that of ENSO, suggesting that persistent warm-water conditions over several years is important. The PDO effect is much less on Puget Sound salmon than on stocks that exit directly from rivers into the open ocean, suggesting that the gradual increase of salinity experienced by juveniles passing through an estuarine environment can increase their resilience.
Climate models presently lack the detail to project changes in many specific factors in the marine environment that are most important for salmon. But for factors for which climate model results have relatively higher certainty, in salmon's inshore and estuarine habitats, their projections are uniformly unfavorable. Increased winter flooding, reduced summer and fall flows, and warmer stream and estuary temperatures are all harmful for salmon. Earlier snowmelt and peak spring streamflow will likely deliver juveniles to the ocean before there is adequate food for them, unless the beginning of other food producing events also happens earlier under climate change. While it is possible that the warming of the oceans could directly pose a problem for salmon, some research suggests that the effects of ocean temperature on salmon are not primarily direct, but instead are through changes in food supply.
Societal and Economic Impacts
Many natural and man-made factors have brought regional salmon stocks to widespread decline. In fact, over the past century, Pacific salmon have disappeared from about 40% of their historical breeding range in Washington, Idaho, Oregon, and California. Many remaining populations are severely depressed. The decline is not universal: populations from coastal rather than interior streams, from more northerly ranges, and with relatively short freshwater rearing periods have fared better than others. In many cases, the populations that have not declined are now composed largely or entirely of hatchery fish. Supplementation of wild stocks with hatcheries and efforts to restore habitat have all been unsuccessful in stemming the overall decline of salmon in the region. These massive restoration efforts reflect salmon's status not just as a commercially important fish but as a regional cultural icon.
In March 1999, eight new salmon stocks were listed under the Endangered Species Act (ESA) as threatened and one as endangered, bringing the number of salmonid stocks listed to 26. The new listings included Puget Sound Chinook, the first-ever ESA listing of a species inhabiting a highly urbanized area. Prior listings of Columbia and coastal Oregon stocks severely constrained forest and water management activities because of the effect on the salmon streams. It is possible that the effects of this new listing on the Puget Sound area economy will be large, but it will be some years before these impacts are known.
Salmon are already beset with a long list of human-induced problems, to which climate change is a potentially important addition. At sea, fishing pressure has recently been sharply reduced, but wild salmon face intense competition from hatchery fish, some of which are being released at ten times the rate of natural smolt (young salmon) migrations. Onshore, the effects of present climate variability on salmon in streams are swamped by the negative impacts from human activities such as clear-cutting, road building, and dams. The effects of future climate trends, and their potential to interact with other stresses, are not known. Neither is the extent to which current and proposed measures to protect salmon by restoring stream habitat and changing dam operations will restore depleted stocks and increase their resilience to climatic stresses.
If endangered-species listings and public concern prompt a strong restoration response, it would likely take the form of far-reaching restrictions on land-use near rivers and streams. Such restrictions would very likely have far-reaching social and economic impacts, which could cost considerably more than the direct cost of decreases in salmon abundance.
Strategies to Address Potential Impacts on Salmon
Strategies for addressing potential impacts on salmon under changing climate conditions vary in focus and in cost. Climate change impacts will vary stream by stream, depending on whether and how stream flow is affected. This list of response options is not exhaustive and should be used as a starting point for discussion and thought.
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