As New Zealand eels have never been seen spawning, this part of their life cycle is based on what scientists think happens.

In autumn, adult eels leave fresh water and swim from New Zealand to tropical seas somewhere in the South Pacific. The females release their eggs, the males fertilise them, and the adults die after spawning. The eggs hatch into larvae that float to the surface and drift back towards New Zealand. They may take about 17 months to arrive.

Larvae then change into glass eels – transparent juvenile eels. These enter estuaries and turn darker, from which point they are known as elvers. Elvers move upstream and find a suitable place to live, where they grow into adults.

Over a decade (or more) later, adult eels head out to sea to spawn, and the cycle continues.