They ought to be wriggling through briny water and marshy flatlands in their hundreds of thousands right now.

But the mystery of the vanishing eels is troubling fisheries officials, conservationists and fishermen who for generations have hunted the curious animal.

A conference in Somerset on the plight of the eel, which was attended by experts from across Europe, has been hearing this week that the eel is in crisis.

The number of European eels across the continent has declined by as much as 95% in the last 25 years, the Environment Agency says. Officials report that the number of young eels arriving in Britain's estuaries, rivers and streams this spring is significantly down on last year. Andy Don, an Environment Agency fisheries officer who has studied the eel for 20 years, said: "There is no doubt that there is a crisis. People have been reporting catching a kilo of glass eels this year when they would expect to catch 40 kilos. We have got to do something."

But the action the Environment Agency is about to take is upsetting those who rely on the eel for their livelihoods. A ban on exporting eels out of Europe - they are a popular dish in the far east - is proposed, along with a plan to severely limit the fishing season and the number of people who will be allowed licences. Some argue that such moves will effectively kill off eel fishing.

Don admitted that it was not at all clear why eels seemed to be vanishing in such large numbers. "The bottom line is we just don't know why they are struggling so badly," he said.

One reason may be that man-made structures such as weirs and dams are stopping glass eels - young eels a few centimetres long - reaching the freshwater habitats where they mature. If this is true, the plight of the eel could get much worse as hundreds of hydro projects are planned in Europe.

Another theory is that a parasite may be killing them off, while some blame illegal fishing methods. At one point a kilo of young eels was worth as much as £500, tempting some fishermen to use illegal nets to scoop as many up as possible. A kilo is still worth £210.

Many believe the shifting of the Gulf Stream means that not so many glass eels are being swept from the Sargasso Sea close to Bermuda, where they are born, to the shores of Europe, while others say there was a surge in the number of eels a quarter of a century ago and the population is now returning to normal.

The Environment Agency has launched schemes - such as building fish passes - across England and Wales to help the glass eels. In East Anglia traps are set up to catch eels, which are released further upstream ahead of impassable obstructions. In the south west of England CCTV cameras record the glass eels coming in from the estuaries on to the Somerset Levels.

Not everybody is convinced. Peter Wood, managing director of UK Glass Eels in Gloucester, called for much more investment in measures to protect the creature rather than fishing being restricted. "In some European countries they spend hundreds of thousands of pounds," he said. "Here they spend hardly anything."

Wood was opposed to measures such as limiting the season. He said that his main business now involved catching glass eels and selling them for farming and restocking projects in Europe. "We need every glass eel we can get hold of," he said.

David Bunt, vice chairman of the Institute of Fisheries Management, which speaks on behalf of commercial and recreational fishermen and women, said the organisation supported action to protect the eel but warned that many individuals would be angry if their livelihoods were affected.

Roger Castle, who has fished for silver eels - the mature eel that returns to the Sargasso Sea to breed after growing in European waters for as long as 40 years - believes the Environment Agency is paying only "lip service" to the fishermen and really wanted to see an end to eel fishing.

Castle, who has fished the Rivers Avon and Stour in Hampshire and Dorset for 30 years, said: "Limiting the season will mean it is not worthwhile for people like me." The eels, he argued, do not arrive and depart at set times to tie in with a restricted season.

"I would much rather they imposed a quota that limited the number of eels we take out but allow us to go whenever we want," said Castle, whose eels are sold on to a smokery. "They will put people like me out of business if they try to restrict us. And it would be a shame to lose those old skills."

A fish's tale

The eel remains one of the world's most mysterious creatures. It is generally accepted that European eels - Anguilla anguilla - are born in the Sargasso Sea near Bermuda.

As leaf-like larvae, they are swept by the Gulf Stream towards Europe, a journey that may take a year. When the larvae reach the continental shelf they change into "glass eels" and in the spring begin to move through estuaries and into freshwater.

The animals develop pigmentation, at which point they are known as elvers and are similar in shape to the adult eel. Elvers continue to move upstream and again change colour to become brown or yellow eels.

When the fish reach full maturity - some can live to 40 and grow to 1m long - they migrate back to the ocean. Females are reported to carry as many as 10m eggs. They return to the Sargasso Sea, spawn and die.