Fish migration

Fish migration is the mass relocation of fish from one area or body of water to another, occurring on time scales from daily to annually and over distances from a few metres to thousands of kilometres. These movements are usually for better feeding or reproduction, though in some cases the reasons are unclear. Migrations involve schools of fish on a scale larger than normal daily activities and include anadromous (sea to fresh water to spawn) and catadromous (fresh water to sea to spawn) types. Salmon and striped bass are well-known anadromous species, while freshwater eels are catadromous, undertaking large migrations. The bull shark is a euryhaline species that can move freely between fresh and salt water. Many marine fish perform a diel vertical migration, rising to the surface at night to feed and descending to deeper waters by day. Some fish, such as tuna, move north and south seasonally following temperature gradients. The longest freshwater migration belongs to the dourada catfish, which travels up the Amazon River. Marine forage fish, like herring and capelin, often make large triangular migrations between spawning, feeding, and nursery grounds, movements linked to ocean currents and seasonal food availability. This migratory behavior may partly prevent cannibalism, as fish cannot distinguish their own offspring. Some large pelagic fish are classified as highly migratory species by the United Nations Convention on the Law of the Sea, moving in and out of different nations' exclusive economic zones. Freshwater migrations, often upriver to spawn, are increasingly disrupted by dam construction. Zoologists classify migrations further: potamodromous fish migrate wholly within fresh water, oceanodromous fish live and migrate entirely in the sea, and amphidromous fish move between fresh and salt water as larvae or juveniles, not for breeding. The fishing industry closely studies these migration patterns.