The following text is for questions 17 to 20.

    Migration, in ethology, the regular, usually seasonal, movement of all or part of an animal population to and from a given area. Familiar migrants include many birds; hoofed animals, especially in East Africa and in the Arctic tundra; bats; whales and porpoises; seals; and fishes, such as salmon.

    The migration cycle is often annual and thus closely linked with the cyclic pattern of the seasons. The migration of most birds and mammals and many of the fishes are on a yearly cycle. In many cases (e.g., salmon and eels), animals with a relatively long life span return to their place of birth in order to reproduce and eventually die. In other cases, as in certain invertebrates, where the animal has a relatively brief life span and reproduces rapidly, migrations may not occur in every generation.

    Most migrations involve horizontal travel The distance traversed may be a few miles or several thousands of miles. Some migrations take a vertical direction and involve no appreciable horizontal movement Certain aquatic animals, for example, move from deep water to the surface according to the season. Certain birds, mammals, and insects migrate altitudinally in mountainous areas, going from the upper zones, where they breed, to the foothills or plains during seasons when the weather is severe and unfavourable. Such vertical travels involve essentially the same type of environmental change as horizontal, or latitudinal, migrations over long distances.

(Adopted from: https://www.britannica.corn/science/migration-animalVFish (April 4, 2020) 

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