Eel populations in the River Thames have fallen by 98% in just five years, scientists have said.
The numbers of eels – which have been a traditional east London dish for centuries – fell from 1,500 in 2005 to just 50 last year.
Conservationists fear this could have a knock-on effect for other species in the river’s ecosystem.
The Zoological Society of London records the numbers by capturing eels in traps before releasing them.
They are thought to take up to three years migrating as larvae from the Sargasso Sea to European rivers, where they spend up to 20 years before making the 4,000 mile (6,500km) return journey across the Atlantic to spawn and die.
Conservationists are concerned the species is not returning to the Thames, or is facing problems in the river and its tributaries.
European eels and flounders were the first species to recolonise the Thames Estuary after being considered “biologically dead” in the 1960s.
Dr Matthew Gollock, tidal Thames conservation project manager at ZSL, said: “Eels are mysterious creatures at the best of times but we are very concerned about the rapid disappearance in the Thames.
“It is difficult to say what is going on – it could be due to a number of potential factors …