Habitat Requirements for Stream-Type Chinook Salmon Populations
General Life History
Chinook salmon are widely distributed throughout North America with over a thousand
spawning populations scattered along the coast. Spawning occurs from near tidal influence to
over 3,000 kilometers upstream near river headwaters. Most individual spawning populations
are relatively small and in British Columbia, 80 % of surveyed streams average fewer than one
thousand spawners. In general, northern populations tend to spawn earlier with peak spawning
times ranging from July to September while southern populations spawn between November and
January. However, within river systems, individual populations may spawn at different times
and in different reaches of the river.
Chinook salmon display a wide variety of life history strategies that include variation in
the age at which juveniles go to sea, length of freshwater, estuarine and ocean residence, ocean
distribution as well as age and timing of the spawning migration. Stream-type chinook have an
extended freshwater rearing phase that lasts from one to two years. While ocean and immediatetype chinook dominate in most systems in British Columbia, stream-type fish make an important
contribution to runs in larger rivers. They are generally more northerly in location or are more
distant from the sea as is the case in the upper tributaries of the Fraser River. In systems where
more than one chinook life history type spawns, stream-type fish tend to enter the river earlier
and spawn in headwater areas. Stream-type chinook typically return to freshwater during the
spring and early summer after between one and four years in the ocean. Since stream-type fish
migrate to headwater tributaries to spawn, their early entry to freshwater permits them to take
advantage of high summer flows to reach remote areas. Since chinook have the largest eggs of
all Pacific salmon and therefore, the smallest surface-to-volume ratio, their eggs are more
sensitive to reduced oxygen levels. As a result, adequate subgravel flow is the over riding factor
in the choice of redd sites by all chinook. Provided the conditions of good subgravel flow are
met, chinook will spawn in water of almost any depth or flow velocity and over a wide range of
substrates.
When mature chinook reach the spawning grounds, the female selects a nest site with the
appropriate features and begins digging a pit referred to as a redd, where eggs will be deposited.
The digging process removes sand, silt and fine gravel from the nest site creating a favourable
environment for incubation of the eggs. Once the nest is complete, the female deposits the eggs
which are fertilized by one or more males and then moves to the area immediately upstream of
the nest and begins digging another pit. The material removed by this digging action covers the
fertilized eggs to protect them from predation and from being washed away by the scouring
action of the river or stream. This process may be repeated several times resulting in multiple
nests containing eggs from one female.
The length of time required for the eggs to incubate is partially dependent on water
temperature. In general, the lower the water temperature, the longer the incubation period
required. Upon hatching, the juvenile chinook salmon (called alevins) move within the spaces
between the gravel particles varying distances depending on gravel size. The newly hatched fish
have an attached yolk sac that provides the required nutrition. Towards the end of incubation in
the spring, alevins move up through the gravel to emerge as fry. This process occurs at night at
lower latitudes which helps to minimize predation and generally coincides with the complete
absorption of the yolk sac. The survival of chinook salmon eggs from spawning to emergence
varies widely between systems and years and is influenced by stream flow, water temperature,
dissolved oxygen, gravel composition, spawning timing and spawner density. Survival to
emergence is highly variable but studies suggest that the survival of chinook eggs is relatively
high and likely averages about 30 percent.
An immediate downstream migration is typical of chinook fry in most populations and is
generally associated with high river flows. For stream-type chinook, this migration serves as a
dispersal mechanism that distributes fry among suitable rearing habitats. In some cases,
competition with other stream rearing species such as coho may also play a role in distribution as
coho grow faster than chinook in similar habitats and may force them out of some stream
reaches. In many cases, habitat segregation has been observed occurring between stream-type
chinook and coho salmon or steelhead. Juvenile chinook are most often found where substrate
size is small, velocity relatively low and depth shallow. They seem to prefer main river channels
and are not often found in off-channel habitat. However, upper Yukon River chinook salmon
make extensive use of small, non-spawning streams for rearing. During their time in freshwater,
stream-type chinook juveniles feed primarily on larval and adult insects. They typically remain
in their home stream or river for a period of one to two years before migrating to sea as smolts.
Northern populations tend to spend longer rearing in freshwater due to the shorter growing
season and reduced freshwater productivity.
During the winter, stream-chinook fry in larger systems typically move out of tributaries
and into the river mainstem, where they seek out deep pools or spaces between boulders and
rubble. In systems with lake access, juveniles may overwinter in these habitats. Considerable
overwintering of Yukon River chinook occurs in smaller spawning and non-natal streams. In the
early spring, smolts begin an active migration to the sea traveling at rates of up to 37 km/d
during moderate discharge. Downstream migration is generally most intense from February to
May in southern streams, extending into June for more northerly watersheds.
In larger rivers, juveniles migrate close to the river edges where velocity is reduced. In general,
while migrations are influenced by flow, the movement of fry in rivers and streams is an active
behaviour. Mortality is significant during downstream migration and loss to predators is
generally considered to be the most significant cause of mortality during this phase.
