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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.
Migratory fish are classified according to the following scheme:
- diadromous fish travel between salt and fresh water
- anadromous fish live in the sea mostly, breed in fresh water
- catadromous fish live in fresh water, breed in the sea
- amphidromous fish move between fresh and salt water during some part of life cycle, but not for breeding
- potamodromous fish migrate within fresh water only
- oceanodromous fish migrate within salt water only
The best-known anadromous fish are salmon, which hatch in small freshwater streams, go down to the sea and live there for several years, then return to the same streams where they were hatched, spawn, and die shortly thereafter. Salmon are capable of going hundreds of kilometers upriver, and humans must install fish ladders in dams to enable the salmon to get past.
The most remarkable catadromous fish 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 reproduction).
Vertical migration is a common daily behavior; many marine types move to the surface at night to feed, then return to the depths.
Freshwater fish migrations are usually shorter, typically from lake to stream or vice versa, for spawning purposes.
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