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The PDO is a pattern of climate and ocean condition regimes occurring in the north Pacific Ocean that results in shifts in sea surface temperatures and plankton abundance on a long time scale (20 to 40 years). The PDO regimes have been shown to relate directly to the abundance of salmon populations that spend their marine lives in the Gulf of Alaska. A shift occurred in 1977, which resulted in warmer coastal sea temperatures, cooler central Pacific sea temperatures, and more abundant plankton resources. These ocean conditions likely contributed to general increases in production of those Washington salmon populations that migrate to the Gulf of Alaska, most notably pink and chum salmon. The PDO can also have a major influence on the local freshwater environment, and since 1977 coastal Washington has experienced a general stream flow pattern that includes summer/fall droughts and extreme flooding in early winter.
While the environmental conditions since 1977 have been generally favorable for chum and pink salmon, the warmer coastal sea temperatures and freshwater drought conditions have had negative consequences for local populations of Chinook and Coho salmon. Overlaid on the PDO effects are ENSO events, which begin as warming episodes in the tropical Pacific zone and can result in large scale intrusions of anomalously warm marine water northward along the PNW coastline. The impacts of these warm water intrusions are felt along the Washington and British Columbia coast for a one to two year duration in an irregular periodicity of every two to seven years. The combined impacts of the 1977 PDO shift and frequent recent ENSO events have created generally hostile freshwater and ocean environments for the coastally oriented Chinook and Coho populations over the last two decades. When combined with the effects of altered freshwater and estuarine habitats, dams, and fishing impacts these environmental changes have contributed to the recent low abundance of Chinook and Coho salmon.
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