Tuna/eel release to keep population thriving

Article by Elizabeth Hibbs

Aotearoa’s tuna/eels are in trouble. We have two native species, the longfin and the shortfin, and populations of both are steadily shrinking due to habitat loss and commercial fishing. Dams in streams and rivers can also prevent tuna from completing their lifecycle, which involves long trips up through the Pacific Ocean. The shortfin is found in other countries, but the longfin is endemic to Aotearoa – found nowhere else in the world. Its conservation status is ‘at risk: declining’.  

The tuna kuwharuwharu/longfin eel, as endangered as our national bird the kiwi, is a taonga species for Māori. For generations tuna, while also being an important food resource, have been respected as kaitiaki of the streams, rivers and lakes they live in, and as environmental indicators in those waters.  

In ZEALANDIA ecosanctuary’s lower reservoir Roto Kawau and its streams there are several longfin tuna/eels. They’re safe from fishing in the sanctuary. But they have a very particular problem. Tuna breed only once, at the end of their lives. When the time comes, they swim downstream, driven by an urge to return to where their lives began, thousands of kilometres away in the Pacific Ocean where they gather in a mass spawning event.  

The tuna at Zealandia, though, are land locked. The lower lake, Roto Kawau, flows out into the Kaiwharawhara Stream through an outlet valve, dating back to when it was a reservoir for Wellington’s water supply. When some of these tuna were tiny elvers, they climbed up and over this machinery, just as they climb up natural rocks and waterfalls on their way to find a stream to call home. Others were rescued and placed in the reservoir many years ago. But when they’re ready to spawn, and swim back the way they came decades before, they will not be able to swim through the outlet valve.  

Once released into the Kaiwharawhara Stream, they’ll be free to live out their lives in the habitat that suits them and leave to breed when they’re ready. No-one knows for sure where the eels’ spawning grounds are, but we know they swim thousands of kilometres, congregating way out in the Pacific Ocean. Their eggs hatch into tiny leaf-shaped larvae which drift back to Aotearoa on the ocean currents, like dandelion seeds on the breeze, following the scent of freshwater at the coast to find a river mouth.  As tiny transparent ‘glass eels’ the new generation hide in the estuary at the river mouth, slowly growing into elvers, juvenile eels that are strong enough to swim and climb upstream to find a home.  

It could be that one day a new generation of eels find their way upstream to the sanctuary of ZEALANDIA, but for now we must let them go so they can play their part in keeping this rare and wonderful species alive. 

References:

https://en.wikipedia.org/wiki/New_Zealand_longfin_eel 

https://www.doc.govt.nz/nature/native-animals/freshwater-fish/eels/