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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: 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). Diel vertical migration is a common behavior; many marine types 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.
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