As New Zealand eels have never been seen spawning, this part of their lifecycle is based on what scientists think happens. In autumn, adult eels leave fresh water and enter tropical seas somewhere in the South Pacific, where in deep water females release eggs. Males fertilise them. Adults die after spawning. Eggs hatch into larvae that float to the surface and drift back towards New Zealand. They may take about 17 months to arrive. They then change into glass eels – transparent baby 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.