Eels (family Anguillidae) are considered a delicacy in some countries, while in others they are not eaten at all or have only limited demand. Traditionally, Western Europe and Japan have been the main areas where there is high demand for eels. Probably the earliest form of eel culture, as distinct from the stew or holding ponds in Roman times, is the rather extensive system of lagoon farming along the Mediterranean coast. In Italian lagoons, eels form an important polyculture species, with grey mullets, seabream and seabass. The rapid expansion of eel farming in Japan from about the middle of the 19th century aroused considerable interest in intensive farming of this group and eel culture enterprises have developed in a number of countries in Europe, especially Italy, Germany and France. Taiwan has become a major exporter of cultured eels to Japan. As will be discussed later, aquaculture of eels continues to be based on seed eels collected from the rivers. Although some laboratory-scale progress has been made in maturing and fertilizing the eggs of some species of eels, it has not yet been possible to develop a system of artificial propagation. Reliance on natural supplies has led to periods of scarcity of elvers, restricting the expansion of culture enterprises. For instance, for almost a decade from 1952 Japan had to depend on the import of elvers from abroad, even to maintain the existing farms. As a consequence of this, the collection and export of elvers to Japan became an industry of some magnitude in a number of countries.
Although there are some 16 species of eels, the most important ones from the point of view of large-scale aquaculture are Anguilla anguilla(= vulgaris) in Europe (fig. 19.1) and A. japon-ica in Japan and Taiwan. They are known to becatadromous species and migrate from rivers and other inland water bodies into the sea for breeding, and the glass eel or leptocephali return to inshore waters and eventually migrate up the rivers. The Japanese eel spawns not very far from the coast, but the European eel migrates far out to the Sargasso Sea area of the Atlantic to spawn. The leptocephali of the European eel (A. anguilla) reach the continent three years after hatching and enter the rivers after they have metamorphosed into elvers; whereas the Japanese eels (A. japonica) enter the rivers as elvers within a year of hatching. The upriver migration starts when the temperature of the river has risen, in about May, and continues until the end of August. Once they start the upriver migration, they show remarkable endurance and ability to overcome barriers and obstacles and form thick shoals.
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