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Many species of Salmon are anadromous and migrate long distances up rivers and streams to spawn.

Many types of fish undertake migrations on a regular basis, on time scales ranging from daily to annual, and with distances ranging from a few meters to thousands of kilometers. The purpose usually relates to either feeding or breeding; in some cases the reason for migration is still unknown.

Classification

• potamodromous fish migrate within fresh water only (Greek: Potamos is river).
• oceanodromous fish migrate within salt water only (Greek: 'Oceanos' is ocean).
• diadromous fish travel between salt and fresh water (Greek: 'Dia' is between).

• anadromous fish live in the sea mostly, breed in fresh water (Greek: 'Ana' is up; The noun is "anadromy")
• catadromous fish live in fresh water, breed in the sea (Greek: 'Cata' is down)
• amphidromous fish move between fresh and salt water during some part of life cycle, but not for breeding (Greek: 'Amphi' is both)

Examples

One of the best-known anadromous fish are the five species of Pacific salmon, which are Chinook (King), Coho (Silver), Sockeye (Red), Chum (Dog) and Pink (Humpback). They hatch in small freshwater streams, go down to the sea and live there for two to six years, most staying in the ocean for around four years, then return to the same streams where they were hatched, spawn, and die shortly thereafter to feed fauna and replenish the earth. Salmon are capable of going hundreds of kilometers upriver, and humans must install fish ladders in dams to enable the salmon to get past. Other examples of anadromous fishes are sea trout, three-spined stickleback, and shad.

Life cycle of anadromous fish. From a U.S. Federal Government pamphlet. (Click image to enlarge.)

The most remarkable catadromous fishes are freshwater eels of genus Anguilla, whose larvae drift on the open ocean, sometimes for months or years, before travelling thousands of kilometres back to their original streams (see eel life history).

An example of an amphidromous species included the Bull sharks living in Lake Nicaragua of Central America and in the Zambezi river of Africa. Both the aforementioned lake and river are fresh water, yet Bull sharks (a known man eater, and very aggressive shark) will live and feed in these waters as well as migrating to and from the sea (the Indian Ocean in the case of the Zambezi Bull shark and the Atlantic Ocean in the case of the Lake Nicaragua Bull shark).

Diel vertical migration is a common behavior; many marine species move to the surface at night to feed, then return to the depths during daytime.

A number of large marine fishes, such as the tuna, migrate north and south annually, following temperature variations in the ocean. These are of great importance to fisheries.

Freshwater fish migrations are usually shorter, typically from lake to stream or vice versa, for spawning purposes.

See also

• DSSAM Model
• Fish ladder
• Hydrology transport model
• Ocean Tracking Network
• Salinity
• Stenohaline
• Euryhaline

References

• Carl E. Bond, Biology of Fishes, 2nd ed. (Saunders, 1996), pp. 599-605.
• Michael Blumm, J.D., Sacrificing the Salmon: A Legal and Policy History of the Decline of Columbia Basin Salmon, Bookworld Publications, 2002.

Wikipedia content modification information:

• This page was last modified on 4 October 2008, at 13:29.

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