Migration is the movement of large number of animals from one place to another for feeding, reproduction or to escape weather extremes. When large numbers of fishes come together and move socially it is called shoaling. But sometimes migrating fishes exhibit high degree of coordination in their movements and carry out synchronized manoeuvres to produce different types of shapes. This is called schooling, as seen in tunas and sardines.
Feeding or alimental migration takes place in fishes for feeding. In high populations fishes exhaust food resources in an area quickly and therefore must migrate constantly in search of new feeding resources. Salmons, cods and sword fish constantly migrate for food from one place to another in the sea.
Spawning migration takes place in breeding season in those fishes which have spawning grounds far away from feeding places. Migratory fishes such as eels and salmons and a large number of riverine fishes spawn in tributaries of river in hills and migrate in large number for laying eggs in these oxygen rich waters.
Juvenile 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 takes place when large number of larvae moves from nursery habitat to the habitat of adults which may sometimes be distinctly different. Adults of eels live in rivers in Europe and America but their larval stages live and grown in sea and migrate to reach rivers which may take one to two years.
Seasonal migration takes place in fishes that inhabit arctic areas where in summer climate is conducive and food abundant but as winter approaches temperatures fall below zero and food becomes scarce. Hence fishes must migrate towards subtropical and tropical areas to escape extremes of weather conditions.
TYPES OF MIGRATION IN FISHES
Fishes live in two different types of aquatic habitats, namely, freshwater and marine habitats, which pose different osmotic problems because of which it is difficult to migrate from one type of habitat to another. Nevertheless, some fishes do migrate.
POTAMODROMOUS MIGRATION
When fishes migrate from one freshwater habitat to another in search of food or for spawning, it is called potamodromous migration. There are about 8,000 known species that migrate within lakes and rivers, generally for food on daily basis as the availability of food differs from place to place and from season to season. Fishes also must 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
This migration is from sea water to sea water. There are no barriers within the sea and fishes have learned to migrate in order to take advantage of favourable conditions wherever they occur. Thus there are about 12,000 marine species that regularly migrate within sea water. Herrings, sardines, mackerels, cods, roaches and tunas migrate in large numbers in search of food by way of shoaling (migrating together socially but without much coordination) or schooling (swimming with high degree of coordination and synchronized manoeuvres).
DIADROMOUS MIGRATION
When fishes can migrate from fresh water to sea or from sea to fresh water, it is called diadromous migration. There are about 120 species of fishes that are capable of overcoming osmotic barriers and migrate in these two different types of habitats. This migration is of three types.
Catadromous migration
This type of migration involves movement of large number of individuals from fresh water to sea water, generally for spawning as happens in the case of eels (Anguilla) inhabiting European and North American rivers.
Both European eel (Anguilla anguilla or Anguilla vulgaris) and the American eel (Anguilla rostrata) migrate from the continental rivers to Sargasso Sea off Bermuda in south Atlantic for spawning, crossing Atlantic Ocean during the journey and covering a distance of about 5,600 km. The adult eels that inhabit rivers are about a metre long, yellow in colour and spend 8-15 years feeding and growing. Before migration the following changes take place in their bodies:
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 female lays about 20 million eggs which are soon fertilized by males.
First clue about life cycle of eels was given by two Italian scientists Grassi & Calandruccio in 1896. Details of migration and life cycle were later studied by Johann Schmidt (1905). Eggs hatch into leaf-like, semitransparent, larvae having small head called Leptocephalus. Leptocephali of American eels take about 10 months to fully grow while those of European eels take about 18 months. Upon reaching coastal waters leptocephali metamorphose into another larval stage called Elver or Glass eel. Female elvers ascend to the rivers and metamorphose into yellow-coloured adults, while males stay back in the river mouth and wait for the females to return for spawning journey.
Anadromous migration
Adults of anadromous fishes live and feed in ocean waters but their spawning grounds lie in the tributaries of rivers. Salmons, sturgeons, Hilsa and lampreys are some of the marine fishes that undertake anadromous migration 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 rivers of Asian countries.
Salmons living in sea are metallic silvery grey in colour but before migration they turn reddish-brown in colour. During fall, they enter rivers and swim energetically against water currents (contranatent), clearing all obstacles, including waterfalls and reach tributaries in hilly areas where they make a saucer-like pit in which female lays eggs and male releases smelt over them. Eggs take 2-3 months to hatch in the following spring, when the juvenile stage called Alvin emerges out but remains within the nest, obtaining its nourishment from the yolk sac attached to its belly. Alvin then transforms into Fry which feed on planktons. Fries are denatant (they swim along with water current) and feed and grow into fingerlings which take the shape of adult fish. They change into Smolt which congregate at the river mouth in large numbers and then enter sea water in to metamorphose into adult salmons.
Problem of navigation
How fishes find their way in huge expanses of sea and reach their destinations which lie thousands of kilometres away has been a mystery. It is believed that they orient by the positions of stars and moon in the night sky and sun in daytime to find the direction of swimming. However, it has been experimentally proven by A.S. Hasler that salmons are guided by the odour of their parent stream during return journey. Eels can also migrate to Sargasso Sea using similar odour maps but how leptocephali find their way back to the river mouths, crossing vast stretches of Atlantic Sea is a mystery.
Anadromous migration in lampreys
Adult lampreys are parasitic on other fishes and live in sea for 3-4 years and grow to become 30 cm long. For breeding, they stop feeding and migrate in rivers hundreds of miles upstream. Males make nest in sand and gravel in which female lays eggs and male fertilizes them. In about 3 weeks time, eggs hatch into 7 cm long, yellowish-brown ammocoete larva that lies buried in sand and feeds on detritus by filter feeding method. Larva lives in river for 3-7 years and grows from half centimetre to 17 centimetres long. Then these fully grown larvae start their downstream journey and enter the sea to metamorphose into adults.