In the early hours of Saturday morning, the CSIRO research vessel was working hard to survey the sea floor south of Montague Island. Associate Professor Martina Doblin of UTS, Professor Iain Suthers of UNSW and Amy Nau of the CSIRO Marine National Facility were aboard. Ms Nau noticed something different.

“A rapid change in the profile of the sea floor captured by the multi-beam sonar system showed that the ship had passed over a reef, popular with locals and known as Twelve-Mile Reef,” the trio said. 

“Even more distinctive was the output from the fisheries echo sounder. The bioacoustic signals of this sensitive equipment showed plankton and smaller fish in surface water, as well as schools of larger fish in bottom water, close to the reef. 

“The reef, 4km south of the listed position, was revealed in unprecedented detail by the ship’s state-of- the-art swath mapper. The reef appears to be the remnant of an old volcano, submerged more than 110m below current sea level. At the top of the reef is a circular depression approximately 200m across, only 15m above the surrounding continental shelf.”

The search began after a tip from someone familiar with Narooma waters. The team had an approximate position and set to work. “Amy saw the relief of the ocean floor change quite quickly,” Prof Doblin said.

“The plotter amplifies the vertical height in colour. All of a sudden the colours are changing to magenta, which indicates there is a significant change in depth.”

Moving at eight knots, the crew watched something extraordinary show itself. “We are slowly seeing this thing being uncovered,” Prof Doblin said.

“It is almost like when you rub something out with a coin. We turned back and it basically grew out of the ocean. We felt really pleased we had uncovered something that would have relevance for locals in particular.

Prof Suthers is excited by its history. “It’s entirely feasible that this ancient eroded feature hosted indigenous Australians over 10,000 years ago, when sea level was lower during the last ice age,” Prof Suthers said.

“It’s also remarkable that such a shallow rocky reef can influence the distribution of pelagic fish. Clearly the reef generates an ecosystem.”

It is that crater that is a bounty for larval fish, Prof Doblin said. “It creates an upwards lift in the water and helps feed the reef with nutrients,” she said. “It stimulates plankton and larval fish feed on the plankton.”

The volcano is eroded; more of a hill than a mountain, but remains important for marine life. “We want to understand the food web better in that region,” Prof Doblin said.

“Even a small feature that stimulates plankton production in that area is important.

The bottom survey forms part of a wider study on linking the oceanography of the region with ecosystem functioning and the marine foodweb.

Scientists on board are researching the impact of the East Australian Current and its eddies on biological processes driven by microbes, plankton and larval fish. 

Given the projected increase in the southward extent of the current, determining how the ecosystem is functioning now will help them understand how it will function into the future.

“Given that a change at the base of the foodweb will have a cascading impact on ocean productivity, our research has important implications,” Prof Doblin said.