Eels were once widespread in Carmarthenshire’s rivers but there has been a rapid decline of this mysterious creatures about which there is still a lot left to learn.

Eels are a major part of the diet of wildlife like otters and herons and form an vital part of the Welsh freshwater ecosystems. In addition eels also provide an income to 250,000 fishermen across Europe.

It is now generally thought that European eels (Anguilla anguilla) are born in the Sargasso Sea near Bermuda and as larvae are swept by the Gulf Stream towards Europe, a journey that may take a year. Changing into "glass eels" when they reach the continental shelf, in the spring they begin to move through estuaries and into freshwater.

Now the eel larvae develop pigmentation, at which point they are known as elvers and look like smaller versions of adult eels. These elvers continue to move upstream and again change colour to become brown or yellow eels.

When eels reach full maturity - some can live up to 40 years and grow to 1 m long - they migrate back to the ocean where they return to the Sargasso Sea, spawn and die.

Across Europe eels have declined by as much as 95% in the last 25 years, according to the Environment Agency. Reports show that the number of young eels arriving in Britain's estuaries, rivers and streams each spring is significantly down on previous years. Catches of glass eels are well down – expected catches of 40 kilos have been reduced to 1 kilo. This important fish species certainly seems to be in trouble - which is of concern to fisheries officials, conservationists and fishermen.

One reason for their decline may be that artificial structures such as weirs and dams are stopping glass eels reaching the freshwater habitats where they mature. Other theories are that a parasite may be killing them off, or that illegal fishing methods are taking their toll.

The Environment Agency are installing special river "ladders" in parts of Wales, including at Dolbantau weir on the River Clettwr in Llanfihangel-Ar-Arth, Carmarthenshire in an attempt to halt a decline in the eel population. The ladders are brush-like in design and provide eels with the grip to climb up and pass weirs that block their way. Using these special ladders, they can get past these weirs and reach the wetlands and habitats upstream where they can mature and thrive.