Fish are champions migrators
Migratory fish are true athletes: some of them swim thousands of kilometres during their migration from their birthplace to the places where they feed and reproduce. In order to complete their life cycle, they need free-flowing, unimpeded rivers. On their way, they have to overcome many obstacles, such as rapids, waterfalls and fishing nets, but also the claws and jaws of predators such as fish-eating birds, otters and bears.
The European eel makes the longest migration of all migratory fish. The eel’s eggs are laid and hatch in the Sargasso Sea, near the Bermuda Islands. After hatching, each small larva (leptocephalus) is shaped like a willow leaf and their transparency makes them difficult for predators to see. As soon as they are born, the larvae begin their long journey carried by the Gulf Stream, crossing 6000 kilometres of the Atlantic Ocean towards Europe. Near the coast, the larvae undergo their first transformation, changing shape and becoming ‘glass eels’, a small, transparent version of the adult eel. In Europe, eels enter river mouths and turn brown: at this stage they are called ‘yellow eels’. Eels explore rivers, lakes and canals in search of a habitat to grow. During the day eels hide, while at night they come out and prey on invertebrates and small fish. Once mature, they are called silver eels; these are the eels that make their way back to the Sargasso Sea, where they reproduce in deep water and then die. This species is now critically endangered. Dams, weirs and locks have made their migration to freshwater habitats difficult or impossible, and eel numbers have declined dramatically in recent decades. A specific European regulation was introduced in 2007 to reverse this trend.
Giant migratory fish
Some migratory fish can become very large, such as the pyraiba, an Amazonian catfish, which may grow to 3 metres in length and undertakes a journey of 4000 kilometres through the Amazon basin; it is said to be capable of swallowing a man whole! The Congo River in central Africa is home to the tigerfish, which has large, crocodile-like teeth, preys on other fish and sometimes jumps out of the water to grab passing swallows! The taimen, which is very popular with anglers, is the largest salmonid in the world and lives in the rivers of Siberia and the Amur River; it can grow to over 2 metres in length and live up to 50 years.