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The bright red bodies and green heads of the spawning sockeye
make it a poster fish of salmon and river conservation. It
prominence in the commercial fishery triggered decades of
research so that today the sockeye is one of the most
researched fish species on the planet.
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).
Distribution and Migration
Juvenile sockeye entering the ocean swim north
along the coast of British Columbia and Alaska and then move
offshore into the Gulf of Alaska where they spend the next two
years. Sockeye gather in cold regions of the Gulf of Alaska
where planktonic food is abundant. After two to three years,
Canadian sockeye migrate toward the coast of British Columbia
arriving between late June and August. Fraser River sockeye
enter either Johnstone Strait or Juan de Fuca Strait and
arrive at the mouth of the Fraser River in early July through
to early September. Fish entering the Fraser River mouth
arrive in early summer in time to reach the upper Fraser
watershed. Late arriving sockeye linger for three to six weeks
near the mouth of the Fraser River before entering the river
to spawn in the mid Fraser watershed. The spawners migrating
to the lower Fraser River tributaries takes only a few days
compared to several weeks for fish returning to the upper
Fraser River watershed. They arrive at spawning grounds in the
lower Fraser River Valley in November. Department of Fisheries
and Oceans estimated that 20-30% of the sockeye run escape the
combined commercial and native fisheries.
Sockeye Spawning Behaviour
The typical life cycle of a Fraser River sockeye salmon is
completed in four years. Adult sockeye return to river
tributaries connected to lakes to spawn between late summer
and autumn. Females dig nests or redds in the gravel bottom
where they deposit fertilized eggs where they leave them to
incubate for several months. Eggs hatch into young fish in the
winter months to emerge in spring as fry. The fry move to a
lake to feed and grow for a year before they migrate into the
Fraser River.
The carcasses of the adults provide an important source of
nutrients to otherwise nutrient poor lakes (Larkin and Slaney
1996). The decaying carcasses deposited into the lake are
later recycled into their offspring through the lake plankton
they eat. The loss of sockeye to lakes might deprive the
ecosystem of an important source of nutrients. Larkin and
Slaney (1996) estimated that nutrients increased in enhanced
streams (i.e. streams with hatchery fish, sockeye lake
fertilization, spawning channel creation, and other methods)
but declined in unmodified streams. The combined historical
effect of logging in spawning rivers and streams, overfishing
of adult fish by commercial and recreational fishers, and
reduced survival at sea during shifts in ocean productivity
might spell long term trouble for many small streams that have
become starved of nutrients when the salmon did not return
(Larkin and Slaney 1996).
References
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.
Larkin, G.A., and P.A. Slaney. 1996. Trends in marine-derived
nutrient sources to South Coastal British Columbia Streams:
Impending
implications to Salmonid production. Province of British
Columbia, Ministry of Environment, Lands and Parks, and
Ministry of Forests.
Watershed Restoration Management Report No. 3:56p, Victoria.
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.
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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