In this article we will discuss about the migration process in fishes with the help of a suitable diagram.
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 place to place in search of food or for breeding. This movement of a large number of fishes for the purpose of feeding or spawning, is known 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 downstream.
Thompson (1952) defines a true migration as seasonal movement that implies return to the starting point. Harden Jones (1968) has defined migration as a class of movement which impels migrants to return to the region from which they have migrated. This definition envisages a return to the original place. Baker (1978) has given a more general definition, and defines migration ‘as the act of moving from one spatial unit to another’.
There is no restriction of return journey. More recently, Dingle (1980) has defined the term migration as a ‘specialised behaviour especially evolved for displacement of the individual in space’. This definition implies evolutionary changes leading to specific migratory patterns. Dingle has opined that accidental or unintentional movements cannot be included in the definition of migration.
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There are two opposing trends of migration in fishes:
1. Catad