For the first time scientists are analysing Fiordland's iconic black corals in deep waters to see how changes, such as marine heatwaves, impact them.
The corals help remove carbon from the ocean and create important habitats for fish and other organisms that feed our fisheries.
The National Park's majestic mountains plunge straight into the ocean and are carpeted in lush beech forest. But dive below the surface and you'll find another kind of tree, called black coral. It actually looks white, but that's because the tissue covers its black skeleton.
Victoria University's Marine Biologist, Professor James Bell told Newshub it's likely the ancient organism has been growing in Fiordland since the 1600s.
"We've seen just this morning one that was nearly four metres across that's probably hundreds of years old and with all kinds of cool things growing over it", said Bell.
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It's estimated about 10 million black corals live in Fiordland on its reefs and sea floor, creating 3D structures for small fish and a home for many different organisms.
"There are species living on dead parts of the black coral, which we think is completely natural, they're associated species that we don't find elsewhere, they seem to really like living on black corals", he said.
By using an underwater drone Victoria University's marine biologists are the first to ever survey the abundance of the corals in deep waters of between 50 and 100 metres and collect samples from there too. The drone is funded by the George Mason Charitable Trust and has been crucial in researching many of New Zealand's deep water habitats including Northland's Poor Knights Islands Marine Reserve.
And Wellington's reefs.
"A really cool part of the project is we are exploring the really deep water parts of Fiordland. We are finding toward the outer parts of the Fiords the black corals extend all the way down," he said.
The project is investigating if Fiordland's black corals are genetically linked to each other, which will determine if the risks they face from marine heatwaves and changes in rain fall, could completely wipe the iconic species out.
"If there are populations that are isolated and they get damaged or destroyed there is possibility they may take long time to come back, or not come back at all - if there are no sources of new babies into that population", Bell said.
Which would be a major loss for Fiordland's biodiversity. "And a major failure by humans to look after our oceans", Bell said.
They have a DOC permit to collect samples of the protected species, which are being DNA sequenced to see where the baby corals come from and whether deep black coral populations can seed those in the shallows, if the shallow ones die.
Victoria University PHD Student Mirriam Pierotti is slicing the samples into small fragments, then placing them into little test tubes of preservative solution to protect the DNA structure for future analysis.
"We are trying to look at different genetic population analysis. We want to see if the populations in inner part, middle part and outer part of Fiords are connected somehow," said Pierotti.
In the 1980s and 90s researchers analysed Fiordland's black corals, but focussed on shallower waters where SCUBA divers could reach, so this recent research is also looking at whether abundance has changed since then.
The group hopes to build a clearer picture of black coral health, and how to protect them in future.
"We are also collecting information to try and find out more about black coral growth and mortality rates, to build a model which will help understand how changes in marine conditions might impact them," said Bell.
It's unusual to see the black corals in such shallow water, but that's because of Fiordland's high rainfall. The ocean has a deep freshwater layer that sits above the seawater and is full of tannins from the beech forest run off, which creates a much darker environment for the corals.
So instead of living in thousands of metres of water, you can find the corals as shallow as just five metres.
"It kind of tricks them into thinking they are in deeper water," said Bell.
The research project is partly funded by the Department of Conservation, who provides the boat for the scientists live on during their week long trip. DOC's senior ranger for Fiordland is Richard Kinsey, who's worked in the area for 15 years and is excited to learn more about black corals.
"It's a habitat-forming species and one of those species that everyone thinks about when they think of Fiordland. It's really useful for us to know more, to help understand it and safeguard it for the future", he said.
After consecutive marine heatwaves, many of Fiordland's sea sponges died from bleaching, which is why DOC wants to keep a close eye on all its species - including black corals.
"There are a bunch of impacts that we need to keep on top of and understand better. With climate change and the warming oceans there's starting to be an effect on species and it'd be nice to know if that travels through to black corals and other species", said Kinsey.
He said the underwater drone is a game-changer, because it's gathering data that hasn't been possible until now.
"We start to understand how deep the corals are and not just looking at the shallower band of them, but the much deeper ones as well", he said.
They're hoping that future projects will further investigate the impacts of climate change on black corals and other special species.
