Migration is one of the innate behaviors of animals. Some animals move from their permanent habitat to temporary habitat for a certain period of time to escape various biological needs or environmental adversities and return to permanent habitat at the end of time. This particular phenomenon is called migration.
Different scientists have defined migration in different ways. Some of the notable ones are:
According to scientist Chon (1935), “migration is the movement of animals from one place to another at regular intervals.”
According to scientist Heapel (1931), “migration is a movement process that attracts migratory animals to return to their permanent habitat from their temporary habitat or destination.
In the words of scientist Woodbury (1931), “migration is the movement of an animal into a permanent or temporary habitat at a particular time in the environment or at a particular moment in the life cycle.”
Like other animals, several species of fish complete the migration. Migratory fish are successfully adapted to the environment through migration and various factors play an effective role in performing migration.
Environmental condition varies from place to place due to seasonal changes. As a result, some places become uninhabitable at certain times for fish. When conditions are threatened for survival of fish, they move away from the hostile environment to a convenient place. Migratory behavior has evolved due to responses to various environmental pressures (ecological pressures).
Migration originates from the Latin word ‘migrare’ which literally means to travel and the technical meaning is migration. In zoology, however, ‘migration’ does not mean aiming or traveling aimlessly. Rather, migration is the movement of an animal from the feeding ground to the breeding ground at intervals of time (seasonally) for searching of fish food, breeding, finding a suitable environment or safe shelter. It is possible to avoid intra-specific competition for food, shelter and other necessities through migration.
Some Notable Migratory Fishes
|English Name of Fish||Scientific Name of Fish|
|Pacific Salmon||Oncorhynchus tshawytscha|
|Atlantic Salmon||Salmo salar|
|European Eel||Anguilla rostrata|
|American Eel||Anguilla vulgaris|
|Sword fish||Xiphias gladus|
|Cod fish||Gadus morhua|
|Milk fish||Chanos chanos|
Types of Migration
There are different types of migration among fish. These are mentioned below:
A. Migration for nutrition, breeding and favorable environment: The migration of fish for biological needs include nutrition and breeding. This type of migration includes:
1. Alimental migration: Fish take part in such activities for food intake and nutrition.
2. Gametic migration: Fish make such migration for the purpose of growth and development of the gonads as well as to acquire reproductive capacity, to perform successful reproduction.
3. Climatic migration: Fish completes such migration in search of a favorable environment to get rid of adverse surroundings.
4. Osmoregulatory migration: In order to maintain the balance of salt and water in the body, fish perform this type of migration.
B. According to the migration cycle, migration can be further divided into three types, namely:
1. Spawning Migration: Different species of fish move from their feeding ground to spawning ground. This migration occurs for accelerating egg production and reproduction and embryonic development in a favorable environment. This accelerates the emergence and development of eggs and larvae. Moreover, the child’s condition is also protected from the predators.
Salmon and Cyclostomes (Lampreys) are accustomed to collecting food at sea. But while laying eggs, they enter the river. Eel fish, on the other hand, collect food in the river but run to the sea to lay their eggs.
2. Feeding Migration: After performing the breeding function, the fish returns to its permanent habitat or migration through such activities to collect and consume food. In this case, the movement of fish from the spawning ground to the feeding ground occurs.
3. Wintering Migration: In this case, fish migrates from the place of feeding or laying eggs to the over-wintering ground. Through such migrations, fish move to favorable environments far from food intake and breeding grounds to survive with environmental adversities.
C. Based on water current: The migration of fish is divided into 2 types based on water current. These are:
1. Denatant migration: This type of migration is done in favor of water flow.
2. Contranant migration: This type of migration occurs against or against the flow of water.
D. Myers (1949) divided fish migration into the following three categories overall:
The migratory fish are accustomed to travel between the sea and freshwater. This type of migration can be of three types, namely:
i) Anadrormous: Diadromous fish that spend most of their times in the ocean, but only migrate to freshwater to spawn during the breeding season. Salmon, Hilsa, Sea-lampray, etc. migrate long distance from the sea to the river(freshwater) to lay their eggs. After laying eggs, all these fish go back to their original habitat (sea).
