Migration is the phenomenon of movement of large populations of animals from one place to another for feeding or reproduction or to escape extreme weather conditions. When high populations of fishes come together to move socially it is called shoaling. But sometimes migrating fishes exhibit high degree of coordination in their movements and carry out synchronized maneuvers to produce different types of shapes. This is called schooling, as seen in tunas and sardines.
Feeding or alimental migration: This kind of migration takes place in fishes for feeding. In high populations fishes use up the food resources quickly in a given particular area and therefore they must migrate constantly in search of new food resources for the population. This type of migration is called as feeding migration or alimental migration. The best examples of feeding migration are Salmons, cods and sword fish. These fishes constantly migrate from one place to another in the sea in search of food source.
Spawning migration: Spawning migration takes place in breeding season in fishes which have spawning places far away from feeding places. The best examples of spawning migration are eels and salmons. Also a large number of riverine fishes spawn in tributaries of river in hills.
Juvenile migration: This kind of migration involves larval stages of fishes which hatch in spawning grounds and must migrate long distances in order to reach the feeding habitats of their parents.
Recruitment migration: This kind of migration takes place when large number of larvae moves from nursery habitat to the habitat of adults which may sometimes be distinctly different from juvenile migration. Adult eels live in rivers of Europe and America but their larval stages live and grown in sea. They travel for around two years to migrate and reach rivers.
Seasonal migration: This kind of migration takes place in fishes that inhabit arctic areas where in summer climate is favorable and food abundant but as winter approaches temperatures fall below zero and food becomes scarce. So these fishes must migrate towards subtropical and tropical areas to escape extremes of weather and food scarce conditions.
Generally fishes live in two different types of aquatic habitats namely, freshwater habitat and marine habitat. These different habitats pose different osmotic problems due to which it is difficult to migrate from one type of habitat to another. Myers classified the fishes into the following three types depending from and to which type of water the fishes migrate,
Potamodromous migration
Potamodromous migration is the migration where fishes migrate from one freshwater habitat to another in search of food or for spawning. There are about 8,000 known species of fishes that migrate within lakes and rivers for food on daily basis as the availability of food differs from place to place and from season to season. Sometimes fishes also migrate to lay their eggs in places where oxygen concentration in water is more and where there is abundance of food for juveniles when they hatch from eggs.
Oceanodromous Migration
Oceanodromous migration is the migration of fishes from sea water to sea water. There are no barriers within the sea and fishes have learned their best to migrate in order to take advantage of favorable conditions. Thus there are about 12,000 marine species of fishes that regularly migrate within sea water. The best example of oceanodromous migration can be Herrings, sardines, mackerels, cods, roaches and tunas. They migrate in large numbers in search of food by way of shoaling.
Diadromous migration
Diadromous migration is the migration when fishes migrate from fresh water to sea or from sea to fresh water. About 120 species of fishes are capable of overcoming osmotic barriers and migrate in these two different types of habitats. This migration is of three types.
Catadromous migration: Generally his type of migration involves movement of large number of individuals from fresh water to sea water, for spawning. The best examples are as Eel fishes inhabiting European and North American rivers.
Both European eel (Anguilla vulgaris) and the American eel (Anguilla rostrata) migrate from the continental rivers to Sargasso Sea off Bermuda in south Atlantic for spawning. They cross Atlantic Ocean during the journey and cover a distance of around 5,600 km. The adult eels that inhabit rivers are about a meter long, yellow in colour and spend 8-15 years feeding and growing.
The following changes take place in their bodies before migration:
They migrate through the rivers and reach coastal areas of the sea where they are joined by the males and then together they swim in large numbers, reaching Sargasso Sea in about two months. They spawn and die. Each of the female lays around 20 million eggs which are fertilized by males.
According to the studies conducted by Johann Schmidt, these Eggs hatch into leaf-like, semitransparent, larvae with small head called Leptocephalus. Leptocephali of American eels take about 10 months to grow fully while Leptocephali of European eels take about 18 months to grow fully. Upon reaching coastal waters leptocephali metamorphose into next stage called Elver or Glass eel. Female elvers ascend to the rivers and metamorphose into yellow-coloured adults, while males stay back within the river mouth and await the females to return for spawning journey.
Anadromous migration: Anadromous migration is carried on by adult anadromous fishes which live and feed in ocean waters. As their spawning grounds lie in the tributaries of rivers they migrate. The best examples of anadromous migration are Salmons, sturgeons, Hilsa and lampreys to spawn in rivers.
Atlantic salmon (Salmo salar) migrates to the North American rivers for spawning while six species of Pacific salmon (Onchorhynchus) migrate to various other rivers of Asian countries.
Amphidromous migration: This kind of migration takes place from fresh water to sea or vice versa. The main example of fishes falling under this class of migration is Gobies.
It has been a mystery how fishes find their way in huge expanses of sea and reach their destinations lying thousands of kilometers away. It is believed that they orient by the positions of stars and moon within the night sky and sun in daytime to seek out the direction of swimming. However, it has been experimentally proven by A.S. Hasler that salmons are guided by the odor of their parent stream during return journey. Eels can also migrate to Sargasso Sea using similar odor maps.
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