Eel populations in London’s river Thames have declined 98% in 5 years, say scientists. The Zoological Society of London (ZSL) is concerned that the species is not returning to the Thames, or is facing problems in the river and its tributaries. Other rivers in the UK have also seen declining eel populations. The eels take up to 3 years to migrate as larvae from the North Atlantic 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.