Habitat Requirements for Coastal Coho Salmon Populations

Habitat Requirements for Pink Salmon Populations
General Life History
Pink salmon are the most abundant of all of the Pacific salmon species accounting for
about 60% in numbers of all salmon caught commercially in the North Pacific Ocean. One of
the distinguishing characteristics of pink salmon is their fixed two year life span. In many river
systems, pink salmon are produced in either even or odd years only while other systems have
distinct populations returning every year. Major spawning runs occur throughout coastal British
Columbia, the largest of which returns to the Fraser River on odd years. Mature adults ranging
in length from 35 to 60 cm return to their river of origin to spawn during the late summer or early
fall. In general, pink salmon tend to spawn closer to the sea than other Pacific salmon species
and may even select spawning sites in areas that are influenced by salt water. As they prepare to
spawn, pink salmon undergo a major change in body shape. This process is most prominent in
males which develop an enormous hump on the back, a compressed body, a greatly enlarged
head, and large teeth on both the upper and lower jaws that form a pronounced hooked shape.
These changes are likely related to mating practices, species recognition and swimming
Pink salmon prefer spawning areas situated on moderately shallow riffles with clean,
coarse gravel along with a mixture of sand and silt. When fish reach the spawning grounds, the
female selects a nest site 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 depends to a large extent on water
temperature. In general, the lower the water temperature, the longer the incubation period
required. Upon hatching, the juvenile pink salmon (called alevins) move within the spaces
between the gravel particles varying distances depending on gravel size. At this point, the young
fish have an attached yolk sac that provides the required nutrition. Towards the end of
incubation (late winter to early 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. Many factors influence survival to this life stage but under
average conditions, 10 to 20% of the fertilized eggs will emerge from the gravel.
After emergence, pink salmon fry, (ranging from 28 to 35 mm in length), migrate quickly
downstream often traveling at or near the surface of the river or stream. In most cases, migration
is direct and completed quite quickly as pink salmon juveniles spend less time in freshwater than
other salmon species. During this migration, strong schooling behaviour is adopted as a strategy
to reduce predation. Little or no feeding occurs during downstream migration and in short
coastal systems sampling has revealed that most stomachs are empty. In larger rivers, some
feeding may occur due to an extended downstream migration period.
Upon reaching the estuary, pink salmon fry show a range of dispersal behaviours. Some
stocks reside in the estuary environment for several months before moving to offshore waters. In
large systems like the Skeena and Fraser rivers, fry may be transported by the river current for
distances of up to 50 km where they then seek out protected areas. Early marine schools of pink
fry tend to follow shorelines and spend much of their time in shallow water where they feed
aggressively for two to three months before migrating to the open ocean. Their diets in these
important nursery areas include various zooplankton species, adult and larval insects, fish larvae
and the eggs of various invertebrates and fishes. The fry consume large amounts of food during
this period and growth is rapid.
As the pink salmon move away from shoreline areas into deeper, offshore waters they
begin consuming larger food items including fish, squid and various large zooplankton species.
Sexual maturation occurs during their second summer at sea when the journey back to the river
of origin begins. This is a period of aggressive feeding and active migration enroute to home
rivers. Mature adult pink salmon complete their ocean rearing phase after about 16 months at
sea when they are two years old.
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. Pink salmon populations require quality
freshwater spawning and incubation habitats to thrive and remain productive. Since pink salmon
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. Near shore and
open-ocean environments are also vital and marine survival rates determine the return run size of
pink salmon stocks in most cases. 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 pink salmon populations.
Adult salmon require unimpeded 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 will mill about in the vicinity of the river
mouth, 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. Many
large die offs of pink salmon have been recorded due to poor river entry conditions and poor
conditions on the spawning grounds. As a result, it is important to critically assess any activities
that impact river flows or water temperatures when pink salmon are returning to spawn and to
ensure that fish have unimpeded access to spawning grounds.
