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Ocean Climate Effects (PDO and ENSO)
in the Pacific Northwest


In recent years, it has become apparent that the growth and survival of local salmon populations can be profoundly affected by fluctuations in ocean conditions caused by two ocean processes; the Pacific Decadal Oscillation (PDO) and the El Niño-Southern Oscillation (ENSO).

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.

There is increasing evidence that the PDO has recently shifted to a regime of different ocean and climate conditions. Local patterns of salmon production are now consistent with environmental conditions similar to those that existed prior to the 1977 regime shift. In general, over the last several years chinook and coho populations have shown increased abundance, while chum and pink populations have contracted somewhat from the large returns of the last two decades. These changes, if they continue, are a normal part of the long-term abundance cycles of local salmon populations, and should not be considered to reflect a change in the status of the various salmon species.

Sockeye Salmon Ecosystems

A sockeye salmon ecosystem includes the interrelated complex of biological communities and environment conditions that contribute to population success.

Sockeye salmon generally spawn in streams that are tributaries to large lakes. These streams can vary in type, ranging from small tributaries to large mainstem rivers and side-channels. Additionally, some sockeye stocks spawn along the shorelines of lakes. Sockeye spawning in Washington State begins as early as August and can extend through February. Those stocks spawning from August through October need adequate stream flows to provide proper spawner distributions on the spawning grounds. All sockeye stocks require extensive, quality spawning riffles for optimum production. Successful egg and alevin survivals are dependent on clean spawning gravels and low to moderate winter stream flows. Those sockeye that spawn on lake shores need access to undisturbed shallow water shorelines, and clean gravels with upwelling ground water.

Sockeye migrate downstream to the deep waters of nursery lakes upon emergence from spawning sites, at a size of approximately 25 to 32 millimeters (1.0 to 1.25 inches). At this small size, sockeye fry are vulnerable to predation by other fishes and birds, and survivals can be lowered substantially by aggregations of natural or artificially produced predators Juvenile sockeye rear in the nursery lake for 1 or 2 years, and continue to be subjected to predation by other fish species. They also face competition for available food resources with other fish. The production of food organisms is particularly important at this life stage because faster growth rates can increase the survival of the young sockeye.

Sockeye smolts emigrate to sea in spring at a length of approximately 4 - 6 inches and are subjected to intense predation by a variety of fish and bird species. Squawfish and trout have been identified as especially significant predators during this outmigration life phase, and gulls and grebes are some of the significant avian predators of sockeye smolts.

The freshwater/saltwater transition zone provided by estuary habitat can be important to the success of sockeye smolts. A natural, productive estuary provides the food resources necessary for the smolts to transit the area, and can offer refuges from numerous fish and bird predator species. In the near shore and open ocean environments, predation by fish, birds, and marine mammals, and competition for food resources with other fish species affects growth and survival of sockeye salmon. Ocean growth and survival of all species of Pacific salmon can be affected by periodic warm water events (El Nino) in local waters, and by cyclic changes in ocean Click here to read about climate conditions in the North Pacific Ocean.

Habitat Factors

Stream habitat

Low stream flow can impede summer-run spawners, and high flows can disrupt the spawning of fall and winter sockeye.

Low stream flows can limit early run sockeye spawner distribution to sub-optimal stream reaches, and force fish to spawn in the center of the stream channel, which can increase egg and alevin mortalities during winter floods. Sockeye that spawn after mid-November are frequently affected by high flows, which can disrupt upstream migration and interfere with spawning activities. All sockeye stocks can be negatively impacted by high flows during the fall and winter incubation period. The erosion and downstream movement of spawning gravels is a major cause of egg and alevin losses, and severe flooding can cause mortalities exceeding 90%.

Land use practices and natural events that introduce substantial amounts of silt into spawning streams affect sockeye intergravel survivals by reducing the permeability of the gravel, which can affect the survival of incubating eggs and alevins by interfering with the delivery of oxygenated water and the removal of metabolic wastes. Channelization and bank armoring reduces the amount, quality, and diversity of sockeye spawning areas by narrowing and deepening the stream channel.

