Lecture notes on fish migration

Migration is the of mass movement of animals from one place to another for food,shelter,mates and escaping from extreme environmental condition.+The reason for migration vary accordingly with the types of animals.In fishes migratory behaviour is a regular phenomenon. 

  

Types fish migration on the basis of needs:

1. Feeding or Alimental migration: migration in search of food and feeding ground. It occur when food resources get diminished.
2.  Spwaning or gametic migration: it occur during breeding period  in search of the suitable breeding ground.
3. Seasonal or climatic migration: migration in search for suitable climatic condition.
4. Osmo-regulatory migration: migration for maintaining water and ionic balance.It occurs  from sea to fresh water and vice-versa.
5. Juvenile migration: it is migration of larvae from breeding ground to the feeding grounds of their parent.

Examples for migratory Fishes

The following are the examples for migratory fishes:

1. Eels- Anguilla Anguilla,A. vulgaris, A. rostra, etc.

2. Salmon - Salmo solar,Oncorhynchus nerka

3. Indian shad - Hilsa hilisa

Causes for Fish Migration

The migration of fish is caused by the following factors:

1. Sexual maturity is a stimulus for migration

2. Hormones play significant role in migration.

3. Instinct is an important causes of fish migration.

4. To avoid predators and competition

5. Scarcity of food resources

6.Environmental factors like  light, temperature, salinity, pH, water currents, turbidity, etc.

Speed of Migration

The average speed of migration is 3 times the length of the fish per second. If the length of the fish is 12 inches, the speed of migration is 12x3=36 inches per second.

Types of Migration

Fish migration is classified into four main types. They are as follows:

1. Oceanodromous,

2. Potamodromous,

3. Catadromous,

4.Anadromous.

5.Latitudinal migration:

6. Vertical migration: 

7. Shore ward migration:

1. Oceanodromous migration

Long journeys within the sea for breed- ing is called oceanodromous migration. Eg.

Herrings (Clupea), Mackerels, Tunas, etc.

2. Potamodromous migration: Long travels within freshwater is called potamodromous migration. Eg. Carps and trouts and catfish

3. Catadromous migration:The journey of freshwater fishes to the for spawning is called catadromous migration. Eg. Eels.

4.Anadromous migration: The journey of marine fishes to fresh- water is called anadromous migration. Eg.Salmon, Indian shad, etc.

5.Latitudinal migration:In this type fishes  migrate to north in spring and to south in autumn.This is performed by fishes like Sphyraena and swordfish  of the warm tropical seas. 

6. Vertical migration:Vertical migration is  performed by many marine and freshwater fishes and is related to light, search of food, protection and also to spawning. Example:Mackeral rises into the surface waters when there is a rich development of plankton.After eating plankton they go to deep layers after feeding.

7.Shore ward migration:In this type of migration there is a temporary movement of fishes from water to land.  The common eel travel from one pond to another through moist meadow grass.

 Migration in Eel

(Catadromous Migration)

Eels are freshwater fishes. They travel long distance into the sea for breeding. Hence they are said to be catadromous fishes and the migration is called catadromous migration.Sexually mature female eels, migrate to- wards the sea.Males follow the females. During the travel the reproductive organs ripen and the alimentary canal shrinks. They stop feed-They travel 5000-6000 kms in the sea and reach a depth of about 500 metres. Here they spawn and die after spawning. The eggs hatch into a larva called Leptocephalus. It looks like a nerium leafand is transparent. It leads a pelagic life.Certain leptocephalus larva grows to alength of 62.5 inches. After 2 or 3 years it becomes a cylindrical larva, called Elver's larva

Elver's larva, after some time, migrates to fresh-water, reaches the place where their parents lived and grows into adults. It is a mystery how he young eels in the absence of their parents find their way and reach the same place wheretheir parents lived.

Migration of salmon

(Anadromous Migration)

Salmon is a marine fish. It migrates to reshwater for breeding. Hence it is an anadroaous fish and the migration is called anadromous migration. They migrate in pairs.Theytravel thousands of kilometres to reach the breeding ground. The males develop red spots and the females develop black spots.On entering freshwater, they stop feeding anddepend on the body fat for energy. On reaching the breeding ground, the female makes a saucer-like depression in the river bed and lays the eggs. Then the male releases the sperms and the eggs are fertilized. After spawning the parents return to their feeding ground. The fertilized eggs hatch into young salmons which after a few days migrate to the sea. These offspring, after attaining sexual maturity, take the same route and reach the same breeding ground for spawning.

Significance of fish migration

1. Migration helps to find suitable feeding and spawning ground
2. Migration gives protection from predators
3. Migration helps in survival from extreme climatic conditions
4. Migration increases genetic diversity
5. it is an adaptation for the survival of species.