Stream-type chinook smolts generally spend some time in the estuary of their home
rivers. In most cases, they concentrate in the outer delta areas and residence times tend to be
relatively short. Stream-type chinook smolts are the first chinook juveniles to disperse seaward
from their home streams. They tend to remain in sheltered, coastal areas through the spring and
early summer before moving to outer coastal areas and to offshore waters early in their ocean
life. Stream-type chinook appear to have a much wider distribution than ocean or immediatetype fish and constitute a larger proportion of the high-seas population regardless of latitude.
The variety of food items consumed varies over time and location but fish (primarily herring)
dominate the diet with crab larvae, squid and large zooplankton also contributing. For the
majority of chinook salmon, sexual maturation occurs during their third, fourth or fifth year of
life with four-year-old fish being dominant in most years. For some stream-type populations,
five year olds dominate the female component of the run.
Habitat Requirements by Life History Stage
The health of all Pacific salmon is closely linked to the availability of productive
freshwater, coastal and marine environments. Stream-type chinook salmon populations require
quality freshwater spawning, incubation, rearing and overwintering habitats to thrive and remain
productive. Estuary and near shore environments are also vital and the rate of survival through
the rearing period spent in these habitats greatly influences the return run size. Healthy habitat is
challenged by human competition for accessible land and fresh water, for ocean spaces and for
the interconnecting estuarine and coastal areas. The loss or degradation of quality habitat in any
of these important zones will have a negative impact on stream-type chinook salmon
populations.
Spawning
Adult chinook salmon require access to their home spawning grounds in order to
successfully reproduce. Features such as dams, debris jams, waterfalls, or rock/mud slides that
block upstream migration can limit access to spawning areas and impact production. Also, if
conditions such as high water temperature or extreme high or low flows are encountered when
spawners arrive at their river or stream of origin, fish often mill about in the vicinity of the river
mouth for long periods, waiting for conditions to improve. This delay in river entry can have a
detrimental affect on survival and on spawning success as fish are exposed to predation from
marine mammals and, since feeding has stopped in preparation for spawning, vital energy
reserves are used up. As a result, it is important to critically assess any activities that impact
river flows or water temperatures when chinook salmon are returning to spawn and to ensure
that fish have unimpeded access to spawning grounds.
Chinook salmon require spawning sites within the stream or river where water velocity,
depth and gravel size are optimal for the incubation of developing eggs. Stream-type chinook
require about 16 m2 of gravel per spawning pair. The substrate must be small enough to be
moved by the fish and large enough to allow good intragravel water flow to the incubating eggs
and developing alevins. Redds of stream-type chinook tend to be in areas of coarser gravel and
are often characterized by having a few large cobbles in the bottom of the nest. Since chinook
eggs are the largest of all the Pacific salmon and therefore have a small surface-to-volume ratio,
good subgravel flow is vital to egg survival. A lack of prime spawning habitat can limit
chinook salmon production as later spawners may be forced to build redds in secondary
locations or on top of previously constructed redds resulting in reduced overall production.
Reports indicate that when spawner densities are high or suitable spawning gravel is scarce,
chinook will spawn in areas of sand or silt that are unsuitable for successful incubation.
Incubation
The survival of chinook salmon eggs from spawning to emergence varies widely
between systems and years and is influenced by stream flow, dissolved oxygen, gravel
composition, water temperature, spawning timing and spawner density. Research indicates that
survival of chinook salmon embryos and alevins is higher in more stable flow regimes. In one
study, the best predictors of return run size included the magnitude of floods experienced during
incubation. In another study, survival to emergence ranged from 0.2 to 7.0% before flow control
structures were installed and ranged from 12.0 to 19.8% after the installation. Successful
incubation requires stable flow rates that are adequate to supply the required level of oxygen
but not high enough to cause gravel movement and streambed scour which could expose eggs
to predators or wash them downstream.
The percentage of chinook salmon eggs and alevins that survive depends to a large extent
on stream and stream bed conditions. Studies have found that higher survivals were correlated
with high gravel permeability which ensured that the developing eggs and alevins were supplied
with a constant current of water that delivered oxygen and removed waste. In one case, 87% of
chinook fry emerged successfully when gravel was large and subgravel flows were adequate
(greater than 0.03 cm/s percolation rate). Chinook eggs are particularly susceptible to low
oxygen levels as they are the largest of all the Pacific salmon species and therefore have the
smallest surface to volume ratio. Spawning areas with slightly larger gravel size and low rates
of sedimentation consistently generate higher survival rates. In cases where large amounts of
silt build up in spawning beds survival rates are greatly reduced. This situation can occur in
areas where streamside activities such as logging, road building, or agricultural practices result in
high sediment runoff into the river or where high flows move sediments from upstream areas
into spawning beds.
