Coho Salmon
Oncorhynchus kisutch
Coho are unique to rivers and streams of the North Pacific
Ocean. After spending their first year in their birth stream,
they mature at sea before returning to the rivers to spawn and
subsequently die. The coho salmon is a lively surface dwelling
predator of small fish. It was formerly numerous along the
south coast of British Columbia where it spawned in streams
and flooded fields, small headwater tributaries, and deep
river channels.
Adults return from the sea to deposit eggs in gravel streams
and then die. The eggs incubate through the winter buried in
the gravel. Hatchlings remain beneath the gravel until their
yolk sacs are absorbed and emerge as swimming, feeding fry in
the spring. The fry remain in the stream for a year or more
before migrating to the sea as smolts. While at sea, coho grow
rapidly for a year and half or more before returning to the
spawning stream of their birth. Most coho reach maturity in
their third year of life after spending up to 6 months in
incubation, 15 months in freshwater, and 16 months at sea. A
small percentage of male and female cohos known colloquially
as ‘jacks’ and ‘jills’ mature one year earlier than the
majority of cohos Sandercock (1991).
Distribution and Migration
Coho are wanderers. Smolts entering the Strait of Georgia in
southern Canada disperse quickly throughout the Strait. Some
stay in the Strait for their adult lives but many exit through
Johnstone and Juan de Fuca straits to either swim north along
the coast of British Columbia to Alaska or swim south into
Washington State waters. Some of the Alaskan cohort swim in a
large counterclockwise direction around the Gulf of Alaska
before returning south toward their home streams while others
remain in coastal waters. In southern British Columbia, coho
usually arrive near the mouths of their spawning streams in
late summer or autumn. They hold in this location until fall
rains raise stream water levels sufficiently that the salmon
can proceed upstream to their spawning grounds.
Population Trend
The trend in coho survival in the Strait of Georgia, British
Columbia, and elsewhere has been downward since 1989. Beamish
et al. (2000) attribute this wide ranging change to a common
feature - the most likely being climate change effects. They
showed that the Aleutian Low Pressure Index (a measure of the
strength of ocean currents that bring nutrients to the sea
surface) and April river flow from the Fraser River changed
abruptly about the same time. Coho smolts might have entered
the sea when their plankton prey was in short supply. The
synchrony of coho survival trends and climate indicators
implies that climate and ocean changes play an important role
in the abundance of coho salmon.
Coho Spawning Behaviour
Spawning takes place from October to January in British
Columbia. Females choose the nest site and defend it against
all comers. Her activities draw a crowd of males but
eventually the largest male dominates the others and
fertilizes her eggs as they are deposited into the nest she
has dug in the gravel. Females in rivers in the Strait of
Georgia lay on average about 2500 eggs. A week or two later,
the adults die. The eggs require about 100 to 115 days before
hatching depending on water temperature. The warmer the water,
the more quickly they mature. The alevins emerge from the eggs
to spend a few weeks in the gravel. Once in the stream they
absorb their yolk sac and begin to feed on aquatic
invertebrates. The fry mill together darting for the cover of
gravel or beneath large stones when frightened. As they grow,
the coho fry spread throughout the stream. Streams with
boulders, woody debris, and a vegetated edge support more fry
than those without these features. While in their natal river,
coho fry are largely dependent on stream invertebrates and
insects that fall from above as food. Coho fry take up
territories along the river and those unable to defend a site
are displaced downstream. The spring rains fill the rivers
discharging both water and young salmon that are unable to
shelter in a slower moving side channel. Young coho that are
washed to sea in the first spring or summer rarely survive.
By late summer and early autumn, juvenile coho swim into the
deeper pools of the river and enter tributaries to avoid being
swept to sea during the fall floods of their second winter.
Many juvenile coho are eaten by such predators as cutthroat
trout and mink while in the river.
By the following spring, coho prepare to leave the river as
smolts, a process that in the Strait of Georgia spread over
about 120 days. Half of the fish depart in the last week of
May usually on a high tide and under the cover of darkness
which likely helps to avoid predators. Many return to a home
stream but recovery of tagged fish is dependent on how far
field one chooses to search. A Quinsam River coho, for example
was recovered 190 kilometers away in the Quatse River (Sandercock
1991).
Coho Feeding Behaviour
Coho salmon are surface dwelling fish; most are found in water
less than 20 meters deep (Quinn 2005). While at sea, coho eat
mostly marine invertebrates such as
krill in the early stages and then prey on small fish as
they grow larger. They are voracious predators of young
herring and
sandlance and are favoured by west coast anglers for their
fight.
