Many types of fish migrate on a regular basis, on time scales ranging from daily to annual, and over distances ranging from a few meters to thousands of kilometers. Fish usually migrate because of diet or reproductive needs; although in some cases reason for migration remains unknown.
Types of Migration
Potamodromous (Greek: Potatoes, river, and dooms, running) fish exemplified by carps, trouts migrate within fresh water only. Oceanodromous (Oceanus, ocean) fish migrate within salt water only. Diadromous (Dia, between) fish exemplified by herring, mackerel and tuna travel between salt and fresh water Anatropous (Ana, up) fish live in the ocean mostly, and breed in fresh water. Catadromous (Cata, down) fish live in fresh water, and breed in the ocean. Amphidromous (Amphi, both) fish move between fresh and salt water during their life cycle, but not to breed. The best known anadromous fish are five species of salmon. Salmon hatch in small freshwater streams, migrate to the sea to mature, live there for two to six years and on maturity, return to same streams, where they were hatched to spawn. Salmon travel hundreds of kilometres upriver Other examples of anadromous fishes are sea trout, three-spined stickleback, and shad. 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. An amphidromous species is bull shark, which lives in Lake Nicaragua of Central America and Zambezi river of Africa. Both these habitats are fresh water, yet bull sharks migrate to and from the ocean. Diel vertical migration is a common behaviour. Many marine species move to surface at night to feed, and then return to depths during daytime. Tuna migrate north and south annually, following temperature variations in the ocean. Gobies undertake such migration. Freshwater fish migrations are usually shorter, typically from lake to stream or vice versa, for spawning purposes.