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A research expedition to uncharted waters in the Indian Ocean

  • Written by  Tommy Trenchard
  • Published in Oceans
Deckhand Laia Pascual looks out across a calm sea for signs of sperm whales Deckhand Laia Pascual looks out across a calm sea for signs of sperm whales Tommy Trenchard
13 Oct
2021
Photojournalist Tommy Trenchard joins a research expedition to the Saya de Malha Bank in the Indian Ocean, where a profusion of marine life waits to be discovered

It's after nightfall in the western Indian Ocean and marine biologist Tim Lewis is staring intently at the computer monitor in front of him, its faint white glow illuminating the look of excitement on his face. On the screen, a complicated pattern of dots and short lines represents sounds being picked up by the hydrophone trailing a few hundred metres behind the ship he’s on. Lewis, a specialist in underwater acoustics, can read sound almost as well as he can hear it, and without even putting on headphones, he can identify without hesitation the unique clicks and whines of one of the ocean’s greatest predators: the sperm whale.

Lewis is among a group of scientists, researchers and campaigners aboard the Greenpeace ship Arctic Sunrise on an expedition to explore what’s thought to be the least-studied shallow tropical ecosystem on the planet. Situated between Mauritius and the Seychelles, hundreds of miles from the nearest land, the Saya de Malha Bank is a vast underwater plateau that hosts what scientists believe is the world’s largest seagrass meadow. Using a mixture of specialist audio equipment, environmental DNA sampling, underwater remote operated vehicle (ROV) footage and a dawn-to-dusk visual-sightings survey, the team hopes to learn more about the species that live on and around the bank, in order to highlight the need for further protection for the high seas.

DSCF2400Marine acoustician Tim Lewis listens to a large pod of dolphins, their calls captured by a hydrophone. Image: Tommy Trenchard

‘Sperm whales are ideal for acoustic analysis because they produce a really loud click every second and we hear them from a long way away,’ says Lewis, now listening through headphones to what he estimates is a pod of at least six individuals. ‘It’s fantastic that we’re hearing them. I was beginning to wonder whether we were going to find any in the area.’

Finding sperm whales in the deeper drop-offs around the bank is a major success for the team because it serves as a marker of the area’s productivity. Where there are feeding sperm whales there are squid, explains Lewis, and where there are squid, there are plankton – the basis of a healthy marine food chain. But finding them at night is far from ideal, as it limits the team’s ability to identify individual whales.

The team had been hoping to approach the whales using small, rigid-hulled inflatable boats (RHIBs). Then, when the whales surfaced, they would attempt to capture so-called fluke ID shots – photographs of the trailing edge of the whales’ tails, or ‘flukes’, which act as a unique identifier, much like a human fingerprint. These ID shots can then be cross-referenced with existing catalogues from around the world, allowing researchers to glean valuable information about the whales’ movements and behaviour over time. For now, however, with the ocean shrouded in darkness, all the team can do is mark the pod’s location on a map and hope to encounter them again in daylight.

listeningTim Lewis uses a directional hydrophone to track a group of sperm whales near the Seychelles. Image: Tommy Trenchard

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For Captain Fernando Romero, who’s been traversing the world’s oceans for decades, navigating the Saya de Malha Bank presents challenges. With parts of the bank rising to less than ten metres below the surface, ships have long circled around it instead of crossing it. This has helped to protect the area, but it has also led to a chronic lack of reliable depth measurements. The area is so poorly charted that some of the depth points on the captain’s map turn out to have been recorded by mariners in the early 1800s, and many are incorrect.

‘We’re in an area that’s basically uncharted waters,’ says Romero, who has been forced at times to reduce speed to just one knot and to send out RHIBs ahead of the ship to feed back a stream of depth measurements. ‘We have some areas here marked 18 metres and our echo sounder is showing 700 metres, so this is the kind of information we’re dealing with.’ 

