A regular journey made by a particular species of fish, on an annual or a lifetime basis, usually associated with propagation patterns but also associated with the seasonal availability of food. Most migrations are mass movements and involve travel over a particular route, usually at the same time annually. Migration is not to be confused with the relocation of fish because of pollution, sedimentation, storms, or the temporary relocation of food sources. Anglers, for example, often refer to fish as making migrations from deep water to shallow water to feed, an action that is really a localized movement. The periodic movement of fish in a water body is not necessarily a migration, although the movement of a fish species to and from breeding grounds (such as walleye in the spring moving from a spawning river back to the main lake) is a migration.

Migrations occur in various species and in both freshwater and saltwater. All freshwater fish that move from lake or river environs to a tributary in order to spawn will migrate to and from the spawning grounds at or around the same time each year. All anadromous and catadromous fish undertake spawning migrations, the former from saltwater to freshwater and the latter from freshwater to saltwater, also around the same time annually. Pelagic ocean species migrate from winter to summer grounds, both for spawning and for food procurement, also around the same time annually.

Migrations occur in north-south, south-north, offshore-inshore, and inshore-offshore patterns, and in combinations of these (some sea organisms migrate up and down in the water column). Some fish migrations cover great distances, even thousands of miles, and some are extremely short, perhaps just a short distance up a river.

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