Unexplored Reefs of the British Indian Ocean Territory

Latest update June 7, 2019 Started on October 30, 2018

Our aim is to survey the remote coral reefs of the British Indian Ocean Territory in order to map and understand the shallow reef ecosystems found there. As this area is so remote it is relatively undisturbed by direct human impacts and so can serve as window to the effects of climate change on similar systems elsewhere.

October 30, 2018
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In The Field

We are now roughly half way into our expedition* within the British Indian Ocean Territory and have successfully completed around ten ‘flights’ down to the mesophotic reef zone of this region using the Trident drone. Thanks to the amazing calm weather conditions and glass-like surface waters, the drone has been easy to control and explore the reef. The initial flights have been down to around 70 m on the seaward reef slopes of the atolls. The reefs are clearly surviving a lot better down at these depths with large amounts of live coral cover and a diverse community of corals and fish compared to the shallower reefs. The shallow reefs down to around 10 metres were heavily damaged by the recent bleaching events (caused by unusually high water temperatures across the globe), and they are still very much in their recovery phase. Many reefs we’ve seen are sadly now in an erosional phase, losing their structure and volume through time, however others, particularly the lagoons have remained largely intact and are starting to regrow.

It’s reassuring to see so much life surviving down at greater depths using the drone as it can be pretty demoralizing working in shallow reefs systems following such a large-scale bleaching mortality event. While we have seen a lot of signs of recovery on the reefs, with many fish still around and many juvenile colonies and new recruits, the bleaching damage is extensive and will take many years to return to something close to its former glory. Hopefully recovery continues to occur rapidly and we start to see new solid reef architecture forming and giving a home for all the species these reefs normally support. However this recovery will only happen if the temperatures in this region remain below around 31 degrees for many years, and this is unfortunately looking increasingly unlikely as global temperatures continue to rise. Whether the corals can adapt fast enough to survive these increasing temperatures remains to be seen, and we will continue to monitor their progress here in the centre of the Indian Ocean. For these reefs at least the benefits of protection from pollution, damaging fishing, sediment run-off and development, which inhibit recovery in many other regions, are likely to give them a good fighting chance for a rapid return to a healthy state.

Over the next few days we will be deploying the drone on some steeper drop-offs to try and get a little deeper down and see what we can find down at around 100 metres! It’s exciting to start to push the limits of our knowledge of these remote reefs and find out what might be hiding down there!

*full disclosure we are now back on dry land, as the internet was not to be found out in the field due to the remoteness of the site, so let's just imagine we're sill there!

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Nice job! How deep was the video taken?
Hi Alexis, Cheers! This video is taken at about 50 metre, but we got down to around 70 m at a few of the sites. Super exciting as due to dive safety restrictions this is likely the first time this bit of the reef has ever been observed!
Hi Daniel, thanks for your prompt reply. It is very interesting to see the habitats you encounter in the upper mesophotic zone in your area. It is quite different than what I have here in Bali on my fieldwork sites. That would be really cool to go do deep dives where you work... Too bad the safety restrictions are too tough. But maybe there is a way to have the access granted?

Expedition countdown begins

We are now just a number of days from the start of the expedition, and are finishing the final bits of kit preparation and logistics planning. The team members from University College London, Bangor, Warwick, and Oxford Universities, Woods Hole, and Dunstaffnage Marine Laboratories will join the research boat in the Maldives before we start the long cruise south to the British Indian Ocean Territory! Over the next few weeks we will be using a range of technologies and sampling techniques to assess the status of these remote reefs and monitor their recovery from the 2016 bleaching episode.

Excitingly, we now have a shiny new Trident OpenROV, which we will use to explore some of the deeper ‘twilight zone’ reefs of this region, most of which have never been surveyed before. We are hoping to find some healthy reefs and habitats and find out exactly which organisms exist here at these depths. We will also be filming some 360 videos and constructing more 3D reconstructions of the reef for people to get a better sense of what we are seeing out here. Dependent on the internet signal we can receive whilst at sea, we will send regular updates of the trip, so keep a look out for our posts. Fingers crossed for a successful trip!

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How deep are you planning to deploy the Trident ROV? Did you get the 100 meters tether cable?

Preparing for a remote expedition at sea

People sometimes ask what is needed to prepare for an expedition to sea in a remote region of the Indian Ocean? So this post is to give you a taste of some of the essentials for an upcoming trip, in this case a 3 week cruise with about 14 scientists from around the world.

Boats (in any colour, so long as they’re red)

For this type of work, you need a pretty big boat! And on-board the big boat you essentially need a mini-fleet of smaller boats! We are very lucky to get to live and work aboard the BIOT fisheries patrol vessel which is well equipped to tick the ‘big boat’ criteria having previously worked the stormy seas of the North Atlantic, and during our last expedition held a total of eight smaller crafts on-deck! The smaller boats let us to navigate close to the reefs without grounding and damaging any of the very shallow corals and banks found here, as well as spread the team out across a much larger area. The smaller boats are sturdy inflatable dinghies, which we have to (carefully) crane into and out the water at each site before moving on.

