Generally fishes restrict their movements within small territorial limits and do not go out of their home ranges. However, a few species travel long distances moving from fresh to sea water or vice versa. This movement of a very large number of fishes for the purpose of feeding of spawning, is know as migration. It may take place in vertical direction, as from the deeper to the surface water, or it may be in horizontal direction, either upstream or down stream. Heape (1931) has defined migration as “a class of movement which impels migrants to return to the region from which they have migrated”. Migration is of the following types:
(a) Alimental migration: This is in search of food and water.
(b) Gametic migration: For reproduction.
(c) Climatic migration: This is to secure more suitable climatic conditions.
(d) Osmoregulatory migration.
The really interesting migrations of mature adult fish are for spawning and feeding. They breed in one area but grow and feed in another area. Examples of such migratory fishes are :
1. Cod (Gadus morhua)
2. Herring (Clupea harengus)
3. Salmon (Salmo sp.)
4. Eel (Anguilla anguilla)
5. Plaice (Pleuronectes platessa)
6. Hilsa (Hilsa ilisha)