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Habitat Requirements for Immediate-Type Chinook Salmon Populations
General Life History
Chinook salmon are widely distributed throughout North America with over a thousand
spawning populations scattered along the Pacific coast. Spawning occurs from near tidal
influence to over 1,000 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 may spawn as
late as 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. Immediate-type chinook
bypass the freshwater rearing phase in their early life history and migrate to the estuary soon
after emergence. Ocean and immediate-type chinook populations dominate most runs
throughout British Columbia. These salmon return to spawn in the fall or early winter after
spending between one and five years in the ocean and tend to select spawning sites in the lower
reaches of rivers. With shorter upriver migrations, immediate-type chinook can delay river entry
until after peak flows and take advantage of a longer ocean feeding period. 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
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.
Immediate-type chinook fry (ranging in length from 30 to 45 mm) move directly to the
estuary after emergence. Downstream migration is generally most intense from February to May
and in larger rivers, fry 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. Since immediate-type chinook typically spawn in the lower reaches of rivers, fry
may arrive at the estuary in a few hours. Many have remnants of the yolk sac still attached and
few have begun feeding before reaching the estuary. Mortality is significant during downstream
migration and has been estimated to reach levels of over 70%. Loss to predators is generally
considered to be the most significant cause of mortality during this phase.
Immediate-type chinook salmon are highly dependent on estuaries for rearing and
preparing for life at sea. Estuaries provide a rich feeding habitat for immediate-type chinook fry
and growth is relatively rapid. Immediate-type fry stay in the estuary longer than the juveniles of
most other salmon species spending up to several months in tidal creeks, sloughs and side
channels where their movements are often coordinated with the pulse of the tides. These fry
continue to rear in estuarine waters until they are about 70 mm in length. As they leave the area
as smolts, immediate-type chinook generally remain close to the shoreline feeding in the
productive, shallow, near shore marine habitat for varying periods. Food items consumed
include various zooplankton species as well as adult and larval insects. As they continue to
grow, the juveniles begin to disperse throughout coastal areas preferring sheltered surface waters
during early marine residence.
While stream-type chinook constitute a large proportion of the high-seas population
regardless of latitude, immediate and ocean-type chinook dominate in coastal waters where they
remain for most of their life at sea. Data suggests that in general, immediate and ocean-type
chinook do not disperse more than about 1,000 km from their home rivers. In the south, many
will remain within the Strait of Georgia or off the west coast of Vancouver Island while central
and northern populations generally inhabit the areas along the rich feeding grounds of the
continental shelf. 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 with four-year-old fish being dominant in most years.
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. Immediate-type chinook salmon populations
require quality freshwater spawning and incubation habitats to thrive and remain productive.
Since immediate-type chinook are not significantly dependent on fresh water as a rearing habitat,
it is the quantity of prime spawning and incubation areas that determine freshwater production
potential. 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 all chinook 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. Immediate-type
chinook require about 24 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. 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 Migration and Rearing
After emergence from the gravel, immediate-type chinook salmon juveniles begin a
directed migration towards the estuary. The period of emergence usually coincides with high
river runoff and most migrations are short with juveniles reaching the estuary on the first night of
travel. Those fry not completing the journey on the first night typically seek out refuge areas as
predation during seaward migration can be a major sources of mortality on chinook fry. In one
study, researchers estimated that up to 70 % of emergent chinook fry were lost to predators
during migration. Although cover requirements are not well understood, the fry are known to
hide in the spaces between bottom substrates during the day as they migrate downstream. As a
result, immediate-type chinook fry traveling to the estuary require an unimpeded migration
path with cover from predators.
The period of residence in estuarine waters is a critical phase in the life history of
immediate-type chinook salmon and appears to play a significant role in determining the size of
the subsequent adult return. Coastal estuaries are important as they provide an environmental
transition zone, early opportunities for feeding and growth, and refuge from predators. As
environmental transition zones, brackish estuaries allow chinook fry the opportunity to acclimate
from freshwater to saltwater and between waters of differing temperatures. They provide the
first substantial opportunities for feeding and growth for species like immediate-type chinook
salmon that migrate to sea soon after hatching, 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 immediatetype chinook salmon and emphasize the need to protect these fragile areas from any activities
that may be detrimental.
Ocean Phase
Immediate-type chinook salmon require productive nearshore marine habitats and
survival during the period of early ocean residence in these environments can greatly
influence total production. Immediate-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
plankton and insects, both major dietary components for juvenile chinook. Therefore, the health
of coastal ocean ecosystems plays a key role in the production of immediate-type chinook
salmon stocks.
While stream-type chinook constitute a large proportion of the high-seas population
regardless of latitude, immediate and ocean-type chinook dominate in coastal waters where they
remain for most of their life at sea. Data suggests that in general, immediate-type chinook do not
disperse more than about 1,000 km from their home rivers. As a result, any factors that impact
the productivity of coastal regions also have an impact on immediate-type chinook salmon
populations. 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, immediate-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 immediate-type chinook utilize
coastal and off shore habitats during a period of rapid growth that is critical to reproductive
success.
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