Migration characteristics and properties of fish

The migration of fish is a special form of fish movement, which is completely different from general movement. General movements are conditioned reflex movements, often caused by external stimuli. Migration is the active, regular, directional, swarming, and species-specific horizontal movement of some fish. Migration is also a cyclical movement that repeats every year as each stage of the fish's life cycle progresses. Migration is the result of fish's long-term adaptation to changes in external environmental conditions. It is also an inevitable response to external stimuli when the internal physiological changes of fish develop to a certain extent. Through migration, the living waters of each life period are replaced to meet the needs of living conditions in different life periods and successfully complete various important life activities in the life history. The distance of migration varies with species. In order to find suitable external conditions and specific spawning places, some species have to travel thousands of kilometers.

There are many factors that affect fish migration, so the purpose of each migration is also 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 off-land migration, downriver (sea) migration and upstream migration. According to the role in the life activities, it can be divided into reproductive migration, feeding migration and overwintering migration. These three types of migration together constitute the migratory cycle of fish. But not all migratory fish have the same migration cycle. Some fish only have reproductive migration and feeding migration. Some fish overwintering grounds are together with or near feeding grounds, and some feeding grounds are near reproductive grounds. The migratory cycles of fish at different growth stages are also different, and the migratory cycles, migratory routes, and migratory times of juvenile and adult fish are often different.

1. Reproductive migration of fish

Reproductive migration is also called spawning migration. The spawning habits of fish are diverse. In order to seek suitable spawning conditions and ensure that eggs and young fish can develop in a good environment, some fish often have to move from wintering grounds or breeding grounds to spawning grounds. cluster movement. Migration distances vary from species to species. Eels living in fresh water have to migrate thousands of kilometers to the depths of the ocean to spawn, while salmon living in the ocean have to swim thousands of kilometers in the opposite direction to enter the upper reaches of rivers to spawn. Some fish only move short distances, such as the large yellow croaker that migrates from Dongtou Yangtze in northern Fujian to Lusi Yangtze in Jiangsu waters; some carp fish in my country's freshwater, such as herring, grass carp, silver carp, bighead carp and other fish, migrate from still water to Lakes migrate to spawning sites in specific river sections to lay eggs. The distance can range from hundreds of kilometers to dozens of kilometers.

The movement direction of reproductive migration varies depending on the species, including swimming from the deep sea or open sea to shallow sea or near shore; there is anadromous migration from the ocean to rivers or from the lower reaches of rivers to the upper reaches; there is the downriver migration from rivers to the ocean; Fish swim from the open central area of the lake to the coast to spawn. It can also be regarded as a spawning migration, but the migration distance is very short. The characteristics of reproductive migration are that the migration route, direction and time are relatively fixed, the distance is long, the swimming speed is fast, the phenomenon of migration in groups is obvious, the density of fish schools is high, and the characteristics of fish groups are fixed; during the migration, the gonad development and physiological state of the fish body change greatly. Some fish have variations in body shape and color. Some fish stop feeding during their reproductive migration.

Young fish and adult fish after overwintering, and adult fish that have undergone reproductive migration or reproductive activities, need intense feeding to supplement nutrition, and swim in groups to feed-rich waters, forming a feeding migration, also known as feeding migration. Some fish feed intensively after overwintering, causing migration from the wintering ground to the fattening ground. For example, carp that overwinter in deep river troughs will migrate to lakes with abundant food to fatten when the weather gets warmer in early spring. Feeding migration also occurs simultaneously with spawning migration. For example, tuna undergoes a long-distance reproductive migration from the Paracel Islands and feeds intensively along the way. In fact, the reproductive migration of tuna is also a feeding migration.

After reproducing, some fish conduct a feeding migration in autumn to accumulate nutrients in preparation for overwintering and reproduction in the coming year. The feeding and migration of juvenile fish is also very obvious. After breeding in rivers, fish such as grass carp, herring, silver carp, and bighead carp have developed into young fish that can swim freely and feed freely. That is, they go upstream along the coast and enter lakes or small tributaries leading to the river for fattening. Because these attached water bodies The food in the river is much richer than in the main river.

2. Fish migration for feeding

The feeding migration is different from the reproductive migration and overwintering migration. Some fish often achieve the purpose of migration during the migration process. The characteristic of bait migration is that the migration route, direction and time change with the distribution and density of bait biota, the number and status of bait fish groups.

Baitfish schools are less stable and dispersed. The length of the feeding period mainly depends on the quantity and quality of bait organisms, the fullness, fat content of the fish, and water temperature conditions.

3. Overwintering migration of fish

Overwintering migration is the migration of fish from the fattening place or the place where they live to the wintering ground. Overwintering migration is also called seasonal migration or temperature-friendly migration. Before winter comes, changes in the hydrological environment, especially the drop in water temperature, will reduce the activity of fish. In order to ensure suitable habitat conditions in the cold season, fish tend to move in groups to moderately warm waters.

