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. |
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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.
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.
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.
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
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.
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.
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