Salmon run

thumb|right|A [[grizzly bear ambushing a jumping salmon during an annual salmon run]]

A salmon run is an annual fish migration event where many salmonid species, which are typically hatched in fresh water and live most of their adult life downstream in the ocean, swim back against the stream to the upper reaches of rivers to spawn on the gravel beds of small creeks. After spawning, most Atlantic salmon and all species of Pacific salmon die, and the salmon life cycle starts over again with the new generation of hatchlings.

Salmon are anadromous, spending their juvenile life in rivers or lakes, and then migrating out to sea where they spend adult lives and gain most of their body mass. When they reach sexual maturity, the adults return to the upstream rivers to reproduce. Usually they return with uncanny precision to the natal river where they were born, and even to the very spawning ground of their birth. It is thought that, when they are in the ocean, they use magnetoreception to locate the general position of their natal river, and once close to the river, that they use their sense of smell to home in on the river entrance and even their natal spawning ground.

Trout, which are sister species of salmon, also perform similar migrations, although they mostly move potamodromously between creeks and large freshwater lakes, except for some coastal/estuary subspecies such as steelhead and sea trout that migrate seasonally between salty/brackish and fresh water just like salmon do. There are also landlocked populations of some salmon species that have adapted to spend their entire life in freshwater like trout.

In Northwest America, salmons are keystone species, which means the ecological impact they have on other wildlife is greater than would be expected in relation to their biomass. Most salmon species migrate during the autumn (September through November), which coincides with the pre-winter activities of many hibernating animals. The annual salmon run can be a major feeding event for predators such as grizzly bears and bald eagles, as well as an important window period for sport fishermen.

The post-spawning death of salmon also has important ecological consequences, because the significant nutrients in their carcasses, rich in nitrogen, sulfur, carbon and phosphorus, are transferred from the ocean and released to inland aquatic ecosystems, terrestrial animals (such as bears) and the wetlands and riparian woodlands adjacent to the rivers. This has knock-on effects not only for the next generation of salmon, but to every wildlife species living in the riparian zones the salmon reach.

Anadromous salmon are Northern Hemisphere fish that spend their ocean phase in either the Atlantic Ocean or the Pacific Ocean. They do not thrive in warm water. There is only one species of salmon found in the Atlantic, commonly called the Atlantic salmon. These salmon run up rivers on both sides of the ocean. Seven different species of salmon inhabit the Pacific (see table), and these are collectively referred to as Pacific salmon. Five of these species run up rivers on both sides of the Pacific, but two species are found only on the Asian side.

There are various theories about how this happens. One theory is that there are geomagnetic and chemical cues which the salmon use to guide them back to their birthplace. The fish may be sensitive to the Earth's magnetic field, which could allow the fish to orient itself in the ocean, so it can navigate back to the estuary of its natal stream.

There is little evidence salmon use clues from the sun for navigation. Migrating salmon have been observed maintaining direction at nighttime and when it is cloudy. Likewise, electronically tagged salmon were observed to maintain direction even when swimming in water much too deep for sunlight to be of use.

Skilled predators, such as bears, bald eagles and fishermen can await the salmon during the run. Normally solitary animals, grizzly bears congregate by streams and rivers when the salmon spawn.

Black bears also fish the salmon. Black bears usually operate during the day, but when it comes to salmon they tend to fish at night. and these deteriorating salmon are sometimes colloquially called "zombie fish". This is because upstream freshwater bodies (especially creeks) typically do not have sufficient food available for the adult salmon diet, and they have used large amounts of energy swimming upriver, thus exhausting their own internal nutrient reserves. Spawning salmon also have programmed senescence, which is "characterized by immunosuppression and organ deterioration", making them more vulnerable to diseases.

In 2009, NOAA advised that continued runoff into North American rivers of three widely used pesticides containing neurotoxins would "jeopardize the continued existence" of endangered and threatened Pacific salmon.

Bristol Bay (Alaska)
Chilkat River (Alaska)
Columbia River (British Columbia, United States)
Copper River (Alaska)
Fraser River (British Columbia)
Kenai River (Alaska)
River Spey (Scotland)
River Tana (Norway, Finland)
River Tay (Scotland)
River Tweed (border of Scotland and England)
River Tyne (England)
Snake River (United States)
Yukon River (Alaska, Yukon, British Columbia)

References

Cited sources

External links

Putting a Price on Salmon True Slant, 9 July 2009.
Fish passage at dams Northwest Power and Conservation Council. Retrieved 17 December 2011.
Mystery Disease Found in Pacific Salmon Wired, 13 January 2011.
Pacific Salmon: Anadromous Lifestyles US National Park Service.
Study takes long-term, diversified view of salmon issues Mount Shasta News, 30 September 2009.

See also

References

Cited sources

Further reading

External links