Fish Migration in Aquatic Ecosystems

Numerous fish species are annually wandering through a specific area of the ocean. In that, some species are true migrants, they travel regularly over the great distant. Usually, young fishes leave the spawning ground and reach the feeding ground, while developing into young ones. Meanwhile, the adult starts moving from the spawning ground to the feeding ground. 

The migratory patterns of the fishes completely depend on oceanographic factors and ocean currents. Eggs, young drifts, and larvae move according to the ocean current. Although the migration of adult fish can move against the current, it starts moving towards the current. It completely depends on the environmental changes. 

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Usually, the Migration Animal Fish are Classified Into Three Categories:

1. Oceanodramous fish

2. Anadromous fish

3. Catadromous fish

Oceonodramous Fish 

The Oceanodromous fishes are widely spread throughout the world’s ocean. These live and migrate whole seas. They differ mainly from other species through the boundary extent of their migration. 

The best know oceanodromous fish is taken for a study ie., Herring (Clupea harengus). The reason for specifically choosing it is, this is one of the economically important species. The oceanodromous fish can be further classified into two types based on their migratory behaviour and it’s ability to mix with others freely. 

1. Several populations 

2. Local Racer

It is possible to observe distinct groups of species in different seasons on different grounds in the North sea.  Like, Buchan herring fishes can notice on the Coast of Scotland from August to September. Later it starts to migrate to the coast of Southwestern Norway. Likewise, the dogger bank herring fishes stay in the central part of the North Sea to the English Coast during September and October. It starts to migrate to Skagerrak, which is between Denmark and Norway. The Downs herring spawn stays on the French coast from November To January ie, between Dunkirk and Fécamp. These species start intaking species from the middle and northern parts of the North Sea during summer, they are sharing the feeding grounds with other populations.  This migration and reproductive seasons of closely connected with the oceanographic conditions of the North Sea. 

The Cod (Gadus morhua) have the same migration pattern as herring. This is one of the important species in the list of Oceanodramous fish. Many species cover greater distances in the Atlantic sea. For example, White tuna (Germo alalunga) can encounter during winter around the Azores and the Canary Island. Then they start to migrate northward near the Gulf of Gascogne and arrives water of Iceland during July. The population of Red tuna (Thunnus thynnus) surrounds the Mediterranean Sea and the Eastern Atlantic. Red tuna lay spawn in the western Mediterranean during May and June.  During summer, they spread northward for feed and reach the Arctic Ocean. Similar migrations occur in the Pacific from North American Coast.

Anadromous Fish 

The Anadromous fishes live in the sea and migrate to freshwater for breeding. These feeds have the capability to adopt both saltwater and freshwater. 

Salmon (Salmo, Oncorhynchus) spawn in the cold, clear water of lakes or upper streams. Fish lay in gravel beds. The young Atlantic salmon stay in freshwater for two to three years and sometimes stay still six years. But, Pacific salmon migrates to sea even in its first year.  The adult dish remains in the sea for one to three winters. These fishes again return to freshwater as grilse (adolescent stage of fish) or as adults to spawn. This fish changes its colour and other external features to adopting the changes. Some Atlantic salmons die after single spawning in freshwater.  The salmon cross the European country from Norway To Scotland.  

The Pacific Salmon are widely distributed over the Bering Sea and the Pacific Ocean, between the latitudes of  45° N and 65° N with the surface water temperature ranges from 2 to 11 °C (36 to 52 °F).

While experimenting Ocean environment of Canada and the United States, Here, a high proportion of young fishes are migrated to the Pacific ocean from freshwater has been noted. The Atlantic salmon migrates to sea even three times from the particular river in the same year.  Adult fishes may experience the hatched rate with eggs laid rate. The celestial features, topographical features are playing an important role in habitat development.  The sense of smell helps the fishes to identify the places. The experiment has shown that migrating salmon get attracted to the waters. Also, the grown fish respond to water depends on odour and temperature. 

Catadromous Fish

The life cycle of Catodromous fish is just an inverse of Anadromous fish. These fishes spend most of their life in fresh water and migrate to the sea for breeding.  Eel belongs to the genus Anguilla is the best-known example of catadromous fish.  Also, the popular breeds are the North American eel  (A. rostrata) and the European eel (A. anguilla).