Fish Migration in Ecology

• Migration in ecology refers to the mass movement of animals from one location to another.
• The purposes of migration vary depending on the animal species.
• Fish migration is a common behavior mainly for feeding and reproduction.

Types of Fish Migration Based on Needs

• Alimentary or Feeding migration occurs when fish migrate to search for feeding grounds due to exhausted food resources.
• Gametic or spawning migration happens during breeding season as fish seek suitable spawning grounds.
• Climatic or seasonal migration involves fish migrating to find suitable climatic conditions.
• Osmo-regulatory migration is when fish migrate to balance water and electrolytes between sea and fresh water environments.
• Juvenile migration refers to the migration of larvae from spawning grounds to the feeding habitats of their parents.

Movement of Fishes During Migration

• Drifting movement is a passive movement of fish along with water currents.
• Dispersal movement involves random locomotion of fish from a uniform habitat to diverse directions.
• Swimming movement is an oriented movement of fish either toward or away from a stimulus source.
• Denatant and Contranatant movement is active swimming; denatant means swimming with the water current, while contranatant means swimming against it.

Types of Fish Migration

• Diadromous migration is the migration of fish between sea and fresh water.
• Most fish species are restricted to either fresh or saltwater, habitat changes can cause osmotic imbalance.
• Some fish regularly migrate between sea and fresh water and have perfect osmotic balance, making them true migratory fish.
• Diadromous migration has two types: anadromous and catadromous.

Anadromous Migration

• Anadromous migration is when marine fishes migrate from sea to fresh water for spawning.
• Fish spend most of their lives feeding in the sea.
• They typically migrate to rivers for spawning during the breeding season.
• Examples include Salmon, Hisla, and Lamprey.
• Salmon migrate for breeding during winter from sea to river, and physiological changes occur during migration.
• They stop feeding, change color from silver to reddish-brown, and their gonads mature.
• They construct a saucer-like nest for spawning, where the female lays eggs and the male releases sperm.
• Juvenile larva hatch from the eggs and are known as Alevins, which transform into parr, then metamorphose into adults before returning to the sea.

Catadromous Migration

• Catadromous migration involves freshwater fishes migrating from rivers to the sea for spawning during the breeding season.
• Examples include Eel (Anguilla spp).
• Both European eel (Anguilla anguilla or Anguilla vulgaris) and American eel (Anguilla rostrata) migrate from continental rivers to the Sargasso Sea off Bermuda for spawning, crossing the Atlantic Ocean.
• Before and during migration, physiological changes occur: fish deposit large amounts of fat, which serve as reserve food.
• Color changes from yellow to metallic silvery grey, the digestive tract shrinks and stops feeding.
• Eyes get enlarged, vision sharpens, and other sensory organs become more sensitive.
• Skin serves as a respiratory organ, and gonads mature and enlarge.
• Fish lay eggs in suitable spawning grounds, which are then fertilized by males; after spawning, the fish die.
• Larva hatch, develop into young eels, and eventually return to the river.

Potamodromous Migration

• Potamodromous migration is the freshwater migration of fish from one habitat to another for feeding and spawning.
• Examples include Carps and catfish.

Oceanodromous Migration

• Oceanodromous migration involves the migration of fish within the sea in search of suitable feeding and spawning grounds.
• Examples include Clupea, Thunnus, and Tuna.

Latitudinal Migration

• Latitudinal migration is the migration of fish from north to south and vice versa.
• It is a climatic migration.
• Sward fish migrate north in spring and south in autumn.

Vertical Migration

• Vertical migration is the daily migration of fish from deep to the surface and vice versa for food, protection, and spawning.
• Sward fish usually move vertically downward to greater depths for food.

Shoreward Migration

• Shoreward migration is the migration of fish from water to land, but it is a temporary migration.
• Eel can migrate from one pond to another via moist meadow grass.

Significance of Fish Migration

• To find suitable feeding and spawning grounds.
• For protection from predators.
• To survive extreme climatic conditions.
• Increases genetic diversity.
• It is an adaptational characters for survival and existence.

Fish migration is a common behavior, mainly for feeding and reproduction. Different types of fish migration exist based on the needs of the fish. These include feeding, spawning, seasonal, osmo-regulatory, and juvenile migrations.