Pisces, keep moving forward! What is driving you?

Have you ever seen something like this?

A group of fish swam tirelessly along the river, as if participating in a marathon in water, moving forward all the way and not daring to stop.

(Photo source: veer photo gallery)

Could it be that there is some mysterious treasure ahead of them?

What are the fish doing?

In fact, this is the fish "migrating".

So, what is "migration"?

Fish migration is a special form of fish movement, which is completely different from general movement. General movement is a conditioned reflex movement, often caused by external stimuli. Migration is the active, regular, directional, clustered, and species-specific horizontal movement of some fish. Migration is also a cyclical movement that repeats every year as the various stages of the fish life cycle progress. Migration is the result of fish's long-term adaptation to changes in external environmental conditions, and it is also an inevitable response to external stimuli when the internal physiological changes of fish develop to a certain extent.

(Photo source: veer photo gallery)

Through migration, fish can change their living waters at different stages of their life to meet the needs of living conditions at different stages of their life and successfully complete various important life activities in their life history. The distance of migration varies with species. In order to find suitable external conditions and specific spawning sites, some fish have to swim thousands of kilometers, which is really a big project!

What drives fish migration?

There are many factors that affect fish migration, so the purpose of each migration is different. According to the driving force of migration, it can be divided into passive migration and active migration; according to the direction of migration, it can be divided into landward migration and away from land migration, downstream river (sea) migration and upstream migration, etc. According to the role in the life process, it can be divided into reproductive migration, foraging migration and wintering migration. These three types of migration together constitute the migration cycle of fish. However, not all migratory fish have the same migration cycle. Some fish only have reproductive migration and foraging migration. Some fish have wintering grounds and feeding grounds together or near them, and some feeding grounds are near the reproductive place. The migration cycle of fish at different growth stages is also different. The migration cycle, migration route and migration time of young fish and adult fish are often different.

(Photo source: veer photo gallery)

Fish that make reproductive migrations often immediately make foraging migrations, and some make wintering migrations when winter comes. However, not all migratory and semi-migratory fish make all three stages of migration. Some cold-water fish, such as whitefish, only make reproductive migrations and foraging migrations, and do not make wintering migrations. Whitefish mostly spawn in late autumn and early winter, and do not stop feeding during the winter. Some fish that live in shallow lakes feed and spawn in the lake area, and sometimes only make wintering migrations to the deep riverbed. Young fish only migrate for feeding, and continue to feed in winter, and do not make wintering migrations.

(Photo source: veer photo gallery)

Fish migration is characterized by regularity, orientation, clustering and periodicity. The reason why we can see spectacular migrating fish is that almost all migrations are clustered, but the size of the clusters of different species and different natures is different, which is related to ensuring the most favorable migration conditions. The distance of migration is related to the size of the migratory fish and its own state. Large body, high fat content, long migration distance, such as sturgeon, salmon, eel, etc., all travel thousands of miles.

In addition to genetics, the directionality of migration is also related to the highly sensitive and selective sense of smell, which plays a great role in guiding salmon, eels and other fish to return to their birthplaces after traveling thousands of kilometers for several years. The extremely tiny magnetic particles in the skull of tuna prevent them from getting lost when migrating in the ocean. The sensitive flow sensing ability of the lateral line also plays a role in guiding the direction of migration.

Under what circumstances do fish begin to migrate?

The start of fish migration depends not only on the state of the fish itself, but also on the influence of the surrounding environmental conditions. For example, the sex hormones secreted by the mature gonads of fish stimulate the nervous system to become excited and produce reproductive needs. Changes in environmental conditions are natural stimulation signals for the start of reproductive migration. For fish in temperate regions that have reached a certain degree of fullness, the natural stimulation of falling temperatures becomes a signal for the start of winter migration.

(Photo source: veer photo gallery)

Therefore, if most fish have poor gonadal development, even if they have reached reproductive age and are strongly stimulated by the external environment, they will not have the desire to migrate for reproduction. Fish that have not reached a certain degree of plumpness or fat content will continue to feed or feed while migrating despite a sharp drop in the external water temperature. For example, when the fat content of European anchovies is below 14%, even if the water temperature drops sharply and the wintering season has arrived, they will not start to migrate, but continue to feed. When the fat content is 14-17%, they will start to migrate, but the movement is slow. When the fat content reaches 22%, in the wintering season, no matter how much the water temperature drops, they will immediately start to migrate for the winter. They can be said to be masters of body management!

(Photo source: veer photo gallery)

Among the environmental conditions, water temperature, water flow, and water chemistry have the most significant effects. The faster the water temperature rises, the earlier the reproductive migration begins. Changes in water temperature will affect the occurrence and distribution of bait organisms, thereby affecting the foraging migration of fish. The early or late and fast drop in water temperature in late autumn directly affects the start time and migration speed of wintering migration. Water flow is the dominant factor causing passive migration and active migration. Water flow carries fish eggs and fry away from their birthplaces, forming a recurrent migration of adult fish. Changes in the chemical composition of water, especially salinity, cause changes in osmotic pressure in fish, leading to changes in blood composition and properties, which in turn induce physiological changes in fish and promote the occurrence of sea or upstream migration.

Conclusion

Migration is the result of natural selection of fishes during their long evolutionary period and is consolidated through inheritance. For migratory fishes, if they cannot complete this migration or if it is hindered, their life cycle will be destroyed, affecting the proliferation of the group and even endangering the survival of the species. It is really a "mission must be accomplished"!

(Editor: Sun Chenyu)

Produced by: Science Popularization China

Author: Life Pulse Team

Producer: China Science Expo