May, once the temperature has reachedabout 10 degrees celsius, the glass eels make their migration from estuaries to freshwater.

Populations of eels have become increasingly threatened as the number returning to European rivers has declined dramatically over recent decades. In one study, it was estimated that numbers had fallen by 90% since the early 1980s, and recent studies suggest the number of new glass eels entering British rivers has now dropped to 1% of former levels. Eel populations are now considered to be “outside safe biological limits”
The crash happened over the whole European continent with no single, obvious cause. Suggestions for possible causes have included over-exploitation, inland habitat loss, climate and ocean current change, disease and pollution.

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