Deep reef dive reveals new discoveries
Deep reef surveys of the Coral Sea Marine Park have found widespread coral cover at depths approaching 100 metres underwater, populated by dozens of fish species, in a surprise discovery for James Cook University researchers.
Working with Parks Australia, JCU researchers undertook three Remote Operated Vehicle (ROV) surveys at 15 reefs within the Coral Sea Marine Park (CSMP) in 2021.
Some 68 species of fish not previously known to live in what is referred to as the mesophotic zone were also identified, providing reef researchers with valuable insights into what goes on in coral habitats living amongst cooler, deeper waters.
“A lot of what we are seeing on the shallower parts of these reefs, from the surface to 15 metres deep, is that coral cover is going down. When we sent the ROV much deeper in certain spots, we discovered coral cover up around 80 to 90 per cent at depths between 80m to 90m which was unexpected,” said project lead and JCU Professor of Marine Ecology Andrew Hoey.
“We’re also not seeing localised human-caused stressors such as reduced water quality or fishing pressure on these remote offshore reefs. However, the impact of climate change on these reefs is evident.”
Prof Hoey suspected the clearer water surrounding these isolated oceanic reefs could be allowing coral to thrive in deeper environments.
“From what we’ve seen in the video footage, the deep-water coral colonies tend to be larger, which suggests they’re older and they haven’t been subjected to the disturbances we regularly see in shallow water corals,” he said.
The highest numbers of reef fishes were recorded at depths between 40m and 60m during the surveys.
The research team observed many coral habitat types including Mesophotic Coral Ecosystems, high complexity soft coral and sea fan habitats, Halimeda beds, unconsolidated soft sediments, and patch coral bommies.
Fellow JCU researcher and ROV pilot Dr Gemma Galbraith said it was “mind-blowing” to discover these ecosystems in the CSMP.
“It was incredible to see those abundant and diverse fish communities,” she said.
“It has taken the better part of six months to go through the videos and classify the different fish species and count coral sites.”
Dr Galbraith used a special set of forward-facing stereo-video cameras on the ROV, which enabled the team to assess the size and species of fish surveyed.
Film makers from production company Biopixel were also on hand to film, producing stunning footage of the deep coral habitats.
In addition to these discoveries, the JCU team used acoustic tags to track the movement of sharks within the CSMP.
They found small and large sharks made significant movements among reefs within the CSMP – most notably a grey reef shark which travelled about 760km across open oceanic waters from Osprey Reef in the CSMP to the Great Barrier Reef Marine Park and back in 25 days.
That journey is so far the largest recorded movement for a grey reef shark.
Prof Hoey said the exciting results of the surveys opened possibilities to explore even deeper reef habitats on future surveys in the CSMP, using a suitable high-tech ROV.
Prof Hoey and Dr Galbraith worked with a team from JCU, including Research Associates Dr Eva McClure and Dr Victor Huertas, Postgraduate Researcher and ROV pilot Ben Cresswell, Professor of Marine Ecology Morgan Pratchett and Dr Adam Barnett on the reef surveys and tagging study.
This Our Marine Parks Grants project received grant funding from the Australian Government.
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