Climate Impacts in Brief
Climate Impacts on Pacific Northwest Salmon
There are seven species of salmon in the Pacific Northwest (PNW): pink, sockeye, chum, chinook, coho, steelhead, and sea-run cutthroat, with a total of 36 substantially isolated breeding populations, or evolutionarily significant units (ESUs). Each of these ESUs has developed a distinct migratory rhythm, giving it a somewhat different sensitivity to climate variations at different stages of life. Today, wild populations of PNW salmon are at historically low numbers, with many ESU's now listed as either Threatened or Endangered under the Federal Endangered Species Act.
Impacts of Climate Variability
Climate variability plays a large role in driving fluctuations in salmon abundance by influencing their physical environment, the availability of food, the competitors for that food, and the predators that prey on small salmon. The complexity of influences on salmon, both climate and otherwise, combined with the scarcity of observations of factors important to salmon in estuaries and the ocean, make it challenging to identify the links between salmon and climate. We and other researchers have examined long-term salmon harvest records along with extensive climate records and the limited estuary and ocean ecosystem data, to infer some of the connections between climate and salmon abundance.
Mantua et al. (1997) demonstrated a remarkable connection between salmon abundance and a North Pacific climate variation that they named the Pacific Decadal Oscillation (PDO). Warm phase PDO is associated with reduced abundance of coho and chinook salmon in the Pacific Northwest (PNW), while cool phase PDO is linked to above average abundance of these fish (Figure 1). This relationship was confirmed in recent years of cool phase PDO in which salmon returns have rebounded to values not seen since the 1970s.
Figure 1 Selected Pacific salmon catch records with PDO signatures. Black (gray) bars denote catches that are greater (less) than the long-term median. The dotted vertical lines mark the PDO polarity reversal times in 1925, 1947, and 1977. The October-March averaged PDO index is included at the top (from Mantua et al. 1997).
A diverse set of climatic factors helps determine salmon abundance. Warm phase PDO favors:
- warm coastal ocean conditions, which are unfavorable for chinook and coho marine habitat (a negative impact),
- warmer stream and estuary conditions, which could cause thermal barriers to adult salmon upstream migration and stress for resident juveniles (negative impact), and
- reduced spring and summer river flow, which could make river flow barriers less likely (positive impact).
Although El Niņo/Southern Oscillation (ENSO) has climatic effects that are similar to PDO, the state of ENSO varies considerably during the salmon life cycle and its effects are consequently more muted than those of the more persistent PDO.
Climate Change Impacts
Salmon productivity in the PNW is clearly sensitive to climate-related changes in stream, estuary, and ocean conditions. In the past century, most PNW salmon populations have fared best in periods having high precipitation, deep mountain snowpack, cool air and water temperatures, cool coastal ocean temperatures, and abundant north-to-south "upwelling" winds in spring and summer.
Global warming's expected impact on PNW climate includes many negatives for PNW salmon. Increased winter flooding and decreased summer and fall streamflows, and elevated warm season stream and estuary temperatures will clearly degrade in-stream and estuarine salmon habitat in the PNW. These changes will likely cause severe problems for the salmon stocks that are already stressed from already degraded freshwater and estuarine habitat.
It is unclear how PNW coastal ocean conditions will respond to global warming. Warmer temperatures are likely to increase ocean stratification, yet possible increases in winds may counter that in ways that mitigate or even increase the wind-driven upwelling of nutrients that fuel a productive food web.
From the global warming scenarios the CIG has examined, the likelihood for many positive impacts on PNW salmon is low. Where winter temperatures are now cooler than optimal for juvenile salmon and/or incubating eggs, warming may improve stream productivity. However, such conditions are now limited to a very small number of inland, high elevation salmon bearing streams.
Will PNW salmon be able to adapt to a rapidly changing chain of habitats in the coming decades? Pacific salmon populations have shown an amazing capacity for adaptation. Management activities that enhance this capacity to adapt will play a vital role in determining the answer to this key question.
For More Information
For more publications on climate impacts on PNW salmon and aquatic ecosystems, please see CIG Publications.
Francis, R. C., and N. J. Mantua. 2003. Climate and extinction risk for salmon populations of the Northeast Pacific. pp. 37-67. In A.D. MacCall and T.C. Wainwright (eds.). Assessing Extinction Risk for West Coast Salmon. U.S. Department of Commerce, NOAA Technical Memo, NMFS-NWFSC-56, 198 pp.
Lawson, P.W., E. A. Logerwell, N. J. Mantua, R. C. Francis, and V. N. Agostini. (in review). Environmental factors influencing freshwater survival and smolt production in two Pacific Northwest coho salmon (Oncorhynchus kisutch) populations. Submitted to Canadian Journal of Aquatic and Fishery Sciences.
Mantua, N.J., S.R. Hare, Y. Zhang, J.M. Wallace, and R.C. Francis. 1997. A Pacific interdecadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society 78: 1069-1079.
Page last updated: July 2, 2004