In this article we will discuss about the migration process in fishes with the help of a suitable diagram.
Generally fishes restrict their movements within small territorial limits and do not go out of their home ranges. However, a few species travel long distances moving from place to place in search of food or for breeding. This movement of a large number of fishes for the purpose of feeding or spawning, is known as migration. It may take place in vertical direction, as from the deeper to the surface water, or it may be in horizontal direction, either upstream or downstream.
Thompson (1952) defines a true migration as seasonal movement that implies return to the starting point. Harden Jones (1968) has defined migration as a class of movement which impels migrants to return to the region from which they have migrated. This definition envisages a return to the original place. Baker (1978) has given a more general definition, and defines migration ‘as the act of moving from one spatial unit to another’.
There is no restriction of return journey. More recently, Dingle (1980) has defined the term migration as a ‘specialised behaviour especially evolved for displacement of the individual in space’. This definition implies evolutionary changes leading to specific migratory patterns. Dingle has opined that accidental or unintentional movements cannot be included in the definition of migration.
There are two opposing trends of migration in fishes:
1. Catadromous migration, i.e., movement from freshwater to salt water (sea) for spawning. The most famous example of catadromous fish is the freshwater eel, Anguilla.
2. Anadromous migration meaning the reverse movement of the fishes from the salt water (sea) to the freshwater (river system) for spawning. Examples of anadromous fishes are salmon, shad, striped bass, sturgeon, Alosa, Hilsa, and some trout. Completely free movement between fresh and marine water without the purpose of breeding is called amphidromous migration exhibited by fishes like Megalopa, Chanos, etc. (Fig. 17.10).
The best example of catadromous migration is furnished by two common species of eels, Anguilla rostrata of European freshwater rivers and Anguilla vulgaris of America. With the advent of autumn, their colour changes from yellow to metalic silver. Feeding stops with the shrinking of their digestive tract. Eyes become larger, snout becomes sharper with thinner lips and the gonads are fully mature.
The silvery eels then enter the sea and migrate about 4,500 kilometres westward from Europe or eastward from America. Reaching their breeding place in the Surgasso sea of Bermuda, the adults die immediately after spawning in deep waters.
The eggs hatch into little transparent, leaf-like flattened pelagic larvae, called leptocephalia, less than 6 mm long. They have sharp needle-like teeth for feeding. During their long homeward journey, they grow into elvers or glass eels about 8 cm long, with cylindrical bodies.
On reaching land, the males remain behind in brackish waters near coasts, while the females ascend freshwater streams and rivers. The elvers feed and grow to become yellow eels in some years. How the elvers without parents are able to find their way across the sea towards homes of their parents remains an unsolved mystery. Until their real identity was discovered, the leptocephali larvae were called glass fishes and placed in the genus Leptocephalus. The biggest larva of some unknown species was 184 cm long which was captured in 1930 by Dana Expedition west of the Cape of Good Hope (Africa).
There is a single species of Atlantic Salmon (Salmo salar) and five species of Pacific salmon (Oncorhynchus). They furnish the best example of anadromous migration. In winter, both the sexes have their feeding grounds at sea to ascend the freshwater mountain streams, reaching the identical spot where they originally grew some years before. They stop feeding, change to a dull reddish brown from silver, and excavate shallow saucer-like pits in bottom gravel.
After spawning the adults die, but some of the Atlantic species (Salmo) may survive, return to sea and spawn for a second or third time in life. After hatching, the larvae fish feed and grow out for some time in the streams before going out to sea. The young salmons grow faster, in the ocean because of abundant food there.
Experimental evidence shows that some strong olfactory sense of salmon determines its homing into original birth place, for different streams have different odours. Many marine fishes migrate to the river from sea for spawning. This anadromous migration is observed in Acipenser, Alosa, Salmo, Hilsa. These fishes usually produce huge numbers of eggs to compensate the risk to which eggs are subjected.
Causes of Migration:
Several authors have given various reasons as to why fish migrate.
According to Northcote (1978) this is:
(i) To optimise feeding,
(ii) To avoid unfavourable conditions,
(iii) To enhance reproductive success, and
(iv) Possibly to promote colonisation.
Specific fish migrations may not fulfill all these requirements, and might be for only one of the above reasons.
Migrations are brought about because reproductive habitats are not likely to be the best areas for other activities such as feeding and over wintering. Unfavourable chemical, physiological and biological conditions may occur in any environment at different times due to seasonal changes, and migration is a means to escape such conditions.
The timing of migration pattern appears to have evolved in such a way that the young fish arrive at feeding areas when the environmental conditions are optical for food production. The strategy of the fish is to exploit rich food source, to enhance food intake which is necessary for increased growth rate, fecundity and survival.
Advantages of Migrations:
According to Nikolsky, migration is an adaptation towards abundance. The spawning or nursery grounds may not have enough food to maintain both the mature and immature members of a large population. Hence, it would be an advantage to have separate spawning, nursery and feeding grounds. The fact that many commercial species are migratory, supports the view that migration is an adaptation towards abundance.
Further, there appears to be some advantage to a species whose adults return to spawn in an area where the environmental conditions were similar to those under which they themselves survived when young. A return to the parent spawning ground provides a means by which these favourable conditions may be exploited. Thus, a better egg and larvae survival would lead to a greater number of spawners on a particular ground.
Fish are often dispersed widely over large areas while feeding, but congregate at specific spawning grounds to enhance reproductive success. Precise timing of spawning migration ensures simultaneous arrival of sufficient members of both the sexes at one place to ensure reproductive success of the group.