Different types of the fishes migrate, on the basis of their requirement, ranging from daily to annually, and the distance ranging from some meters to thousands of kilometres. They migrate usually in the search of food or to reproduce on the basis of their requirement but in some of the cases, the reasons are unclear.
Here migration means the movement of the fish from one place to another on a larger scale and duration than those arising during normal activities. The particular types of migration that can be seen are anadromous migration (here adult fish live in the sea and they migrate into freshwater to spawn), and catadromous migration (in this adult fish live in freshwater and migrate into saltwater to spawn).
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Marine forage is the fish, which makes larger migrations for their spawning, feeding and nursery grounds. The movement of these species is directly related to ocean currents and the availability of food in the different areas at different stages of the year. The migration of fish is directly connected to the fact that the fish cannot identify their own offspring and moving this way prevents cannibalism (consuming another individual of the same species as food). Some of the species have been described by the UNCLOS (United Nations Convention on the Law of the Sea) as highly migratory species. These are large pelagic fish that move in and out of the exclusive economic zones of different nations, and these are covered differently in the treaty from other fish.
Classification of the Migratory Fishes
Depending on the various aspects of fish life, scientists have developed some empirical relation
for fish migration, particularly in two terms, that has been in wide usage for a long duration:
Anadromous Definition - It is the fish that migrate from the sea up into freshwater to spawn, such as salmon, striped bass, and the sea lamprey.
Catadromous Definition - It is the fish species that migrate from fresh water down into the sea to spawn, such as eels.
Diadromous, Amphidromous, Potamodromous, Oceanodromous - In a 1949 journal article, George S. Myers coined the inclusive term diadromous to refer to all fish that migrate between the sea and freshwater. Like the two well-known terms. Diadromous proved to be a useful word, but terms proposed by Myers for other types of diadromous fish did not catch on, which includes amphidromous (fish that migrate from freshwater to the seas, or vice versa, but not for the purpose of breeding), potamodromous (fish whose migrations occur wholly within freshwater), and oceanodromous (fish that live and migrate wholly in the sea).
All these classifications are made on the basis of fish and their principle, so they are applicable to only aquatic organisms.
Catadromous fish species spend most of their lives in freshwater, but they migrate to the sea for the purpose of breeding. The best examples of catadromous fish are eels of the genus Anguilla, numbering 16 species. The best-known species among them is the North American eel (A. rostrata) and the European eel (A. anguilla).
European and North American eels breed in the warm, salty waters of the Atlantic, at depths of 400 to 700 feet (approximately 1,300 to 2,300 m), at an altitude of 26 ° N longitude 55 ° W called the Sargasso Sea.
Pelagic eggs that grow into leptocephali - transparent, leaf-shaped forms with small heads - are carried by the Gulf Stream to the shallow water on the continental shelves. When it is about two and a half years old and about eight inches [8 inches] long (slightly more than three inches), a transformation takes place. The leptocephali are transformed into something called elvers, which are seated, coloured, and in cylindrical form. They reached the shoreline as glass eels and began to swim upstream in the freshwater rivers. Their migration upstream is spectacular, as young fish gather in the millions, forming a dense weight several miles long. In freshwater the eels grow to full size, becoming yellow eels. They live like this for 10 to 15 years before turning into eels of silver, with enlarged eyes; they swam to the bottom of the sea, returned to the breeding grounds (Sargasso Sea), and then died. The migration of these eels is not fully understood, especially their return to the Sea of Sargasso. European and North American eels may be similar.
Example of the Catadromous Species
The catadromous migration is remarkable to the freshwater eels, some of the examples are the American eel and the European eel. They migrate larger distances from freshwater rivers to spawn in the sargasso sea, and whose subsequent larvae can drift in currents for months and even before coming back to the natural rivers and streams as glass eels (elvers).
Interesting Facts About Migrating Fish
Marine fish are also called diadromous, meaning they spend part of their life in freshwater and partly in freshwater. Diadromous fish or:
Anadromous, spends most of their adult life on the ocean, but returns to freshwater to reproduce, or
Catadromous, spends most of their adult lives in freshwater but returns to the sea to reproduce.
Herring, herring is actually two different species of fish: alewives (Alosa pseudoharengus) and blueback herring (Alosa aestivalis).
Measured by rapids, herring does not jump above them like salmon. They swim very fast and with short bursts.
Alewives prefer to lay eggs overnight in slow-moving water and blueback herring prefer to breed on rocks during the day and in fast-moving waters.
During the 1800s people ate most of the harvested alewives because they were well preserved in salt or when smoking. As the refrigerator expanded in the 20th century, demand for alewives for human consumption declined and other fish species were introduced.
Atlantic sturgeon (Acipenser oxyrinchus) can grow up to 14 feet [14 m] in length, weigh 500 pounds [800 kg], and live up to 60 feet [60 m].
Atlantic sturgeon is cartilaginous, meaning that its bones are mainly cartilage, not bone.
The mouths of the Atlantic sturgeon occur. They can be excluded because their jaws are not really connected to any bone structure in their poles. This allows them to extract the spoils found at or below. Sturgeons also have large muscular belts that are strong enough to crush and separate digestive food. It helps because they have no teeth!
Unlike Pacific salmon that breeds once and then dies, Atlantic salmon (Psalm salar) is iteroparous, meaning it does not die after giving birth and therefore can give birth more than once.
Depending on the size of the female, the Atlantic salmon produces about 2,500 to 7,000 eggs. That is 600-800 eggs per kilogram of body weight! Why does it depend on female sizes? Usually, older females produce more eggs.
In Norse mythology, Loki was the god of deception who tricked Hodr into killing the beloved Baldur. To escape the wrath of other Norse gods, Loki was transformed into a salmon. Thor interrupted Loki's escape by grabbing her by the tail. Thor's catch was so strong it formed a salmon's caudal peduncle, a small part of the fish's body that the tail attaches to the body.
Archaeological evidence seems to confirm the myths - that salmon has been revered for tens of thousands of years. The French Abri du Poisson, or "Fish Rock Shelter'', is adorned with a famous prehistoric 25,000-year-old Atlantic salmon painting. It is one of the oldest known fish representations, the only known fish statue of the Paleolithic period, and the shelter is a UNESCO World Heritage site.