Eel populations in the river Thames have crashed by 98% in just five years, scientists warned today.

The eel, which has been a traditional east London dish for centuries, now appears to be vanishing from the capital's river, according to researchers from the Zoological Society of London (ZSL).

Each year, ZSL's Tidal Thames Conservation Project places eel traps in a number of the river's tributaries, to catch the fish and allow scientists to record numbers before setting them free.

While 1,500 were captured in the traps in 2005, just 50 were recorded last year.

The eels 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 return journey across the Atlantic to spawn and die.

But 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 it was considered "biologically dead" in the 1960s, and there are fears the rapid collapse of the eel population could have knock-on effects for other species in the still-fragile ecosystem.

Other rivers in the UK are also seeing declines in eel populations, ZSL said.

Dr Matthew Gollock, tidal Thames conservation project manager, 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 including changes in oceanic currents due to climate change, man-made structures such as dams and the presence of certain diseases and parasites."

And he said there was a need to find out why the declines were happening, in order to save the eels and help other species in the estuary's food web who would be affected by its disappearance, such as birds which feed on it.

"Time appears to be running out for eels in the river Thames and this could have a domino effect on other species in the Thames," Gollock said.

"The Thames is a very urban, developed estuary. It's much healthier than it was 50 years ago, but there is constant pressure on it.

"It's quite a precarious ecosystem and the fast removal of any species — whether it is a fish or a plant - is going to upset the balance," he said.