Wildlife experts say they are baffled about the rapid decline of eels in Welsh rivers.
They say the number making the two-year migration from the Caribbean has fallen around 70% since 1980.
The Wye and Severn rivers once teemed with eels and 30 years ago they were caught for export to the Middle East.
Environment Agency Wales has brought in limits on eel catches but believes the decline in eel numbers is probably due to conditions out at sea.
Fisheries project manager Ben Wilson said experts still did not know exactly where eels spawn but the smallests eels were found in the Sargasso Sea.
He told Radio Wales: "There's been a dramatic decline in the number of elvers [baby eels] returning to home waters in Europe, 95% lower than they were 20 years ago.
"And in England and Wales, elver return is around 70% down, so not at bad as the rest of Europe, but still very bad.
"There is evidence of reduced numbers of adult eels in rivers as well."
He said a number of climate change factors at play in the Atlantic Ocean, such as water temperature affecting eel reproduction, were having an unknown effect on eel numbers.
He said: "There are lots of things happening out there that we're not entirely certain about.
"There is a chance of eels being netted out at sea. They are netted when they get back to the [Severn] estuary.
"We're not entirely sure that exploitation is the big problem.
"We think there is probably something going on at sea but exploitation is not helping."
Young eels, called glass eels when they are at sea, are carried by the Gulf Stream and reach river systems across Europe.
Special river "ladders" are being used in parts of Wales 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.
They have been fitted in rivers in Carmarthenshire, Ceredigion and Bridgend.