We left the island of Egmont overnight for what the charts suggest is a field of shallow seamounts, rising from 1,000m to less than 100m from the surface, to the northwest of Diego Garcia. We have identified four candidate seamounts that, according to the charts, rise to between 63m and 94m of the surface ... but we’re not sure they are actually there!
We arrive at the first waypoint at dawn with the Marlin running a search pattern over the area. This first seamount is supposed to rise to 94m. Eyes are fixed on the echosounder which can ping to 800m. We track south, then west, then northwest with a 3 nautical mile range. Anticipation turns to quiet disappointment as we are unable to find it. Not even a hint of seabed even at 800m.
But determined, we move to the next location. Eyes still glued to the echosounder, there is a whoop of excitement on the bridge as we start to see the bottom shoaling. Moving at 2.5 knots, we see in rapid succession 800m, 500m, 200m, 150m, 100m and then a plateau at about 65m. We mobilise long ropes, the BRUV rigs and begin dropping across the plateau from the FRC (Fast Rescue Craft) which has been our workhorse throughout this trip under the able hands of Chief Engineer Les Swart.
Twelve BRUV deployments later, with our brawny but exhausted crew having hauled the BRUVS up from 65m to 85m on a beautifully still but swelteringly hot day, the depths were revealed. Crystal clear bright water greeted us even at 85m. The top of this seamount hosted a garden of seawhips, soft corals, and sponges. Inquisitive silvertip sharks were abundant, a large marbled ray ambushed one of our bait bags, and an abundance of emperors crowded the camera’s field of view with trevally swooshing through. Moorish idols, pennants flying, danced in and out, undisturbed by so many potential predators.
Our dive master David Tickler also completed a very quick blue-water snorkel which yielded a magnificent photo of six silvertip sharks suspended in a rich blue. We are very excited about finding this feature as a preliminary assessment suggests it has not been trawled and, according to our Senior Fisheries Protection Officer Andy Deary, its proximity to Diego Garcia means poaching is unlikely. A perfect place to study pristine fish and shark assemblages!
Meanwhile, Captain Neil Sandes had spent the time going in circles, literally. He mapped the 100 km contour, indicating a maximum length of about 2.2km and a maximum width of 1.2km. In recognition of the honour of this accomplishment, we have named it Sandes Seamount (subject no doubt to all sorts of international rules of course!).
We then sailed on to find the remaining two seamounts on our list. Both were found as ornithologist Peter Carr used seabirds to help guide us onto the features. Waypoints were taken but camera deployments would have to wait for another day as the sun was starting to set. We continued our return to Diego Garcia, accompanied by 11 bottlenose dolphins playing off the Marlin’s bow. We couldn’t help but reflect that the day had been a magnificent one of exploration and discovery.
It was also our last day of sampling in the Chagos Marine Reserve. We have had a very successful expedition, with BRUVS deployed at 208 locations around the Archipelago. We have sampled from 6m to over 80m depth, with most of our research undertaken at depths where scientific divers typically do not sample. The team has hand-hauled over 9 km of rope to retrieve these 208 deployments which, as the cameras are stereo, will generate over 400 hours of footage for our image analysts to process.
We are greatly anticipating the results of the image analysis as our preliminary reviews have shown high abundances of sharks and large predatory fish such as groupers along with a huge diversity of schooling fish, both midwater and reef-associated. This survey will help form a baseline to document the current status of the fish community of the Chagos Marine Reserve, thereby supporting reserve managers and helping to ensure that it is effectively protected.
Previously in this series:
About the Author: Professor Jessica Meeuwig, Scientist
Jessica is the Director of the Centre for Marine Futures, University of Western Australia. Her main expertise is marine and fisheries conservation, and quantitative modelling. Her research group works across a range of taxa, from humpback whales to sharks to bony fish and includes some benthic ecology with key questions centring on how animals use habitat and the impacts of human activities on their ecology, population biology, energetics etc. Her group also has a strong interest in the development of video and image based sampling methods and maximising the power of information obtained from these methods. Jessica has worked as a marine ecologist in a wide range of temperate and tropical ecosystems and is a keen science communicator.