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Out of the 25 000 tons of eels annually produced in Europe, 15% come from eel farms. Most of the farmed eel production (85%) is from the Mediterranean. Out of the remaining 15%, four per cent is farmed in heated effluents and 11% in recirculated water (EIFAC, 1987; Heinsbroek and Kamstra, 1990).
The most common method of eel culture is in pond farms (fig. 19.2), the ponds being comparatively small. The elver ponds are about 100–350 m2 and the stocking ponds about 1000–1500 m2. They may be of the still-water type with occasional exchange of water, or running water ponds with a flow-through of varying velocity. The latter type permits intensive stocking and feeding for high production.
Another system practised in areas with abundant supplies of warm water from springs is the tunnel method. Rearing is carried out in concrete tanks of 1 m2 surface area and a depth of 1 m, with an inlet tank and an outlet tank. Water enters the main tank from the inlet tank through a 23 cm diameter pipe and drains out through the outlet tank. This system is suited for highly intensive production.
The recirculation system has also been adopted on a limited scale for eel culture, to enable highly intensive tank culture in areas with a limited water supply.
The systems mentioned above are based on the use of fresh water. Making use of the adaptability of eels, systems of culture in brackish and sea water have also been developed. The extensive culture in the Mediterranean lagoons (valli) is based on elvers which are allowed to enter the impounded areas through the manipulation of tidal flows and which are grown in the fertile lagoon waters. In ponds supplied with sea water, eels are reported to grow faster, even though the higher rate of H2S formation in salt-water ponds creates problems when the oxygen concentration of pH is low.
The culture of eels in net enclosures in sheltered bays, back waters, etc., has been tried to reduce the capital costs of pond construction and avoid problems of water supply. As eels grow better in warm-water environments, wherever possible heated water from thermal stations or industrial sources, as well as from hot springs, has been used.
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