Migration is an important phenomenon in the life activities of fish, which is manifested as a directed periodic movement. Through migration, fish are able to complete various important aspects of their life history, such as reproduction, bait, wintering, growth, etc. The phenomenon of fish migration is very obvious in many fish. The phenomenon of fish migration is very obvious in many fish species, such as most marine fish, anadromous and diadromous fish. Because they require different environmental conditions at different times in the course of their life activities, and the migration of fish is an effective movement for them to find a suitable living environment. Some fish, in the breeding season, after a long distance, from the overwintering place to swim to the spawning ground to spawn, the result of this movement is to make the fish to reach the place suitable for the growth and development of their offspring, thus preserving the race of the continuity of the race, which is essential in the life of the fish. From a biological point of view, the phenomenon of migration is still an intrinsic characteristic of fish that has developed over time as a result of adaptation to environmental conditions, and has become an indispensable part of the life history of certain migratory fish through generations. Due to the migratory period of fish, often gathered into groups, to a certain direction for regular movement, can be in a certain period of time, a certain location of a large number of appearances, and thus the formation of fishing season, and can be observed in the eggs and young fish appeared in the place of origin, mastery of these laws for the development of the marine fishing, the reproduction and protection of resources is very important. The study of fish migration is also very important for the development of marine fish breeding work, we are in the marine fish breeding research, always choose some high economic value, migration distance is small, after the release of high catch rate of species as the object, to find out the migratory pattern of economic fish, so that the breeding work to achieve better results. Section I. Types of Fish Migrations and Factors Affecting Migrations I. Types of Fish Migrations There are different types of migrations of various fish species in different life stages. For the classification of migrations, there are many different classification methods from different perspectives. At present, fish migrations are usually classified according to their different purposes as spawning migrations, bait soliciting migrations and overwintering migrations, which in most cases constitute the interconnection between the three major life processes of fish. (i) Spawning Migration When the gonads of fish mature, the sex hormones secreted by the pituitary gland produce physiological stimuli within the fish body, leading to the requirement for fish to spawn and reproduce, and during this period, fish often gather into schools and swim to spawning grounds to carry out spawning activities. Spawning migration is characterized by the gathering of fish into large groups, within a certain period of time, along a certain route, in a certain direction for rapid migration. The clustering of fish during the spawning period is partly due to the stimulation of gonadal hormones, and partly due to biological adaptations, which allow large-scale reproduction when large aggregations of fish arrive at the spawning grounds. Lebedev (1946) pointed out that grouping of fish can find the migratory route more quickly, it is easier to find some orientation signs, and even the fish that usually live alone, during the migratory period also grouped together into a group. Although the time, route and direction of the onset of migration vary slightly according to environmental conditions, the general trend remains relatively stable. According to the different spawning grounds, there are the following three types of fish spawning migrations: 1. From the deep sea to the shallow sea or near the shore, which is seen in most marine fishes, such as large yellowtail, small yellowtail, herring, mackerel, horse mackerel, tuna, skipjack tuna, and so on. Their spawning grounds are mostly in shallow coastal bays or near estuaries, where natural bait is abundant and the temperature and salinity are very suitable for the hatching of fertilized eggs and the growth and development of young fish and juveniles. 2. Anadromous migration from the ocean to rivers Anadromous migration refers to life in the ocean and spawning in rivers (including estuaries) during the breeding period, and they have to go through two major changes in their lives, one is the migration of their juveniles from freshwater to the marine environment, and the other is the migration from the ocean to the freshwater environment in the adult period to carry out breeding activities. As a result, they have developed effective physiological adaptations to overcome the difficulties in the migration process. The biggest problem encountered by anadromous fish during anadromous migration is the regulation of osmotic pressure. All anadromous fish have good regulation ability, such as Chinook salmon in the sea life period blood freezing point drops to -0.762 ℃, after living in brackish water for a period of time is -0.737 ℃, in the upper reaches of the river spawning grounds -0.628 ℃, the salt in the blood has been significantly reduced, while its gill secretion cell function has been significantly strengthened. Anadromous spawning migratory fish are also quite common, such as the familiar shad, crabs, silverfish, sturgeon, salmon and so on. Hemp salmon usually live in the ocean, to the reproductive period, on the cluster anadromous, their ability to swim upstream against the water is very strong, and even on the way to encounter obstacles like waterfalls, will overcome the difficulties, and strive to leap out of the water, over the obstacles, to reach the destination. Another characteristic of the salmon spawning migration is that it is particularly strong in "return", and it will not forget to return from the ocean to the freshwater rivers where it was born to spawn and reproduce for generations. China's four major fish (green, grass, silver carp, bighead carp) and other freshwater fish, before spawning from the lower reaches and tributaries migrate to the middle and upper reaches of the river spawning, some travel up to 500-1000km or more, this is another type of spawning migration. 3. Downstream migration from rivers to the sea The most obvious representative of this type is the eel, which usually inhabits freshwater, but when it becomes sexually mature, it begins to leave the waters where it solicits bait and grows, and moves downstream to the rivers, where it gathers in large groups at the mouths of the rivers and swims to the deep sea. Where do the eels that enter the sea from our country swim to? Some say they spawn near the Ryukyu Islands. The eels that enter the sea from Europe and America have been proved to spawn in the Atlantic Ocean south of Bermuda at a depth of about 400 meters. The eel has a long period of sexual maturity, 8-10 years for males and longer for females. Eel migration, generally more at night, the beginning of the migration body fat, but in the long-distance migration on the way to consume a huge amount of energy, and do not eat, the body is extremely thin, to reach the spawning ground after spawning, most of the parent fish died of exhaustion. After hatching, the young fish gradually migrate to the original habitat, when the young fish white, head fine, shaped like a willow leaf, known as the willow leaf eel, it drifted on the surface of the water between the waves, back to Europe's willow leaf eel after three years, before entering freshwater, began to change into an eel-shaped line eel. Back to the American willow eel, the trip is shorter, it takes about a year to enter the freshwater perversion. Because of the spawning migration of fish swarm into the spawning grounds, so the fishing for the greatest value, at present, our marine fisheries production, many of the spawning groups of various types of fish fishing, we can be based on the spawning groups of male and female sex ratio, age composition, length composition of the analysis, to determine the movement of the fish stock and the flood of the season. Regarding the mechanism that leads to fish spawning migration, it is generally believed that the stimulation and pressure of the reproductive glands, had to go towards the old way of fish ancestors for thousands of years; and there are explanations for the influence of environmental factors, such as water temperature, current, salinity and other roles. Because of the changes in external factors, prompting the gradual maturation of the reproductive glands, the maturation of the reproductive glands lead to internal physiological changes. However, some fish are able to regulate their internal and external conditions (osmotic pressure in the body is adapted to the external water environment), so they do not need to migrate for spawning and find suitable spawning sites, while some other fish cannot actively regulate osmotic pressure in the body to adapt to the external environment, so they have to migrate to seek for the coordination of this internal and external environment, and it is believed that changes in physiological status caused by the fish during the reproductive period may be reflected in changes in blood osmolality. Some scholars believe that the changes in physiological status caused by fish during reproduction may be reflected in the changes in blood osmolality and blood pressure, and it has been proved that when salmon enter freshwater, the salts in the blood and the blood pressure are much lower than those in the sea, and if they are still in the sea, they may feel that they cannot adapt to it, so they have to enter freshwater. In contrast, eels, such as the eel before the start of spawning migration, the concentration of carbon dioxide in the blood continues to increase, which leads to an increase in blood pressure, forcing it must be in the sea, and the deeper the better, only in this way to adapt to the requirements of the environmental conditions of their spawning.

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