Exploring the twilight zone in the British Indian Ocean Territory

For the last week, scientists on the Bertarelli Foundation’s expedition to the Chagos Archipelago in the British Indian Ocean Territory have been carrying out research in a wide range of ecosystems. Dominic Andradi-Brown, from the University of Oxford, is taking part in the expedition and has provided the following update about the work he has been conducting in the ‘twighlight’ or mesophotic zone:

For the past week Catherine Head and I from Oxford’s Ocean Research and Conservation (ORC) group have been lucky enough to take part in the Bertarelli Foundation funded expedition. We’re involved in studying the health of the reefs, particularly in the face of widespread coral bleaching which is currently occurring across the globe. In addition to this reef health monitoring, a major focus for my work is to conduct some of the first exploration of the twilight zone reefs of the Chagos Archipelago.

The twilight zone, known scientifically as mesophotic coral ecosystems, includes coral reefs from 30m to 150m depth. These reefs are characterised by light dependent ecosystems, but are adapted to very low levels of light. Due to the remote nature of the archipelago, diver surveys have only been listed to a maximum depth of 25m, meaning that most mesophotic reefs have never been scientifically surveyed.

Large fragile sea fans were observed in the mesophotic zone at 58m
Large fragile sea fans were observed in the mesophotic zone at 58m

So why are we interested in the twilight zone?  Many of the impacts that cause most damage on shallow reefs in Chagos, for example processes such as coral bleaching and direct storm damage, are believed to decline in severity at greater depths. This means that twilight zone reefs may act as a refuge for shallow reef life.

We’re using a remote operated vehicle (ROV) to survey the upper twilight zone around the Chagos Archipelago in the 30-60m depth range. Already we’ve had many exciting findings! For example, the charismatic Chagos Clownfish (Amphiprion chagosensis), found only in Chagos, had previously been found down to 25m, we’ve extended that known depth range down to 37m after documenting several individuals in an anemone off Peros Banhos in the north of the archipelago earlier in the expedition.

The endemic Chagos Clownfish (Amphiprion chagosensis) was seen at 37m swimming next to a sea anemone
The endemic Chagos Clownfish (Amphiprion chagosensis) was seen at 37m swimming next to a sea anemone

The structure of the reef changes a lot in the twilight zone. One of the most common corals found on the shallow reefs belong to the genus Porites. On shallow reefs these corals have distinctive rounded boulder shapes. At twilight depths we’ve documented very flattened plate-like Porites colonies. We think this change in shape is an adaptation to the lower light levels on these deeper reefs, as this pattern has been observed on twilight reefs elsewhere in the world. However, researchers are still trying to understand the advantages to corals of becoming flatter, particularly at the fine scale (something Jack Laverick in the ORC group is actively working on).

As well as the seabed reef-specific twilight zone surveys, when deploying the ROV we’ve often found lots of sharks at twilight depths. Mostly these have been grey reef sharks (Carcharhinus amblyrhynchos) that have been interested in the ROV unit, circling in closer to look. On a couple of occasions, during ROV surveys in one of the Chagos atoll lagoons we found black-tip reef sharks (Carcharhinus melanopterus). What is clear from the ROV surveys is that sharks in Chagos are regularly visiting twilight reefs, further reinforcing the importance of these deeper reef habitats to larger mobile predatory species in the marine reserve.

Grey reef sharks (Carcharhinus amblyrhynchos) were seen at 30m taking a keen interest in the ROV
Grey reef sharks (Carcharhinus amblyrhynchos) were seen at 30m taking a keen interest in the ROV



The 2014, Bertarelli Foundation science expedition to the Chagos Archipelago

March 2014 sees researchers from Stanford University, the University of Western Australia (UWA), University College London (UCL) and the Zoological Society London (ZSL), supported by the Bertarelli Foundation, preparing to revisit the Chagos Archipelago in the British Indian Ocean Territory (BIOT).

The research team are heading back to BIOT to service the 30 acoustic listening stations deployed in 2013, which since then have been monitoring shark movements amongst the atolls of the marine reserve. The current expedition will download all the data that has been collected over the last 12 months, as well as perform essential maintenance such as cleaning, reinforcing their moorings and installing fresh batteries.

Additionally, the researchers will be deploying several types of brand new tags on sharks and manta rays to gather even more data. One of the tags we’re hoping to use is a newly developed camera tag that is able to combine video footage with acceleration and movement data – telling us not only where and how the sharks are moving, but more detailed information about their behaviour. This will be the first time these cutting edge tags have been used in BIOT, and the scientists are all very excited to see what they will show us.

Marine conservation and science workshop is held in Geneva’s Campus Biotech

In partnership with the Zoological Society of London, The Bertarelli Foundation convened a three-day workshop in Geneva to develop a coordinated approach to marine science – particularly megafauna science – in the British Indian Ocean Territory’s Marine Protected Area (MPA).

An international team of 25 scientists and conservationists from 18 organisations and six countries met at the Campus Biotech building for over three days of presentations and discussions.  The participants agreed a vision, mission and values for science in the MPA as well as nine key objectives which will help scientists to answer important questions about the territory’s important biodiversity. On the basis of this workshop, the participants developed an outline five-year science plan.

It was agreed the best way to address the knowledge gap in the Chagos Archipelago would be through a collaborative and coordinated approach and the creation of a scientific consortium – an exciting idea which the Foundation will develop in the months ahead.

Chagos has huge potential to teach us more about large marine reserves, about the species that inhabit them, and their role in the context of the Indian Ocean and beyond. The research planned has huge potential to inform how reserves can be more effectively managed and, therefore, to drive conservation globally.

We’ll bring you Mmre on these plans shortly.

Tagging expedition to Chagos

In February and March 2013, a research expedition was undertaken in the Chagos Marine Protected Area (MPA) sponsored by the Bertarelli Foundation, in partnership with Stanford University and the University of Western Australia. The expedition piloted electronic tagging to examine the feasibility of using remote technologies to monitor the movement of important pelagic species in the region. It was a great success, both in terms of results and also important lessons learned for future tagging projects.

Five different types of electronic tags were deployed in this study, with a total of 99 electronic tags placed on 95 animals, along with the installation of an acoustic receiver array around two northern atolls to detect animal movements. The Foundation believes tagging is one of the keys to answering the questions of how large pelagics such as sharks and tuna are utilising the MPA, and how much protection the no-take MPA is providing. It is the Foundation’s hope that the findings of this expedition will help to inform work by marine biologists around the world.

Read a report of the expedition here