Glass eels are juvenile forms of eel that begin life in the ocean trenches of the Pacific and migrate hundreds of kilometres to New Zealand.
Adult eels breed in the tropics and spawn glass eels. Between July and December millions of glass eels drift with ocean currents to freshwater in New Zealand. They do not migrate to the river or stream where their parents come from – recruitment (the addition of new eels to a population) is random. If eels homed to a particular river they would form genetically distinct groups in freshwater, and there is no DNA evidence that this is the case.
It is also unlikely that male and female eels from the same waterway would mate together. Mating is thought to be a highly random process, probably involving several males fertilising a single female.
Eels have a lifespan of decades. Eventually they return to the tropics where they breed and die.
Juvenile eels are between 5.5-6.5 cm long and are known as glass eels because of their transparent bodies.