They reach Spain and Portugal first, France next, and England last. So highly prized are they that at the start of this season in France, live elvers, which go for breeding, were selling for 3,000 Francs (pounds 300) a kilo. Here, as they begin invading the Severn, the price has fallen to pounds 60. Some of the catch will go for consumption, and some will be sent live to China.
Fishermen are out along the river banks every night, trying to catch them in fine-mesh nets, but nobody is sure what governs their movements. After an ocean trip lasting maybe three years, they come up-river on big tides. Much seems to depend on temperature: if the river is below 6C, they tend to stay off-shore. Once established in fresh water, they grow slowly to maturity - a process that may take from three to 10 years. Then, as adult eels, they return to the sea to spawn.
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