The Chagos Archipelago sits alone in the Indian Ocean, 500km from anywhere. Its pristine waters offer sanctuary to our most threatened marine predators.
The Chagos Archipelago is one of the most remote places on earth. This cluster of 55 tropical islands sits in the central Indian Ocean, 500km away from its closest neighbour, the Maldives. Chagos is also one of the most valuable areas for global marine conservation. It is home to the largest no-take marine reserve in the world, and may hold the key to safeguarding our most threatened marine predators.
The British Government, who administer the archipelago, set up the Chagos marine reserve in 2010. The 644,000km² reserve area (about three times the area of Victoria) covers some of the most pristine waters and diverse marine life in the world.
Dr Tom B. Letessier, a marine research fellow at the University of Western Australia, has been on three expeditions to Chagos since 2012. For him, working there is like living inside a nature documentary; animals behave in ways that are unheard of in populated areas.
"Birds are completely tame," says Tom. “They'll try and land on you because they want to perch; they've never seen people before. It's just fascinating."
He describes a hawksbill turtle that dragged itself ashore in broad daylight to lay her eggs right in front of him, moray eels that leapt onto reefs to ensnare crabs, and schools of manta rays feeding in V-formations, just like flocks of birds.
But being a researcher in Chagos is not all fun and games. Organising an expedition in such a remote location, involving scientists from all over the world, requires a great deal of preparation. All his equipment is packed and shipped out months in advance aboard a military supply boat departing from Singapore, reaching the US base at Chagos just in time for the team’s arrival.
“It takes years off your life,” says Tom. “You’re just there worrying about it, thinking 'have I thought of everything? Have I done everything?’ But it's all worthwhile when you're out there".
Tom and his colleagues are researching whether large marine reserves like the one in Chagos can help protect some of the ocean’s top predators.
Shark and tuna populations have declined globally, mostly due to overfishing and by-catch. Shark species commonly exploited for shark-fin trade have declined more than 99% in some regions, and numbers of Pacific bluefin tuna, prized for the sushi market, have dropped by 96%. Effective conservation is urgently needed to prevent their extinction. However, many shark and tuna species migrate seasonally through the marine territories of different countries, and also through the high seas, which don’t belong to any country, so coordinating multi-country management is challenging and mostly ineffective.
As an alternative to regulating fisheries as a whole, providing large areas of refuge from destructive human activity during migration allows sharks and tuna to live and breed in peace, rebuilding populations numbers even if they only stay temporarily.
"In Chagos there is lots of habitat which is favourable to these pelagic [open ocean] species," says Tom.
Protected areas also benefit fisheries, since fish spawned within the sanctuary spill out and replenish stocks in areas open for fishing. For instance, 83% of coral trout fished in the Great Barrier Reef fishing zones were spawned within protected areas. This means sanctuary in the Chagos reserve could help replenish shark and tuna stocks in the rest of the Indian Ocean.
To understand whether large marine reserves aid the recovery of these populations, the research team must establish the abundance of sharks and tuna in the Chagos marine reserve, and what habitats they prefer to use. However, counting large predators in an area is not easy; because of their roaming natures, sightings are few and far between.
“If you go diving in the middle of the blue ocean, and you just wait there for an hour," says Tom, "I mean, you’re probably not going to see very much, are you? You’re just going to see lots of blue.
"But we know that there are sharks and tuna out there, so we need to come up with ways to count them."
Tom does this using underwater camera systems specially designed to attract and monitor creatures swimming in the open ocean. These cameras are set up in stereovision to count each fish and measure their size (similar to how our two eyes allow us to see depth) and the cameras are baited with mashed up pilchards and sardines to attract top predators.
Five cameras, rigged on a line 200m apart, are dropped into the water and left to drift for a few hours, recording any creatures that swim by. The researchers use these data to build a picture of how and why abundances of sharks and tuna vary through space and time.
"Both shallow and deep seamounts [underwater mountains] tend to preferably aggregate these animals," says Tom, "so large migratory species and schools of fish tend to choose these areas, which are geomorphically complex".
Seamounts are hotspots for marine predators, as these habitats may promote primary production in the area, fuelling animals further up the food chain. It is therefore no surprise that Chagos harbours relatively high abundances of marine life, given that there are 86 seamounts of various sizes in the reserve area, attracting and supporting diverse ocean creatures.
Tom and his colleagues haven’t yet collected enough data to fully understand how fish abundances change from year to year, but that, he says, is what future expeditions are for. If patterns in shark and tuna abundances through time improve in Chagos compared to elsewhere in the western Indian Ocean, then this would support the role of large marine reserves like Chagos in replenishing threatened marine predator numbers.
Of course, there's more to Chagos’ marine habitat than just sharks and tuna, and this year’s expedition carried equipment and scientists to study many aspects of the pelagic realm, from tiny plankton to large sharks, to monitoring water temperature and salinity. Their survey work covered seabirds above the water right down to the pitch-black depths of the ocean.
The researchers discovered that Chagos is home to some very rare ocean species.
"We had the first ever records of deep-sea sharks inside the reserve, which was fantastic," says Tom. The species had never before been recorded in that part of the ocean.
Also identified in camera footage were oceanic white tip sharks, a species that has been hammered by longline fishing and reduced in some areas to 1% of its former numbers. Sightings of ocean sunfish, the world’s largest bony fish, are rare in any part of the world, yet these lonely ocean wanderers were also caught on camera in Chagos.
The sheer abundance of wildlife at Chagos, and its pristine habitat, means the reserve can be used as a baseline to understand what reefs look like in the absence of humans.
“[Chagos] as a baseline is a great tool when you’re trying to manage fisheries," says Tom. "One of the first problems is to determine what is the impact of fishing, and having this reference site is of great value."
Today, the archipelago's only residents are the personnel of the US military base on the largest island of Diego Garcia.
Chagos has a controversial history, with Chagos islanders (mostly the descendants of a 1793 French Colony) permanently evacuated by the British government in the 1960s and 70s to make way for the military base.
Since then, the UK have fought several legal battles against the Chagossians’ right to return to the islands. More recently, the UK’s right to claim maritime rights in Chagos has been thrown into dispute and the future of the marine reserve is uncertain.
However, regardless of its sovereignty politics, what is certain is that the marine habitat around Chagos is unique and one of the few corners of the world where life exists with minimal human interference.
The marine reserve has both scientific and conservation value for the whole of the Indian Ocean, and keeping this area free of fishing may hold the key to the recovery of large marine predators threatened with extinction.