Some fish have special requirements on the conditions of life activities at different stages of their life history, so they must regularly integrate large groups in a certain period of time, migrate along fixed routes at different distances, meet the conditions suitable for breeding, feeding and wintering by changing their living environment, and then return to their original places after a period of time. This behavior of fish is called migration. In the process of migrating from the sea to the upper reaches of the river or from the river to the lower reaches of the sea, fish need a process of changing and adjusting osmotic pressure mechanism to adapt to different water quality environmental changes. For example, shad, bonito, salmon, salmon and so on. They are all marine fish that migrate upstream, and their eggs laid in the river are hatched in fresh water. Only when the salt-secreting cells on the gills are fully developed and other physiological changes are completed can the young fish gradually return to the ocean by adjusting the osmotic pressure sequence. These fish and their young fish after living in fresh water for a period of time have to swallow seawater equivalent to 4% ~ 15% of their body weight every day for the first five days, and at the same time, the filtration function of the kidney is sharply weakened, while the salt-secreting function of salt-secreting cells and the reabsorption function of renal tubules are strengthened, and the water displacement is greatly reduced. When living in fresh water, the water displacement is only 65438. Only after fully adapting to the seawater environment can the osmotic pressure of body fluids change from the original hypertonic to hypotonic.

According to the different types of fish migration, it can be divided into breeding migration, feeding migration and wintering migration.