Chum Salmon
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Chum Salmon Ecosystems

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

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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.

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

Various chum salmon stocks spawn from August through March, and in Washington State these stocks are identified by their timing of spawning as summer, fall, or winter stocks. Chum salmon spawn in a variety of stream types, ranging from small tributaries to large mainstem rivers and side-channels. Summer chum stocks, which spawn from August through October, need adequate stream flows to provide upstream access and proper distributions on the spawning grounds. Summer, fall, and winter chum spawners all require extensive, stable spawning riffles, with clean spawning gravels and low to moderate winter stream flows for optimum egg and alevin survival.

This species typically migrates directly to marine waters upon emergence from spawning sites, at a size of approximately 1 to 1½ inches. Since there is no extended freshwater rearing, high levels of fine sediments in spawning gravel, redd scour by high flow, or other habitat factors that reduce survival to emergence can have a major negative impact on chum salmon populations. Because of their small outmigration size, chum fry are particularly vulnerable to predation by other fishes and birds, and survival rates can be lowered substantially by aggregations of predator species.

Water quality is particularly important at the out-migration life stage because the small chum fry lack the energy reserves and swimming speed to quickly move through degraded areas. The freshwater/saltwater transition zone provided by estuary habitat is critical to the success of chum fry. A healthy estuary provides the food resources necessary for early growth, and can offer refuges from numerous fish and bird predator species. In the near shore environment and open ocean, competition for food resources with other fish species has been shown to affect growth and survival of chum salmon.

Habitat Factors

Stream Habitat

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

Summer chum spawner distributions can be limited to sub-optimal stream reaches near tidal areas by the low flows that typically occur in late summer months. These low flows also force fish to spawn in the center of the stream channel, which can increase egg and alevin mortalities during subsequent winter floods. Fall and winter chum spawners are seldom limited by low flows, but frequently are affected by high flows, which disrupt upstream migration and can interfere with spawning activities.

Floods during the incubation period reduce the intergravel survival of eggs and alevins and can significantly affect future production.

All chum stocks can be negatively impacted by high flows (displacement of spawners and streambed scour) 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 as high as 90 percent.

Land use practices and natural events that introduce substantial amounts of silt to spawning streams affect chum intergravel survivals by reducing the permeability of the gravel, interfering with the delivery of water and oxygen to incubating eggs and alevins. Channelization and bank armoring reduces the amount, quality, and diversity of chum salmon spawning areas by narrowing and deepening the stream channel. Chum salmon rarely occur in streams above lakes or reservoirs because upstream migrating adults are often reluctant to pass through fishways and downstream migrant fry have difficulties successfully negotiating their way through still water impoundments.

Estuarine and Marine Habitat

Newly emerged chum fry migrate directly to salt water, and early marine survival is dependent on healthy estuaries providing good quality water, abundant food resources, and refuges from predators.

Juvenile chum salmon spend the first part of their marine lives in estuarine and near shore areas adjacent to their natal streams. These young chum salmon obtain their critical early growth by feeding in tidal sloughs and creeks and other intertidal areas. Most of the estuaries in Washington have been altered in some way by changes like channelization, dredging, diking, filling of wetlands and tidal areas, and degraded water quality. This extensive alteration and/or loss of estuarine habitat has been caused by factors such as urbanization, agriculture, forest land management, and industrial and water resource development. It has been estimated that 39% of the coastal wetlands and 70% of the Puget Sound emergent wetlands have been lost. These modifications tend to reduce the overall amount of habitat, and degrade the general productivity of estuaries (and lower food production), which limits overall utility of these areas for chum rearing. This can result in reduced growth rates, which can affect how fast the juvenile chum salmon grow to a size that reduces their vulnerability to predators.

Most chum salmon enter the open ocean during the summer and fall of their first year. They migrate northward to the Gulf of Alaska where they spend from 1 to 4 years feeding and growing. The North Pacific habitat is primarily influenced by natural climate processes that cause long term changes ocean temperatures and currents, which in turn can affect the production of food organisms utilized by chum and other salmon.

Biological Interactions

Predation

Predation effects on chum salmon are primarily caused by various fish species and birds during the juvenile life stage, and large fish species and marine mammals during the adult life history stage.

A variety of predator species feed on chum salmon throughout their life cycle. Juvenile chum 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 chum populations unless they are subjected to unusual aggregations of predators. The release of hatchery fish of a variety of species is a common reason for large predator aggregations, and in some situations, this practice has been shown to negatively impact the survivals of chum salmon juveniles. Adult chum are subject to predation in marine areas by sharks, lampreys, and marine mammals, and in freshwater by bears, marine mammals, and large predatory birds.

Competition

Competition for food resources with other fish species in the near shore environment and open ocean can reduce chum survival and abundance.

Competition between chum salmon and other salmonids for spawning locations, and with a variety of species for food resources has been shown to impact chum survival rates. Temporal and spacial overlapping of spawning chum with other salmon spawners can result in reduced survivals caused by egg loss from redd superimposition. Competition for food with other fish species in the marine environment influences juvenile chum growth rates and survivals. In particular, the abundance of pink salmon sharing common marine areas with chum has been demonstrated to have a direct impact on chum survival and abundance.

Maintaining Chum Salmon Ecosystems

Instream flows sufficient for the migration, spawning, incubation, and juvenile outmigration of chum salmon 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 instream values such as minimal siltation, stable stream banks, natural flow regimes, extensive spawning riffles, and stable and diverse stream beds. Estuaries and near shore marine areas should be managed for natural habitat values. The maintenance and restoration of water quality and natural tidal and salt marsh environments should be high priorities. In areas frequented by juvenile chum salmon, releases of hatchery salmonids should be evaluated for their potential impact as both predators and competitors.