All species of Pacific Salmon die shortly after spawning. This one was photographed at a spawning site along Eagle Creek in Oregon.

Salmon is the common name for several species of fish of the family Salmonidae. Several other fish in the family are called trout. Salmon live in both the Atlantic and Pacific Oceans, as well as the Great Lakes.

Salmons are born in fresh water, migrate to the ocean, then return to fresh water to reproduce. Tracking studies have shown that the fish return to the exact spot where they were born to spawn, but the nature of how this memory works has long been debated.

The population of wild salmons declined markedly in recent decades, especially north Atlantic populations which spawn in the waters of western Europe and eastern Canada, and wild salmon in the Snake and Columbia River system in northwestern United States. The decline is attributed to the following factors:

• Disease transfer from open net cage salmon farming, especially sea lice.
• Overfishing
• Ocean and river warming which can delay spawning
• Loss of suitable freshwater habitat
• The construction of dams, weirs, barriers and other "flood prevention" measures, which bring severe adverse impacts to river habitat and on the accessibility of those habitats to salmon.
• Other environmental factors such as light intensity, water flow, or change in temperature dramatically affects salmon during their migration season.
• Loss of invertebrate diversity and population density in rivers because of modern farming methods and various sources of pollution, thus reducing food availability.
• Reduction in freshwater base flow in rivers and disruption of seasonal flows, because of diversions and extractions, hydroelectric power generation, irrigation schemes, barge transportation, and slackwater reservoirs, which inhibit normal migratory processes and increase predation for salmon.

Salmon aquaculture is the major economic contributor to the world production of farmed fin-fish, representing over $1 billion US annually. Other commonly cultured fish species include: tilapia, catfish, sea bass, carp, bream, and trout. Salmon farming is very big in Chile, Norway, Scotland, Canada and the Faroe Islands, and is the source for most salmon consumed in America and Europe. Atlantic salmon are also, in very small volumes, farmed in Russia and the island of Tasmania, Australia.

Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)

For more information (background, pictures, experiments and references): Salmon