Ecology of Salmon
One of the
most remarkable ecological stories to emerge in the past few
decades is the vital role of salmon in north Pacific
ecosystems. An excellent review of the story can be found in
Thomas Quinn’s book, The behavior and ecology of Pacific
Salmon and Trout (University of Washington Press, 2005).
Years ago fisheries biologists in Alaska knew that the
carcasses of sockeye salmon enriched otherwise nutrient-poor
lakes where the young sockeye grew before leaving to sea.
Experiments showed that artificial fertilization of lakes
sometimes enhanced the growth of young sockeye. It appeared
that the addition of nitrogen and phosphorous enhanced algal
growth in lakes that became food of zooplankton eaten by young
sockeye. The next breakthrough was the role of salmon
carcasses in rivers and lakes. With the development of
technology to measure isotopes, scientists were able to
measure the relative contribution of marine and terrestrial
sources of nitrogen and carbon in plants and animals along
riverbanks. Kline et al (1990) showed that substantial amounts
nitrogen and carbon in plants, insects and fishes was derived
from marine sources. Biologists had long known that bears
discard salmon carcasses along the edges of salmon spawning
rivers but it was only recently that the significance of
bears became clear. The size of bears and the number of
cubs they raise is directly related to the amount of meat they
consume (Hildebrand et al. 1999) and so large numbers of bears
assemble along salmon spawning streams in autumn. They catch
large numbers of large fish but eat mostly the brains and eggs
and discard about 70% of the carcass. The remaining protein is
left to rot in the forest where it eventually is taken up by
the forest plants, insects,
birds, mammals and fish (Bilby
et al. 1996,
Helfield 2001, Reimchen et al. 2003,
Wilkinson et al 2005).
References
Beamish R.J., D.J. Noakes, G.A. McFarlane, W. Pinnix, R.
Sweeting and J. King. 2000. Trends in coho marine survival in
relation to the
regime concept. Fisheries Oceanography 9: 114-119.
Bilby, R.E., B.R. Fransen and P.A. Bisson. 1996. Incorporation
of nitrogen and carbon from spawning coho salmon into the
trophic
system of small streams: evidence from stable isotopes.
Canadian Journal of Fisheries and Aquatic Sciences 53:164-173.
Bilby, R.E., B.R. Fransen, P.A. Bisson and J.K. Walter. 1998.
Responses of juvenile coho salmon (Oncorhynchus kistusch)
and
steelhead (Oncorhynchus mykiss) to the addition of
salmon carcasses to two streams in southwestern Washington,
U.S.A. Canadian
Journal of Fisheries and Aquatic Sciences 55:1901-1918.
Healey,
M.C. 1991. The life history of chinook salmon. Pp 311-393 in:
Pacific Salmon Life Histories. C. Groot and L. Margolis
(eds).
University of British Columbia Press.
Helfield,
JM, 2001. Interactions between salmon, bear, and riparian
vegetation in Alaska. PhD thesis, U of Washington, Seattle.
Hilderbrand,
G.V., Schwartz, C.C., Robbins, C.T., Jacoby, M.E., Hanley, T.A.,
Arthur, M.S., and Servheen, C. 1999. The importance
of meat,
particularly salmon, to body size, population productivity,
and conservation of North American brown bears.
Canadian
Journal of Zoology 77: 132–138.
Kline, TC et al. 1993. Recycling of elements transported
upstream by runs of Pacific salmon: 1. Canadian Journal of
Fisheries and
Aquatic Sciences 50-2350-2365.
Quinn, T.P. 2005. The behavior and ecology of Pacific salmon
and trout. UBC Press, Vancouver.
Reimchen,
T. 2003. Some ecological and evolutionary aspects of
bear-salmon interactions in coastal British Columbia. Canadian
Jounral of Zoology 78: 448-458.
Sandercock, F.K. 1991. Life history of coho salmon (Oncorhynchus
kisutch). Pp. 397-445 In C. Groot and L. Margolis
(Eds.). 1991.
Pacific salmon life histories. University of British Columbia Press, Vancouver, B. C.
Wilkinson, C.E., M.D. Hocking and T.E Reimchen. 2005. Uptake
of salmon-derived nitrogen by mosses and liverworts in coastal
British
Columbia.
Oikos 108: 85-98.
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