Introduction
- In life history of some fishes, population needs, like foraging & reproduction, can’t be met in single habitat. So, they travel periodically from one habitat to another & back after meeting the requirement, termed as fish migration.
- Causes: Sexual maturity, hormonal changes, instinct, predators, competition, food, light, temperature, pH, water currents, turbidity etc.
- The migration can occur from hundreds of meters to thousands, depending upon fish species.
- Typically occurs seasonally, though some may show daily movements (vertical or tidal).
Classification
1) Oceanodromous Migrations:
- These migrations occur entirely within seawater.
- Fishes example: Herring (Clupea harengus), Tunas, White Sharks (Carcharodon carcharias) etc.
- Oceanodromous fish species & subspecies differ among themselves by method & extent of migration.
- Example:
| S.No. | Fish name | Spawning place (Month ) | Migrate to… |
| 1 | Buchan Herring | Near Coast of Scotland (August & September) | Coast of Southwestern Norway |
| 2 | Dogger Bank Herring | In Central Part of North Sea & along English Coast (September & October) | Skagerrak, North Sea, between Denmark & Norway. |
| 3 | Downs Herring | Near French Coast (November to January) | Northern part of North Sea |
2) Potamodromous Migration:
- Occur entirely within freshwater.
- Fishes example: Carps, Trouts etc.
- Migrations can occur solely within lake, rivers, streams or between lake & fluvial freshwater body:
| S.No. | Freshwater Body (ies) | Example |
| 1 | Lake | Lake trout (Salvelinus namaycush) |
| 2 | Rivers & Streams | Brook Lampreys (Lampetra) |
| 3 | Between Lakes & Fluvial freshwater body | White suckers (Catostomus commersoni) |
3) Diadromous Migration:
- Occurs across freshwater & seawater.
- Types:
- Anadromous: Majority of fish feeding & growth occurs in saltwater & mature adults migrate to freshwater for spawning. Fish example: Salmon spawn in cold & clear lake water. Eggs laid in gravel bed. Young Atlantic salmons remain in freshwater for 2-3 years whereas Pacific salmon remains for one year & then migrate to sea. Adult fish remains in sea for 1-3 winters. Salmon after one spawning dies.
- Catadromous: Majority of fish feeding & growth occurs in freshwater & mature adults migrate to saltwater for spawning. Fish example: Eels (Anguilla), European & North American eels spawn in Atlantic’s warm saline water depths (1300-2300 feet). Their pelagic eggs develop into leptocephali, which are carried by Gulf Stream to continental shelves’ shallow waters. Leptocephali undergo metamorphosis transforming to elvers. Further development forms glass eels which arrive at coastal waters & start swimming upstream in streams, in springs. In freshwater, eels mature & live for nearly 15 years. Mature ones move downstream into sea for spawning & later die.
- Amphidromous: Migration occurs from freshwater to sea water not for spawning but for growth of larvae. Growth, maturation & spawning occurs in freshwater. Fish example: Sicydium sp. Adult lays eggs & larva hatch in freshwater, then larvae moves to sea water, after development, juvenile form starts journey back to freshwater.
