Scientists say a number of factors may be responsible for the decline Eel populations in the River Thames have fallen by 98% in just five years, scientists have said.Every year the Zoological Society of London (ZSL) records the numbers of eels by capturing them in traps at tributaries before releasing them. While 1,500 were captured in the traps in 2005, just 50 were recorded last year. Conservationists fear the number fall could have a knock-on effect for other species in the river's ecosystem. 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 (6,500km) return journey across the Atlantic to spawn and die. 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. 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 re-colonise 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 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 which would be affected by their 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," Dr Gollock added.