Fish populations in rivers

Of all aquatic organisms, fish are among those that possess the greatest mobility, thus offering the possibility of rapid occupation and exploitation of the various biotopes in the fluvial mosaic. For those fishes that migrate in order to reproduce, the longitudinal course of the river represents the principal means of access to spawning grounds. Long-distance migrants may ascend the entire length of the river to reach its headwaters; whereas non-migratory species, such as some cyprinids, use the longitudinal course of the river merely as part of the fluvial network to move between various transversal parts of the hydrosystems (e.g. side channels, ox-bows) for different periods in their ontogeny.