Tilde's Talks: The Mysterious Journey of Aotearoa’s Long-Finned Eels
You loved Charlie’s Chats, but now we are introducing Tilde's Talks! Tilde Sorensen, a Waiheke local and a Te Korowai o Waiheke Field Team Technician, graduated from the University of Auckland in 2022 with a Master's in bio-security and conservation, specialising in monitoring invasive and native skinks on islands. She played a key role in setting up the stoat pilot on Waiheke Island in 2016 and is currently training to become a stoat detection dog handler.
Many of us will have had the pleasure of swimming or sitting on a stream bank and encountering an eel (or tuna). While their serpent-like appearance may not lead them to win a beauty contest, eels are absolutely fascinating creatures. In Aotearoa, there are two native species of freshwater eel: the short-finned eel, also found in Australia, and the long-finned eel, which is endemic. Long Finned eels can be differentiated from their short-finned cousins because the dorsal fin will be longer than the anal fin. In short-finned eels, the fins will be around the same length.
What makes long-finned eels so fascinating is their lifecycle and the mystery surrounding it. The spawning of eels happens outside Aotearoa and has never been observed. The exact location is a mystery but it is somewhere in deep ocean trenches of the Pacific. The eggs will float and many will be eaten by other fish. Once the eggs hatch the eels are tiny, flat and transparent. These larvae will float back to Aotearoa on the ocean currents.
Once they reach the shores of Aotearoa, the eel larvae that have managed to survive the journey transform into the familiar eel shape but will remain clear, at this stage, they are called glass eels. Slowly, they make the transition from saltwater to fresh and gain their darker colouring. The tiny eels now make their way up the rivers and streams past many bossy and territorial older eels until they find a suitable habitat. Here they will live and grow.
Eels are very long-lived with the males sometimes reaching 35 years. Females are larger and have a longer lifespan, with the oldest recorded catch being 106 years and 24 kg!
They feed on insects, fish, and the occasional duckling. Eels are not aggressive but don’t have good vision so may mistake a finger for food.
New Zealand Long fin eel in stream by Brian Scantlebury
When the adult eels reach a ripe old age, their bodies will begin to change: their eyes will widen and get larger and their internal organs will be absorbed, resulting in a streamlined body shape and eyes adapted for the deep ocean. During autumn floods, the eels will move down to the sea, transitioning back to saltwater. Their last journey is back to the Pacific Ocean trenches where they hatched and where they will spawn and die.
The unusual life cycle means that eels will only spawn once at the end of their lives. While this life strategy has historically worked well for them, the advent of commercial fishing has placed them on the IUCN (International Union for Conservation of Nature). red list of threatened species. In 2023, nearly 100 thousand tons of live eels and eel meat were exported. Eels are also affected by habitat loss and dams which block their migration to the sea. Because eels are so long-lived, we may not notice the decline for several decades but fewer and fewer seem to be arriving in Aotearoa each year.
The best thing we can do to protect these special creatures is to restore their habitat by planting stream banks, removing obstacles, and creating fish ladders at culvert overhangs which may prevent the movement of eels, this could be as simple as piling up rocks to make a gentle sloping waterfall or, as featured in this link fishladdersolutions.co.nz
— Written by Tilde Sorensen
