Longfin eels breed only once , at the end of their life. When they are ready to breed, they leave New Zealand and swim 5000 kilometres up into the tropical Pacific to spawn, probably in deep ocean trenches somewhere near Tonga. When they reach their destination, the females lay millions of eggs that are fertilised by the male. The larvae are called leptocephalus and look nothing like an eel - they are transparent, flat, and leaf-shaped. The larvae reach New Zealand by drifting on ocean currents [which can take 1 to 2 years]. Before entering fresh water, the leptocephalus change into a more familiar eel shape, although they remain transparent for up to a week after leaving the sea. These tiny "glass" eels enter fresh water between July and November each year, often in very large numbers. Eels take many years to grow and it could be decades before an individual is ready to undertake the long migration back to the tropics to breed. The average age at which a longfin eel mi...