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Threatened fishes: what future for migratory species?

The 'caged' rivers: damage on several fronts.

Rivers must breathe. In nature, watercourses flow through a riverbed flanked by large areas of expansion into which excess water pours during periods of flooding. The extension of urban centres and inhabited and cultivated areas, the construction of roads and the settlement of industrial plants have taken 'living space' away from rivers and streams. But the reduction and construction of cement cages to rectify the riverbeds has in many cases proved to be detrimental. Cement creates an impermeable layer that denies exchanges between river and underground waters, reduces the amount of oxygen dissolved in the water and shortening the river path causes an increase in the speed of the current and a greater erosive force.
The risk, which is becoming increasingly evident, is that of greater bank instability and flood risk.

The survival of migratory fish.

There is more: dams and non-essential river works block watercourses and prevent migratory fish from going back to where they were born to feed themselves, reproduce and give continuity to the species (often causing their death, as in the case of northern European salmon). Many migratory fish species are therefore seriously threatened by dams and locks, which interrupt the natural flow of rivers and make the usual migratory routes impossible. Migratory fish are a crucial link in the food chain and play an important role in the creation of healthy and productive river systems, as well as ensuring an important food supply for millions of people around the world.

21 April: World Migratory Fish Day.

The idea of a global event to raise awareness of the importance of 'open rivers' and migratory fishes, and to promote the creation of 'ascending stairs' to allow them to pass through, stems from the observation that the problem is underestimated. For this reason, April 21 is the World Fish Migration Day, an event aimed at protecting fish species at risk of extinction. One of the best known cases is that of eels (European eel and North American rostrate eel). The larvae of these fishes, leaf-shaped, are transparent; they are called leptocephalus and live in the sea. In the next stage, the larvae - while remaining transparent - assume a cylindrical shape (and are called blind), live near the coast or in freshwater, which go up to reach great distances from the sea. In these ecosystems their growth ends and pigmentation takes place until they reach sexual maturity, which corresponds to the time of migration back to sea. From research carried out on the larval stages and their distribution, it seems that the reproductive zones are two, well distinct, and are located in the Sargasso Sea. But it may not be the only place of reproduction: in fact, it is considered probable that the eels of the countries bordering the Mediterranean do not go towards the ocean, but mate and lay their eggs in some deep basin of the same sea, perhaps in some ditch of the Tyrrhenian Sea. The role of wetlands.

The protection of migratory fish also includes the protection of wetland habitats, which are fundamental to the life cycle of fish throughout the world. Wetlands provide areas where fish can reproduce. They are also spawning grounds and nurseries for fish, providing hidden niches for eggs between the edges of wetlands and underwater grasses. The vegetation of these formidable habitats provides shelter from predators and bad weather. Fishes also feed on plants and other organic matter in the area. But the list of virtues of wetlands is long: in addition to providing fish species with clean water, as a result of the function of filtering and removing sediments and pollutants, the wetlands also ensure an easier connection between rivers and the sea, necessary for migratory fish.

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