Fish migration

A class of movement which forces the fishes to return back in that place from where they have been migrated is called fish migration. The majority of fishes are restricted with small territorial limits. So, they do not go out from their home ranges. However, some fishes show seasonal migration in search of food, shelter, and better breeding place.

Causes of fish migration

• In search of food (Alimental migration).
• For reproduction or spawning (Gametic migration).
• For protection (Protective migration).
• For better climate (Climatic migration).
• For osmoregulation (Osmoregulatory migration).

Factors affecting fish migration

• Physical factor: Light, temperature, turbidity, depth of water etc.
• Chemical factor: Salinity, pH value etc.
• Biological factor: Predators, competitors, shortage of food, hormonal secretion etc.

Types of migratory fish

Diadromous fishes

They are true migratory fishes which migrate in between the sea and fresh water. They are divided into three subtypes. They are:

(A) Anadromous

They spent the major part of their life in sea water but migrate towards the fresh water for breeding. After laying eggs, they return back to sea water again. Eg; Salmon, Milsa, Lamprey etc.

(B) Catadromous

They spend the major part of their life in freshwater but migrate towards sea water for breeding. After laying eggs they return back to fresh water again. Eg; European eel, American eel, Bam eel etc.

(C) Amphidromous

They migrate from seawater to freshwater to seawater at any time for any reason but not for breeding purpose. Eg; Gobies fishes.

Potamodromous fishes

Their migration is confined within freshwater only. Eg; Carpitrout etc.

Oceanodromous fishes

There is confined within seawater only. Eg; Clupea, Scomber, Thunnus etc.

(a) Alimental or feeding migration: It is the migration with reference to the search of suitable food materials during winter and spawning site.

Types of fish migration

Latitudinal migration:

Fishes migrate towards north during spring and towards south during autumn. Eg; swordfish.

Vertical migration

Fishes show up and down migration in the water body to search food, better climatic condition and to escape from predators. Eg; Mackerel migrate towards the surface of the water to feed planktons.

Spawning migration

Migration from feeding to breeding place. Eg: Hilsa

Feeding migration

Migration from breeding to feeding place. Eg: Eel.

Overwintering migration

Overwintering is the inactive stage of life cycle of fishes in which they stop feeding less consumption of oxygen, low activities etc to search the proper place for that period they migrate. Eg; flatfishes.

Shoreward migration

Temporary movement of fishes on land. Eg; common eel can travel in moist land found in between two ponds.

Advantages of fish migration

• Fish get more food, better climatic condition and breeding place.
• Fish gets better adapting in new places.
• There will be wide distribution.
• Less competition.

Disadvantages of fish migration

• Chances of loss of life due to the shortage of food for predators of higher forms, earthquake, tsunami, cyclone etc.
• Mant migratory birds are exposed to innumerable risks during traveling and die in tremendous numbers.
• They are killed by sudden changes in temperature in weather such as heavy rainfall or snowfall or stormy wind.
• They are killed by hawks nd man during migration.