Passage fish - fish that live in the sea, brackish lakes, and for reproduction go to the rivers, going up them for considerable distances (sturgeon, beluga, starred sturgeon, pink salmon, chum salmon, herring); others, on the contrary, live permanently in rivers and freshwater lakes and go out to spawn in the sea (eel).
Passage fish have a number of characteristics, such as the ability to tolerate strong fluctuations in salinity, for which they must have a certain range of physiological adaptations. Migrations provide favorable conditions for the development of juveniles as well as an abundant food base for adult passage fish. In addition, migrations give passage fish another advantage, which is that when changing the water body from one salinity level to another, they get rid of skin parasites, which die, leaving pale spots on the skin in the places of their former attachment. However, migrations require from fish a large expenditure of energy to overcome various obstacles (fast currents, rapids, waterfalls, etc.), which is possible only due to the accumulated in the body of fish a large stock of reserve substances, mainly fat (in rivers, adult passage fish, as a rule, do not eat). After spawning, many passage fish (salmon, herring) die, but some can migrate and spawn several times in a lifetime. Some of the passage fish have winter and spring races. Winter races enter the river with immature sexual products, usually reach high upstream spawning grounds, and overwinter and reproduce. Spring races enter the river with nearly mature sexual products and spawn the same year; their spawning grounds are located downstream. Many passage fish are important fisheries. Therefore, fish passages are built in dams during the construction of hydroelectric power plants.
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