Fancy cameras

For the work many of us do imaging and surveying the reefs of this region, the essential bits of kit are underwater cameras! My dive buddy and I use three main types of camera. First up we need a very high-spec digital camera that can take super detailed images in pretty much any conditions. We use a Nikon DSLR with an underwater dome lens (roughly the size of my head!) to take wide-angle imagery of the corals to create large-scale image mosaics and 3D models. Despite the extremely clear conditions in the Chagos archipelago, as we go deeper we start to need to use lights / strobes to keep getting crisp colourful images, because the light and spectrum of colours entering the water gradually fades with depth. ‘Fancy camera number two’ is smaller, but uses specialised lights to make the corals fluoresce, and then we capture this fluorescence through a special lens filter! We are using this ability of the corals to facilitate mapping where live coral and juvenile recruits are found! The final fancy camera allows us to take 360 degree ‘virtual reality’ images and video so that people can get a real feeling for what it is like to be here! (check out VR video below and have a look around)

Unexpected engineering and audit skills

One of the less glamorous bits of a scientific field trips to remote spots in the world is the need to spend a lot of your time trouble-shooting, fixing, and even re-making from scratch a whole range of things. This involves engineering skills which you never imagined would be necessary in order for you to just go and count some fish! This might involve a weary scientist trying to fix a temperamental kaput air-compressor in ~ 40 degrees C, or a very confused ‘fish counter’ (me) reassembling and servicing a broken dive tank – in an even hotter (and louder) engine room! The final task pre-departure is making sure you brought all the right gear for a successful expedition, which involves going box-by-box through a warehouse full of equipment, tools and spares. Now, where did we put that spanner again...?

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Despite the remoteness of the British Indian Ocean Territory, in 2015-2016 the region (along with most of the world's tropical coral reefs), experienced widespread 'coral bleaching'. This bleaching response was driven by sea temperatures which were abnormally high. These hotter than normal temperatures cause corals to become stressed and expel the colourful symbiotic algae which live in their tissue. In turn the coral's supply of food, and the bright array of colours we associate with reefs, are lost. When the symbiotic algae leave the coral, we are able to see through the transparent coral tissue to the white skeleton underneath, giving the 'bleached' appearance. If the corals remain in this state for too long they will eventually die, and their intricate structures will erode.

A benefit of the relative isolation of this area is that we can investigate the effects of temperature-induced bleaching away from the normal confounding effects of pollution, or damaging fishing for instance. This allows us to quantify the direct effects of climate change and also investigate if the reefs in these isolated 'wilderness' areas behave differently to more impacted / polluted areas. Do these reefs recover more quickly? Do these reef's communities respond differently? These sorts of questions help us to understand the impacts of change better, and therefore help us manage marine areas more effectively.

The video animation shows a 1000 m2 section of reef, two years following bleaching. Much of the area has died and the table corals particularly have collapsed, however the large massive-form colonies have survived and evidence of juvenile coral recruitment is visible in patches. We are using these imaging methods to map and monitor the changes across the region to assess how the reef community and it's 3D structure is recovering.


Preparations are starting for our next trip out to the British Indian Ocean Territory in April, which means checking and testing camera kit, and planning where we will be surveying in the archipelago over the month long survey. When we are in the field, the focus of our work is to monitor changes in reef benthic composition and structure through time. We have recently begun to use 'structure from motion' photogrammetry in order to reconstruct large areas of reef in high detail photo-mosaics, and in 3D! This allows us to analysis the changes occurring in fine detail and to visualise the whole area easily. I've attached a large-scale photo-mosaic to this post of a reef at Blenheim atoll to give an impression of the scale of most surveys. These mosaics are about 500 m2 and made up of around 1000 HD images stitched together, creating very high resolution imagery. I've also attached a 360 video, which will let you have a look around underwater in Chagos while we image the reef!
Normally we work at ~10 m depth and are restricted to around 25 m, which is why we are hoping to incorporate the use of an ROV this year to explore deeper down the reef slope and see how the reefs are faring in the mesophotic 'twilight' zone!

Expedition Background

The British Indian Ocean Territory, also known as the Chagos archipelago, is situated ~ 500 Km south of the southern tip of the Maldives archipelago, in the centre of the Indian Ocean. This remote archipelago is considered one of the few remaining areas of marine wilderness, and in 2010 was designated as a ‘no-take’ marine reserve, protecting the reefs, open-waters and island species.
The total area protected is roughly the size of France and contains the words largest living coral atoll with the vast majority of the territory still unexplored! Previous expeditions to record what is there and to monitor the state of the reefs have been restricted in their scope by the isolated nature of the region – diving to any deeper than 25 m is considered too high risk, and we typically only have a few weeks to conduct surveys each year. Because these waters have so little pollution or sediment, light is able to reach great depths here, allowing the reefs to extend down a long way past 25 metres! Beyond around 30-40 m these ‘twilight zone’ / ‘mesophotic’ reefs support large amounts of life, of which we currently have very limited knowledge. However recent discoveries include rare deep seagrass systems, and what appear to be large tracts of dense healthy reefs. After the global coral bleaching event of 2015-2016 much of the shallow reefs here were killed off by the elevated temperatures, driven by the mix of an El Nino year and higher average temperatures from climate change. Thankfully below around 20 m, where waters are cooler, the reefs appear to still thrive and so during our next expedition we hope to use a Trident ROV to explore these slightly deeper reefs, find out what’s there, and monitor any changes that are occurring.


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