The characteristic of overwintering migration is that the migration direction is towards the gradually rising water temperature, often moving from shallow water environment to deep water environment, or from the north to the south of the water area, with a stable direction. In China's coastal waters, they mainly move south and east. Many large carp fish in the middle and lower reaches of the Yangtze River usually feed and fatten in lakes connected to the river. Before winter comes, they swim to the depths of the riverbed or pits in the main stream. Overwintering.

The location, migration route and speed of fish overwintering grounds are affected by water temperature conditions, especially the distribution of isotherms in the water area. The water temperature gradient is large, the fish movement range is narrow, and the density is relatively high; the temperature drops quickly, and the migration speed is correspondingly fast. Water temperature conditions are affected by the number and intensity of cold air and cold waves.

4. Characteristics of fish migration

Fishes that undergo reproductive migration often migrate for food immediately, and some migrate for overwintering when winter comes. However, not all migratory fish and semi-migratory fish must carry out all three migration links. Some cold-water fish, such as whitefish, only carry out reproductive migration and feeding migration, and do not migrate over the winter. Whitefish mostly spawn in late autumn and early winter and do not stop feeding in winter. Some fish that live in shallow lakes feed and lay eggs in the lake area, and sometimes only swim to the depths of the river bed for overwintering migration. Juvenile fish only feed and migrate, and they still feed in winter, and there is no overwintering migration.

Migration is the result of natural selection of fish over the long years of evolution, and it is consolidated through inheritance. Fish migration has the characteristics of regularity, directionality, clustering and periodicity. Almost all migrations are cluster migrations, but for different types of migrations with different natures, the size of the migration clusters varies, 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 condition. They are large in size, have high fat content, and migrate long distances. For example, sturgeons, salmon, eels, etc. can travel thousands of miles. In addition to being genetically related to the directionality of migration, the highly sensitive and selective sense of smell plays a big role in guiding fish such as salmon and eels to return to their original birthplace after traveling thousands of kilometers after several years. The extremely small magnetic particles in the skull of tuna allow it to swim in the ocean without losing its direction. The sensitive flow sensing ability of the lateral line also plays a role in guiding the direction of migration.

The start of fish migration depends not only on the state of the fish itself, but also on the influence of surrounding environmental conditions. For example, the sex hormones secreted by mature fish gonads stimulate nervous system excitement and produce reproductive requirements. Changes in environmental conditions are natural stimulus signals to start reproductive migration. For fish that reach a certain degree of plumpness in temperate zones, the natural stimulation of falling temperatures becomes a signal to start overwintering migration. If most fish have underdeveloped gonads, they will not require reproductive migration even if they have reached reproductive age and are strongly stimulated by the external environment. Fish that have not reached a certain degree of plumpness or fat content will continue to search for bait or search for bait while migrating despite the drastic drop in external water temperature. For example, European anchovies, when the fat content is less than 14%, will not start migration even if the water temperature drops sharply and the overwintering season has arrived, but will continue to feed. When the fat content is 14-17%, migration begins, but the movement is slow. When the fat content reaches 22%, during the overwintering season, the overwintering migration will begin immediately regardless of the magnitude of the drop in water temperature. Among environmental conditions, water temperature, water flow, water chemistry, etc. have the most significant impact. The water temperature rises quickly, and reproductive migration begins early. Changes in water temperature will affect the occurrence and distribution of feed organisms, thereby affecting the feeding migration of fish. The sooner and faster the water temperature drops in late autumn directly affects the start time and migration speed of overwintering migration. Water flow is the dominant factor causing passive migration and active migration. Currents carry fish eggs, larvae, and juveniles away from their birthplace, forming a return migration of adult fish. Changes in the chemical composition of water, especially changes in salinity, cause changes in the osmotic pressure in fish, leading to changes in blood components and properties, which in turn induce physiological changes in fish and promote migration to the sea or upstream migration.

5. Nature of fish migration

Due to different habitat environments of fish, the nature of migration is different. People often divide migratory fish into several major categories. Commonly used classification methods are: marine migratory fish, freshwater migratory fish, estuary brackish water migratory fish, estuary migratory fish, etc. According to the nature of fish migration, fish that move long distances and move between seas and rivers are called migratory fish, and those that move between different freshwater bodies or between upstream and downstream rivers are called semi-migratory fish.

The migration of migratory fish is a typical and true migration of fish, and it is a basic attribute of this type of fish. If migratory fish cannot complete this migration or their migration is hindered, the life cycle of these fish will be destroyed, affecting the proliferation of the population and even the survival of endangered species.

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