From the Guardian, 20 April

This is the peak season for the arrival of what is hoped will be millions of glass eels swimming up Britain’s estuaries to reach fresh waters where they can grow into adults. The European eel (Anguilla anguilla) is a curious creature with a complex lifestyle that is still barely understood.

Eels are thought to begin life as eggs in the Sargasso Sea; drifting as larvae for about a year 4,000 miles north-east in the Gulf Stream before turning into tiny transparent eels. Their goal is to find a home in rivers, lakes and ponds while growing darker and larger for up to 10 years. When nearly a metre long they set off back across the Atlantic to breed.

Eels were once common but the species is now critically endangered. This is partly because many rivers have been blocked with weirs and dams and others polluted. Another only recently discovered reason is that tinned and processed foods made with eels, analysed in Hong Kong and thought to be equally scarce Asian eels, turned out to be the European variety. The glass eels had been caught illegally in Europe and shipped to Asia to be grown in eel farms. Despite these setbacks, a Europe-wide effort to protect eels and help them upstream is having some successes.