Pink salmon require spawning sites within the stream or river where water velocity,
depth and gravel size are optimal for the incubation of developing eggs. 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. Pink salmon prefer coarse gravel with a few large
cobbles, along with a mixture of sand and a small amount of silt. Water depth must be sufficient
to cover the redd at all times. A lack of prime spawning habitat can limit pink salmon
production as spawners will be forced to build redds in secondary locations or on top of
previously constructed redds resulting in reduced overall production.
The percentage of pink salmon eggs and alevins that survive depends to a large extent on
stream and stream bed conditions. In order to survive, eggs and alevins require protection from
sediment build up, freezing, drying of the redd, streambed scouring, mechanical injury, and
predation. Studies have found that high survival of pink salmon fry was correlated with high
gravel permeability where flow rates ensured that the developing eggs and alevins were supplied
with a constant current of water that delivered dissolved oxygen and removed waste. Spawning
areas with slightly larger gravel size and low rates of sedimentation generated higher survival
rates. In cases where large amounts of silt built up in spawning beds survival rates were greatly
decreased. This situation can occur in areas where streamside activities result in high sediment
runoff into the river or where high flows move sediments from upstream areas into spawning
Incubation temperatures outside the ideal range can cause hatching and emergent
times that result in reduced survival of pink salmon. In extreme cases, freezing of redds can
result in the loss of all eggs in the affected areas. Studies indicate that while pink salmon eggs
and alevins can withstand a wide fluctuation in temperature, decreased survival and impaired
development occurs at temperatures below 5 0C and above 16 0C.
Pink salmon may spawn in slightly faster currents than those preferred by other
salmonids as pink eggs may require more dissolved oxygen. Successful incubation requires
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. Data shows that the flow range preferred by pink salmon is from 40 to 70 cm/s.
The water surrounding pink 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 pink salmon incubation area.
Juvenile Migration
After emergence from the gravel, pink salmon juveniles typically begin an immediate
migration towards the ocean. The period of emergence usually coincides with the high river
runoff and most migrations are short with juveniles reaching salt water on the first night of
travel. In turbid waters, some daytime migration may occur at the peak of emergence. Those fry
not completing the journey on the first night typically hide in the rocky substrate during daylight
hours and continue the migration the following night. As a result, the presence of appropriate
cover is important to the survival of migrant fry requiring more than one night to reach the
Water temperature is considered by many to be the most important environmental
variable for seaward migrating pink salmon fry. Migration has been observed at temperatures
ranging from 2 to 17 0C with a preferred range of from 10 to 16 0C. Temperatures above 17 0C
have a detrimental effect on fry behaviour and performance with 23 0C considered as the upper
lethal limit.
Ocean Phase
When pink salmon fry enter salt water they begin another critical phase in their life
history. Early marine residence is important in the life cycle of all Pacific salmon and can
influence overall ocean growth, survival and the subsequent return of spawning adults. Coastal
areas are a vital component of the early marine stage because they can provide early
opportunities for feeding and growth, and refuge from predators. Studies have indicated that
survival is largely driven by ocean conditions during early salt water residence. Since survival at
sea is generally size-selective, favourable near-shore ocean productivity is important as it can
result in faster growth and a shorter time to reach the size required to escape from predators.
Therefore, coastal ocean productivity is a key habitat component that impacts the survival of
pink salmon.
The distribution of pink salmon in offshore waters is dependent on ocean environmental
conditions and on food availability. 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, pink 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 pinks utilize coastal
and off shore habitats during a period of rapid growth that is critical to reproductive success.
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Heard, W. R. 1991. Life history of Pink salmon (Oncorhynchus gorbuscha). In: Pacific Salmon
Life Histories. Edited by C. Groot and L. Margolis. UBC Press. P 119 - 230.
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Raleigh, R. F., and P. C. Nelson. 1985. Habitat suitability index models and instream flow
suitability curves: Pink salmon. U.S. Fish. Wildl. Serv. Biol. Rep. 82(10.109). 36 pp.
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Species/Habitat outlines. Prepared for DFO by G.L. Williams and Associates.
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