Fish migrate to spawning areas for the purposes of reproduction. Generally fish begin their migration together, congregating in a group that grows until the impetus to begin the migration overtakes them. Thus many fish enter the mouth of a river together over a short period of time, this is called a fish starting its 'run'. Before the migration the fish have lived in smaller schools dispersed around their feeding areas. Different species of fish run at different times of year. Whitefish start their spawning run in Autumn whilst most Cyprinids (Carp and its relatives) begin theirs in Spring.
Most migrations are from areas of large areas of water, oceans, seas and lakes, into and up rivers, this is called Anadromous migration. Species such as Salmon, Sturgeon, Lampreys and various Cyprinids all have anadromous migrations. The opposite is a Catadromous migration, from the river to the lake or ocean, Eels have catadromous migration. For other species migration is simpler and involves only a move to shallow areas of the lake, sea or ocean to spawn amongst vegetation; sea grasses, mangroves and reed beds are all good spawning sites.
Finally you should note that not all fish migrations are horizontal, some are vertical. Some species that live at depth migrate to the surface in order to spawn, their eggs and larva becoming part of the zooplankton while still other species that live in the surface waters of the sea migrate to the the depths of the sea floor to spawn.
Most fish feed very little, if at all, whilst they are on migration and swim quite quickly. Sockeye Salmon migrating up the Yukon travel 30 km to 40 km a day, while Chum Salmon in the Amur travel about 50 km per day. However there are exceptions, it is known that the Atlantic Salmon that pass through the White Sea have Spring and Autumn races. The Autumn race enters the rivers in late Autumn with their gonads poorly developed, as they travel up the river they move slowly and feed up as they go their gonads developing as they travel. The Spring race enters the same rivers in the Spring with their gonads already fully developed, they feed very little and travel quickly up the rivers, catching up with the Autumn race near the spawning sites.
The distance of a migration is very variable, while Sockeye Salmon make awe inspiring migrations of 3,600 km up the river Yukon, Crucian Carp in Lake Kerkini in Greece migrate less than 1 km up the Kerkinitis river to spawn. The distance a fish has to swim on its migration obviously has an effect on its ecology. It is a lot harder to swim 3,600 km than it is to swim 300. In the USA Sockeye Salmon make massive journeys of 3,000 km or more up the long rivers there, the cost of this is that they are too exhausted to return to the sea after their spawning, they simply spawn and die. In Europe many Atlantic Salmon migrate up much shorter rivers, making journeys of less than 400 km. The benefit of this is that they have the energy to return to the sea after they spawn, here they can feed up and spawn again one or two years later, in fact some Atlantic Salmon may spawn 3 or even 4 times before they die.
If a fish swims thousands of km over a period of 2.5 months and hardly eats anything on the way, where does it get its energy from. The answer is from stored body fat. During the premigration period the fish stores up fat reserves to fuel its journey, the table below shows how the percentage body fat of a Chum Salmon changes over a 1,200 km journey. Notice how the female, who has to carry the heavier eggs and thus has to do more work starts off with a higher percentage of body fat and ends up with a lower one.
|Distance from Sea||0 km||220 km||400 km||570 km||730 km||1,200 km|
|% Body Fat in Females||11.3||10.9||7.7||6.6||5.9||3.0|
|% Body Fat in Males||9.2||9.15||8.25||5.7||4.5||3.4|