Lake habitat

Any limitations of food supplies during lake rearing can reduce growth rates, which in turn can lower survival of the juvenile sockeye in the lake and during their early marine life.

Juvenile sockeye rear for 1 or 2 years in lake habitats before migrating to sea. Limnetic habitats are especially critical because sockeye spend most of their time in these habitats. Good water quality and production of food organisms are important because survival in lakes can depend upon how fast sockeye grow to a size that reduces their vulnerability to predators. In addition, the size of sockeye leaving freshwater has a direct affect on their subsequent marine survival.

Mainstem habitat

The freshwater migration corridors used by emigrating sockeye juveniles (smolts) and by returning adults are generally medium to large sized mainstem streams, providing a direct route between the outlet of the home lake and marine waters. This mainstem habitat on the Columbia River and Lake Washington systems has been altered by the construction of dams, which has had negative impacts on sockeye salmon success. The impacts of Columbia River mainstem dams on the local sockeye stocks have included factors such as: total extirpation (in the case of stocks above Grand Coulee Dam); turbine passage mortalities of outmigrating juveniles; water quality problems (nitrogen supersaturation mortalities); predation on smolts in reservoirs; and between dam losses of upstream migrating. The dam and ship locks on the outlet of Lake Washington causes some impacts on sockeye salmon survivals; primarily through increased susceptibility to predation, and facility related (physical injury) mortalities of outmigrating smolts.

Estuarine and marine habitat

Juvenile sockeye smolts emigrate to salt water after one or two years, and early marine survival is dependent on healthy estuaries providing good quality water, and abundant food resources.

Juvenile sockeye salmon spend the first part of their marine lives in estuarine and near shore areas adjacent to their natal streams, although their residence time in these areas may be the shortest for any of the salmon species. Most of the estuaries in Washington have been altered by changes including channelization, dredging, diking, filling of wetlands and tidal areas, and degraded water quality. This alteration and/or loss of estuarine habitat by factors such as urbanization, agriculture, forest land management, and industrial and water resource development has been extensive. It has been estimated that 39% of the coastal wetlands and 70% of the Puget Sound emergent wetlands have been lost. These habitat modifications tend to reduce the overall amount of habitat, and reduces the general productivity of estuaries (and food production), which limits overall utility of these areas for sockeye rearing.

Biological interactions

Predation and competition for food resources with other fish species in the near shore environment and open ocean can reduce sockeye survivals and abundance.

A variety of predator species feed on sockeye salmon throughout their life cycle. Juvenile sockeye are preyed upon by fish (including other salmonids) and birds in both the freshwater and marine environments. This type of predation does not normally threaten the success of sockeye populations unless they are subjected to unusual aggregations of predators. The release of hatchery salmonids can cause large aggregations of species that are potential predators, and in some situations, this practice has generated concerns about the possibility of negative impacts on the production of each of the salmon species. Adult sockeye are subject to predation in marine areas by sharks, lampreys, and marine mammals, and in freshwater by bears and large predatory birds.

Temporal and spacial overlap of spawning sockeye with other salmon spawners can result in reduced survivals caused by the loss of eggs due to redd superimposition. Competition for food with other fish stocks and species in the marine environment has been shown to influence sockeye survivals. In particular, the abundance of sockeye salmon originating from different regions and sharing common marine areas with other sockeye stocks can negatively impact sockeye survival and abundance.

Maintaining Sockeye Salmon Ecosystems

Instream flows sufficient to meet the needs of sockeye salmon for migration, spawning, incubation, and juvenile outmigration are essential for successful production. Man-made barriers to migration should be removed or made passable wherever possible. Land use practices must be compatible with the maintenance of quality instream conditions such as; minimal siltation, stable stream banks, natural flow regimes, extensive spawning riffles, and stable and diverse stream beds. Lacustrine and estuarine habitats supporting sockeye should be managed to maintain or restore good water quality1 and populations of predator species should not be enhanced either directly or indirectly. In areas frequented by juvenile sockeye salmon, releases of hatchery salmonids should be managed to minimize their potential impact as both predators and competitors.