The European eel may have a snake-like appearance, but is in fact a fish. All European eels hatch from tiny spherical eggs in the Sargasso Sea, in the western Atlantic Ocean and embark on a migration that will take between 1 and 2 years. The tiny eggs float on the sea currents and hatch into larvae, called leptocephali which are are about 5mm long. The leptocephali float on currents towards Europe, on a journey that is over 6,500km.
By the time the leptocephali reach the continental shelf of Europe they will have grown into a glass eel and it is at this stage of the life cycle that they enter the Thames Estuary.
As they move up the Thames, they develop into elvers. The elvers need to spread out within the river system and swim up tributaries to find suitable habitats and food to enable them to grow.