Habitat Requirements for Ocean-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 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 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. Ocean-type chinook have a
limited freshwater rearing phase that lasts from 60 to 150 days. Ocean and immediate-type
chinook populations dominate most runs throughout British Columbia. I most cases, ocean-type
chinook return to spawn during autumn after spending between one and five years in the ocean
and tend to select spawning sites in the lower or middle reaches of rivers. With shorter upriver
migrations, ocean-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.
An immediate downstream migration is typical of chinook fry in most populations and is
generally associated with high river flows. In general, while migrations are influenced by flow,
the movement of fry in rivers and streams is an active behaviour. For ocean-type chinook
spawning in upstream areas, 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 play a role in distribution as coho grow faster than chinook in similar habitats
and may force them out of some stream reaches. Typically, ocean-type chinook fry will rear in
stream habitats for periods ranging from 60 to 150 days. Larval and adult insects are the
principle components of the diet during this period. Migration downstream to the estuary
typically occurs between April and June with southern populations tending to migrate earlier
than those from more northern areas. The juveniles are often referred to as fingerlings at this
stage and are distinguished from immediate-type fry by their larger size (ranging from 50 to 120
mm). In larger systems, fry will usually migrate close to the river edges where velocity is
reduced.
Upon reaching the estuary, the fingerlings occupy deeper waters and remain in the area
for varying periods ranging from a few weeks to several months. Their arrival generally
coincides with the departure of immediate-type juveniles. Estuaries provide a rich feeding
habitat for ocean-type fingerlings and growth is relatively rapid. When they leave the estuary as
smolts, ocean-type juveniles 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 young
ocean-type chinook 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 and sandlance) 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. Ocean-type chinook salmon populations require
quality freshwater spawning, incubation and rearing 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 ocean-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. Ocean-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 Rearing
An immediate downstream migration is typical of chinook fry in most populations and is
generally associated with high river flows. For ocean-type chinook spawning in upstream areas,
this migration serves as a dispersal mechanism that distributes fry among suitable rearing
habitats. During their limited freshwater residence, ocean-type fry tend to utilize back eddies
and other habitats along the 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 adult and larval insects, particularly those floating on the surface of the stream.
During their period of freshwater rearing, ocean-type chinook juveniles require stream
habitats that are moderate in temperature and flow, and that support healthy and productive
insect populations. These criteria are met in natural systems with healthy streamside vegetation,
low sediment loads, high dissolved oxygen levels, and variable substrates.
The period of residence in estuarine waters is an important phase in the life history of
ocean-type chinook salmon. 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 ocean-type
chinook salmon and emphasize the need to protect these fragile areas from any activities that
may be detrimental.
Ocean Phase
Ocean-type chinook salmon require productive nearshore marine habitats and survival
during this period of early ocean residence can greatly influence total production. Ocean-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 ocean-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, ocean-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 ocean-type chinook salmon. 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, ocean-type chinook salmon migrate varying distances while increasing
in size and acquiring the energy reserves required for reproduction. The variety of food items
consumed varies over time and location but fish (primarily herring and sandlance) dominate the
diet with crab larvae, squid and large zooplankton also contributing. While distribution patterns
vary between years and stocks, all ocean-type chinook utilize coastal and off shore habitats
during a period of rapid growth that is critical to reproductive success.
References
Allen, M. A., and T. J. Hassler. 1986. Species profiles: life histories and environmental
requirements of coastal fishes and invertebrates (Pacific Southwest) – chinook salmon.
U. S. Fish Wildl. Serv. Biol. Rep. 82(11.49). U. S. Army Corps of Engineers, TR EL-824. 26 pp.
Brown, T.G. 2002. Floodplains, flooding and salmon rearing habitats in British Columbia: A
review. Canadian Science Advisory Secretariat Research Document. – 2002/07.
DFO. 1999. Fraser River chinook salmon. DFO. Science Stock Status Report D6 – 11 (1999).
DFO. 1999. Lower Strait of Georgia chinook salmon. DFO. Science Stock Status Report D6 –
12 (1999).
Fraser, F. J., P. J. Starr, and A. Y. Fedorenko. 1982. A review of the chinook and coho salmon
of the Fraser River. Can. Tech. Rep. Fish. Aquat. Sci. 1126: 130p.
Greene, C. M., D. W. Jensen, G. R. Pess, and E. A. Steel. 2005. Effects of environmental
conditions during stream, estuary, and ocean residency on chinook salmon return rates in
the Skagit River, Washington. Trans. Am. Fish. Soc. 134:1562-1581.
Quinn, T. P. 2005. The behaviour and ecology of Pacific salmon and trout. Univ. Wash. Press.
278p.
Raleigh, R. F., W. J. Miller, and P. C. Nelson. 1986. Habitat suitability index models and
instream flow suitability curves: Chinook salmon. U.S. Fish Wildl. Serv. Biol. Rep. 82
(10.122). 64 pp.
Riddell, B. 2004. Pacific salmon resources in central and north coast British Columbia.
Vancouver, BC: Pacific Fisheries Resource Conservation Council.
Healey, M. C. 1991. Life history of chinook salmon (Oncorhynchus tshawytscha). In: Pacific
Salmon Life Histories. Edited by C. Groot and L. Margolis. UBC Press. P 308 – 393.
Williams, G. L. 1989. Coastal/Estuarine fish habitat description and assessment manual. Part I.
Species/Habitat outlines. Prepared for DFO by G.L. Williams and Associates.