ABOUT 50 kilometres south of Tasmania’s coastline, the sea-floor falls steeply to depths of a kilometre or more. At the bottom of this drop lies a landscape containing 130 extinct volcanoes or underwater mountains – the largest up to 500 metres high and several kilometres across at the base.
These seamounts, found at depths of 500 to 1500 m, teem with life: hard and soft corals, sponges, crabs, lobsters, seastars, brittle stars, sea urchins and fishes, including the much-prized commercial species, orange roughy.
Scientists believe the world’s oceans hide tens of thousands of such deep-sea seamounts. Yet they don’t know how many others support such a high biodiversity, as they know little about the ecology of the species and communities that populate them.
This lack of knowledge has partly been due to the challenge of getting advanced remote sensing equipment – such as high-definition digital cameras – to work effectively and reliably at crushing pressures of 100 atmospheres or more.
Mapping and documenting life in our deep ocean is becoming a pressing priority, however, as humans dive deeper for Earth’s resources to meet their needs for food and minerals.
“Until recently, the impacts of human activity in the deep sea have mainly been from bottom-fishing, particularly for orange roughy, and that’s been CSIRO’s focus in the past,” says Alan Williams, a deep-sea ecologist with CSIRO.
“But increasing deep-sea activity associated with oil and gas extraction and seabed mining means our research now includes a broader range of environmental impact assessments and ecological monitoring.”
Sea-life in the slow lane
In December 2018, a CSIRO team – led by Williams and based on Australia’s Marine National Facility research ship Investigator – ‘flew’ a hi-tech underwater camera system over 33 of Australia’s southern seamounts.
In total, the one-month Investigator voyage completed 147 transects – most of them 2 kilometres long – netting an unprecedented haul of high-quality images and data from these ecosystems.
CSIRO first began surveying seamounts in the 1990s, when Australia’s orange roughy fishery was very active. At that time, Australians couldn’t get enough of the fish, but populations soon began to decline from the industry’s efforts to meet market demand.
Because orange roughy don’t mature until 25–30 years of age, grow more slowly than most other marine fish, and can live to 100 years or more, the high fishery catches were not sustainable.
The fishery’s early years also left a mark on the hard-coral communities living on the seamounts. These corals build up reefs that provide habitat for soft corals and other life. The result is a complex faunal community and food web, all based around the coral structures.
Again, it was the slow growth rate of reefs that meant the impacts of fishing were expected to be long-lasting.
Signs of recovery?
CSIRO surveys carried out in 1997 and 2007 showed what looked like dead zones, scraped clean of corals and other signs of life, inside many areas trawled in the early 90s.
During the 2018 survey, however, Williams was surprised to see large aggregations of orange roughy on two heavily trawled seamounts.
Importantly, one is now located within a marine park established by the Australian Government 21 years ago. Williams notes that, while the fishery had some impact on coral communities, the design of the marine park design benefited from knowledge provided by that industry.
“The question of ecosystem recovery, particularly the contributions made by marine parks, is key to our surveys,” he notes.
“In terms of stock recovery, it’s a bit premature to make conclusions, but at face value, there are a lot of orange roughy now where there were none 10 years ago.”
Mapping natural refuges
Data collected during the 2018 survey are currently being analysed, while animal specimens have been sent to various museums for identification.
“The full evaluation of fishing impact and ecosystem recovery is some time away – we still have a lot of work to do on the data,” Williams says.
“But when it’s done, we’ll have a much better understanding of what resilience might mean in these deep-sea communities. That’s attributable to the amount and quality of the imagery we collected.”
During the 2018 voyage, Williams noted that, even on the most heavily trawled seamounts, craggy surfaces seem to play an important role in habitat recovery.
“They’re natural refuges with relatively high abundances of corals, from the big fans that grow a metre or more tall, to the most delicate species – despite some areas around them being almost swept clear of any fauna.”
“Mapping these refuges and identifying their role in providing larvae for recruitment to colonise surrounding areas is an important outcome from this survey.”
Unique design and build
The CSIRO towed platform features two high-resolution stereo stills cameras, separate high-definition video, and separate vision system for obstacle avoidance.
“Seamount terrain is steep, rugged and rocky, but now we can safely tow and manoeuvre the equipment at depths of between 700 and 2000 metres,” says Williams.
He also notes that the calibrated stereo high-res digital images allow ecologists to estimate the camera field-of-view and identify, count and measure animals with unparalleled accuracy.
The camera system was custom-designed and built by engineers and technicians at CSIRO’s marine research labs in Hobart, under the guidance of instrumentation and electronics group leader, Matt Sherlock.
Sherlock says CSIRO’s 20 years of experience in designing and deploying sensing equipment was an important factor in the system’s success at sea. Having engineering, software, and build skills under one roof was another.
“It means our systems are reliable and tailored to meet science needs,” he says.
Sherlock’s team designed and tested the various camera system components for a depth tolerance of 6000 metres – well beyond the depth required for the most recent seamount deployment, but ready for future deeper missions. Lens ports are designed to eliminate pressure-related optical distortions that can cause errors in stereo measurement.
At sea, the biggest challenge facing technicians is ‘landing’ the towed platform on a seamount and gliding it down the often-steep and rough slopes.
“The camera system is 1.5 to 2 km behind the ship at the end of the tow cable, and the ship has to start a run-up to the target several kilometres beforehand,” explains Sherlock.
“You have to be able to ‘land’ the camera platform on the seamount peak and then maintain the platform about 2 to 3 m off the bottom by managing ship speed and accounting for deep-sea currents”.
Ripple effect
For ecologists like Williams, the results of the 2018 survey will provide a much clearer picture of the timing and extent of recovery of deep-sea ecosystems after human impacts.
“After three surveys over 20 years, we now have information about trends and indicators of the health of these seamount environments – including knowledge of the biodiversity we’ve protected within our marine parks,” he says.
“Nationally, this information will be incorporated into Australia’s State of the Environment reporting. It’s a measure by which Australia can be compared and assessed internationally on its performance to look after the marine environment.
“Similar communities occur on seamounts and other deep-sea areas throughout the world’s oceans, and many of them are in areas beyond any national jurisdiction.
“The whole question about recovery of these deep-sea communities following human impacts is a big knowledge gap internationally. We hope that what we find here will inform management of deep-sea fisheries around the world.
“And not only fisheries, but increasingly the human footprint in the deep ocean is increasing through oil and gas exploration and seabed mining. Whatever we can learn about impacts and recovery has application across those other areas.”
Find out more about the Deep Tow Camera system