Incubation temperatures outside the ideal range can cause hatching and emergent
times that reduce survival. In extreme cases, freezing of redds can result in the loss of all eggs
in the affected areas. Research shows that while chinook salmon eggs and alevins can withstand
a wide fluctuation in temperature, decreased survival and impaired development occurs at
incubation temperatures below 5.0 0C and above 15 0C. Healthy streamside (riparian) vegetation
helps to moderate extreme high and low temperatures and it is therefore important that natural
growth remains undisturbed along the banks of salmon bearing streams.
The water surrounding chinook salmon redds must be non-toxic and of sufficient
quality to provide the basic requirements of incubation. There are many types of pollution that
can affect water quality including waste water, pesticides, toxic chemicals, petroleum products
and organic compounds. All efforts must be made to minimize the introduction of such
pollutants into salmon incubation area.
Juvenile Rearing
During their freshwater residence, stream-type fry tend to reside in tributaries and along
river margins. As they grow in size, they move to habitats with increasing velocity and depth.
This shift in habitat preference is associated with the search for areas with increased food
abundance and also serves to segregate chinook juveniles from potential competitors such as
coho and steelhead. While in freshwater, juvenile chinook primarily feed on a variety of
invertebrate species as well as on adult and larval insects, particularly those floating on the
surface of the stream. Optimal substrate for the maintenance of a diverse and healthy
invertebrate population includes a combination of mud, gravel and rubble with rubble dominant.
A pool to riffle ratio of about 1:1 appears to provide an optimal mix of food-producing and
rearing areas for chinook in streams. Healthy, natural streamside vegetation is important in
maintaining temperatures, controlling erosion and sedimentation and supplying food items that
are an important component of stream-type chinook diets. During the winter, chinook fry in
larger, more southerly rivers typically move out of tributary systems and into the river mainstem,
where they seek out deep pools or spaces between boulders and rubble. Optimal substrate size
for escape from predators and for winter cover is from 10 to 40 cm. Interior stocks may
overwinter in off-channel habitats. A significant and perhaps major portion of Yukon River
stocks overwinter in smaller natal or non-natal tributaries. In summary during their period of
freshwater rearing, stream-type chinook juveniles require stream habitats that are moderate in
temperature and flow, provide abundant rearing space for chinook and other juvenile
competitors, support healthy and productive invertebrate and insect populations and have
water of sufficient quantity and quality to allow overwintering. These criteria are met in natural
systems with healthy streamside vegetation, low sediment loads, high dissolved oxygen levels,
and variable substrates.
Stream-type chinook spend a relatively short but important period in the outer estuary.
Coastal estuaries are important as they provide an environmental transition zone, extensive
opportunities for feeding and growth, and refuge from predators. As environmental transition
zones, brackish estuaries allow chinook juveniles the opportunity to acclimate from freshwater to
saltwater and between waters of differing temperatures. They provide substantial opportunities
for feeding, and typically have higher food productivities than adjacent ocean or freshwater
areas. Estuaries may thus offer the opportunity for enhanced growth and therefore, larger size at
ocean entry which is known to correlate with higher marine survival. One final role of estuaries
is to provide refuge from predators. The higher turbidity often associated with estuarine areas
limits the ability of visual predators to key on salmon juveniles. Also, the extensive aquatic
vegetation associated with estuaries provides important structural cover. All of these factors
point to the importance of estuaries in the life of stream-type chinook salmon and emphasize the
need to protect these fragile areas from any activities that may be detrimental.
Ocean Phase
Stream-type chinook salmon require productive nearshore marine habitats and
survival during this period of early ocean residence can greatly influence total production.
Stream-type chinook generally remain in sheltered, near shore environments for varying periods
depending on factors such as food availability, competition, predation and environmental
conditions. Coastal areas provide a rich habitat with opportunities for feeding and growth, which
are important since survival in the ocean is size dependent with larger fish surviving at much
higher rates. Throughout this period, kelp and other shoreline vegetation provide an important
refuge from predators as well as a productive environment for insects and plankton, both major
dietary components for juvenile chinook. Therefore, the health of coastal ocean ecosystems
plays a key role in the production of stream-type chinook salmon stocks.
Stream-type chinook constitute a large proportion of the high-seas population regardless
of latitude. Data suggests that in general, stream-type chinook disperse widely throughout the
North Pacific where they feed mainly on small fish (primarily herring and sandlance), with crab
larvae, squid and large zooplankton also contributing to their diet. While migration patterns and
other aspects of their marine ecology remain poorly understood, ocean residence is recognized
as a very important component of the life cycle of all Pacific salmon. During their time at sea,
stream-type chinook salmon migrate varying distances while increasing in size and acquiring the
energy reserves required for reproduction. While distribution patterns vary between years and
stocks, all stream-type chinook utilize coastal and off shore habitats during a period of rapid
growth that is critical to reproductive success.
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May 1, 2007