For example, anadromous migration is observed in Salmo, Hilsa, Acipenser, etc. In winter, both males and females leave the feeding ground of the sea and start migration to the freshwater springs and reach their destination. It then stops eating and changes color to light brown. Before laying eggs, they make nests in small holes like bowls in the breeding ground. In that house, the eggs hatch and the fry come out. It grows by taking food before returning to the sea.
ii) Catadromous: This is the exact opposite of anadromous migration. This type of fish spend most of their times in freshwater. But they return to the sea for a short time to breed. The freshwater Anguilla travels thousands of miles from the river to the sea, and the young larvae begin to swim toward the freshwater bodies. Finally they come to the river and spend his life. For example, Anguilla rostrata (European eel) is a typical example of a catadromas migration. There are four stages in the life history of this fish such as:
1. Yellow feeding phase living in the river,
2. Reproductive stage of silver color,
3. Floating (pelagic) larval stage and
4. Elver phase.
At the beginning of autumn, their yellow color changes and they become like silver. At this time they stop eating. Their eyes are very clear. The chin is sharpened, the lips are thinned and maturity of gonads occur. Then they travel a long distance (thousands of kilometers) in the sea. After reaching the breeding ground in the sea and laying eggs in deep water, they die. The eggs hatch into small transparent leaf-like larvae called Leptocephalia. The length of these larvae is not more than 6 mm. Their teeth are as sharp as needles. The larvae then gradually turn into tubular elver stages. Elver start migration towards the river after about three years and reach its original habitat.
iii) Amphidromous: Some diadromous fish migrate from freshwater to sea or from sea to freshwater. But their migration does not take place for the purpose of reproduction, but migration occurs for other reasons in other stages of the life cycle. Myers mentions that some Gobies fall into this category.
This type of migration is limited to freshwater boundaries. Carp and trout migrate long distances in large rivers to the spawning area from the feeding area and leave the eggs there and return to the feeding area.
Many marine fish such as Herrings (Clupea), Mackerels (Scomber), Tunnas (Thunnus) move from their feeding grounds to the spawning area to lay eggs and return to the feeding grounds. They never come to fresh water, they are confined to the sea.
Techniques of Migration
The migration of fish is completed in different processes. These are:
1. By Drifting: The migration of fish by water current is called drift. If the motion of water is controlled in the same direction, then start of drifting occurs through directional movement. Through drifting, the fish fry go to the nursery ground.
2. Random locomotory movements: If many fish can be released at any point in the same environment, they will spread evenly around.
3. Orientation swimming movements: The fish swims in a particular direction or in a certain direction such as:
a) Stimulus can be towards the source or vice versa.
b) The stimulus can be at an angular distance from the source and any one imaginary line between them.
Regulatory Factors of Fish Migration
Migration is affected by physico-chemical and biotic factors. Physical factors include water depth, pressure, temperature, light intensity, currents, etc. Biotic factors, on the other hand, include sexual maturity, blood pressure, diet, memory, endocrine gland, consumers and predators.
Numerous fish migrate to the feeding ground in search of food. In summer, due to the increase in the temperature of the surface of the sea, the salmon is stimulated in the sea. As a result, when water temperatures in the river continue to rise, then fish migrate against the water current.
The salinity of water is another important issue. Most freshwater fish cannot tolerate major changes in salinity. Therefore, their migration is confined to a limited range. So they migrate in its fresh water. On the contrary, fish like Salmo, Anguilla, Hilsa etc. are accustomed to migrate to any level of salinity.
The factors that directly or indirectly contribute to the control of fish migration are:
(A) Physical factors: The physical factors that influence the movement of fish include water temperature, light, and pressure, nature of the bottom of the water bodies, tides and currents. The depth of the water bodies, the turbidity and the penetration of light into the water also affect the movement of the fish.
(B) Chemical factors: Fish migration is affected due to excess or deficiency of various chemical substances dissolved in water. Chemical factors include water salinity, pH, various contaminants and gaseous substances (O2, CO2) dissolved in water. In addition, the taste and smell of the water also affects the movement to some extent.
(C) Biological factors: Fish can adapt to the aquatic environment. Fish migrate in the interest of adaptation. Notable biological factors that stimulate fish migration are blood pressure, sexual maturity, competition, search for food, secretion of endocrine glands and appetite. These biological factors interact with physical and chemical factors to stimulate the migration of fish.
Advantages of Migration
If there is not enough food in the spawning ground for the mature and immature members of a large population, then if they migrate to different spawning grounds or feeding grounds, as a result, adequate food supply for the population can be ensured. Migration plays a significant role in increasing the population or achieving abundance. It may also be mentioned that matured animals come to lay eggs in the environment where their young stage or larval stage had passed. So they are able to use all the elements there to get back to the previous environment.
Importance of Fish Migration
Migration has special importance for successful adaptation to water. The importance of fish migration is mentioned bellow:
- Abundance of fish population is maintained.
- Maintaining the continuity of reproduction.
- Favorable environment is ensured.
- Ensuring embryonic development with egg protection.
- It helps to find suitable environment for breeding.
- Ensuring the safety of fish from enemies or predators.
- Ensuring adequate food supply.
- The tendency for making shoaling and schooling among the fish occurs.