Why Do Fish Migrate? Understanding Biological Reasons and Types
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:
Oceanodramous fish
Anadromous fish
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.
Several populations
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).
FAQs on Fish Migration: Patterns, Types, and Life Cycle
1. What exactly is fish migration?
Fish migration is a large-scale, regular movement of a fish population from one habitat to another for specific purposes like feeding, spawning, or seeking favourable environmental conditions. Unlike random daily movements, migration is a predictable, cyclical journey that is crucial for the survival and reproduction of the species. A classic example is the journey of salmon from the ocean back to the freshwater rivers where they were born.
2. What are the main categories of fish migration based on their purpose?
Fish migration can be categorised based on the primary reason for the journey. The main types include:
- Alimental Migration: This is undertaken purely for the search for food and better feeding grounds.
- Gametic Migration: This is a journey to specific spawning grounds for reproduction. It is often the most demanding type of migration.
- Climatic Migration: This involves moving to escape unfavourable conditions like extreme temperatures or ice cover, seeking more hospitable waters.
- Osmoregulatory Migration: This migration is driven by the need to move between freshwater and saltwater environments for different life stages, requiring significant physiological adaptation.
3. How do anadromous and catadromous migrations differ?
The key difference lies in the direction of travel between freshwater and saltwater for breeding.
- Anadromous migration involves fish that spend most of their lives in the ocean (saltwater) but migrate to freshwater rivers and streams to spawn. Salmon and Hilsa are prime examples.
- Catadromous migration is the opposite. These fish live most of their lives in freshwater and travel to the ocean to reproduce. The European Eel is a well-known example.
4. What key factors trigger and influence the migration of fish?
Fish migration is triggered by a combination of internal and external factors. Physical factors like changes in water temperature, water flow speed, and light duration (photoperiod) are major cues. Chemical factors, such as changes in water salinity or the scent of their native stream, are also critical. Internally, biological factors like the fish reaching sexual maturity or having low food reserves create the physiological drive to begin the journey.
5. How can a salmon find its way back to the exact stream where it was born?
This incredible navigational feat is achieved through a combination of senses. In the open ocean, salmon are believed to use the Earth's magnetic field as a large-scale map to get close to their home coastline. As they approach the river systems, their highly developed sense of smell takes over. This is known as olfactory imprinting, where the unique chemical signature of their home stream was imprinted in their memory as juveniles, allowing them to follow the scent trail back to their precise birthplace.
6. What major physiological challenges must a fish overcome when moving between freshwater and saltwater?
The primary challenge is maintaining the balance of salt and water in their body, a process called osmoregulation. When moving from saltwater to freshwater, a fish must prevent its body from absorbing too much water and losing essential salts. Conversely, when moving into the ocean, it must prevent dehydration by drinking seawater and actively pumping out excess salt through its gills. These changes require significant, pre-programmed adjustments in their kidney function and gill cells.
7. How do human activities like building dams impact fish migration?
Human activities pose a severe threat to migrating fish. The construction of dams and weirs creates physical barriers that block migration routes, preventing fish from reaching their spawning grounds. This can lead to a sharp decline in population or even local extinction of species like salmon or sturgeon. Other impacts include water pollution, which can mask the chemical cues fish use for navigation, and altered water temperatures from industrial discharge, which can disrupt the timing of migration.