DSCF6610Seagrass and coral on the Saya de Malha Bank. In contrast to the abundance of seagrass here, globally, a football-pitch-sized area of the marine plant disappears every half hour. Image: Tommy Trenchard

Diving over these shallow areas comes with a powerful sense of cognitive dissonance. Above the surface lies a featureless expanse of open ocean that stretches for hundreds of miles in every direction. But hold your breath and dive just a few metres down and you find yourself in an underwater fairyland of swaying emerald seagrass meadows and coral reefs that teem with life. Damselfish in acid shades of blue and yellow dance skittishly around the corals; lilac starfish cling to algae-covered rocks; angelfish flit this way and that, their elongated dorsal fins trailing behind them like kite tails in the wind.

Anywhere else, a thriving ecosystem such as this would be a major attraction for divers, or indeed fishermen. But in this isolated oasis in the middle of the ocean, it’s a fairly safe bet that most of the creatures the team find have never before seen a human being. With no comprehensive studies of the area’s marine life ever having been conducted, each time the divers dip below the surface they have little idea what they’ll discover.

DSCF3613Kirsten Thompson, Mariajo Caballero and Juliana Costa examine nautical charts as the Arctic Sunrise passes a large pod of sperm whales. Image: Tommy Trenchard

Since leaving the port of Victoria in the Seychelles two weeks earlier, the team has encountered dozens of species, ranging from turtles, sharks and rays to rare beaked whales, an endless parade of colourful reef fish and vast ‘superpods’ of acrobatic spinner dolphins. In addition, reams of data, including environmental DNA samples taken from the water and hundreds of hours of hydrophone recordings, still await analysis. 

‘Even though we don’t know much about the biodiversity of the area, it could be a really important habitat for certain species,’ says Kirsten Thompson, a marine biologist from the University of Exeter and the lead scientist on the expedition. ‘I feel there’s a lot of biodiversity here and we’re just scratching the surface. We’re creating a baseline for people to study this area.’

Arguably, however, the Saya de Malha’s most important resource is the seagrass itself. Studies have shown that seagrass meadows act as highly effective carbon sinks, capable of storing more than twice as much carbon per square kilometre as terrestrial forests. And scientists believe that the seagrass beds on the bank may cover an area roughly the size of Switzerland.

Given that coastal seagrass beds have been shrinking rapidly in recent years, remote areas such as the Saya de Malha hold an even greater importance. The United Nations Environment Programme estimates that the world is losing the equivalent of one football pitch of seagrass every 30 minutes due to a combination of pollution, destructive fishing practices, coastal development and climate change.

‘Seagrass essentially draws carbon down through the body of the plant and down into the seabed and stores that carbon for millennia,’ says Thompson, who has been collecting samples for analysis back in the UK. ‘We know that this is a huge area of seagrass and it could be really important for climate change mitigation.’

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DSCF6664Arctic Sunrise deckhand and diver Alessandro Montanari explores the world’s largest seagrass meadow on the Saya de Malha Bank. Image: Tommy Trenchard

The team hopes that the findings of the expedition will strengthen calls for the Global Ocean Treaty, the groundbreaking international agreement being brokered by the UN, which aims to see 30 per cent of the high seas – the waters beyond the territorial claim of sovereign nations –protected by 2030. Currently, less than one per cent of this area is under effective protection, leaving it vulnerable to overexploitation by fishing or extractive industries.

Fifty metres long and painted a deep forest green with a rainbow emblazoned across the hull, the Arctic Sunrise began its life as a seal-hunting vessel in the Arctic before being bought by Greenpeace in 1995 and refitted for research and conservation work. With a rounded hull designed for navigating ice floes, it pitches and rolls wildly in even the tiniest swell and is consequently referred to affectionately as the ‘washing machine’ by some of the two dozen sailors, researchers and campaigners on board.

By the time the expedition enters its third week, life has settled into a familiar rhythm structured around mealtimes, chores and free time spent reading, playing darts or watching flying fish skimming over the tops of the waves ahead of the ship. In between, the data collection rolls on. Thompson collects eDNA samples, both from the seabed and nearer the surface; Lewis continues to monitor his equipment for signs of cetaceans; and divers explore the shallows, together with a pair of ROVs. During daylight hours, lookouts work in shifts to scan the horizon with binoculars for signs of life, noting down the details and coordinates of wildlife encounters. Soon there are hundreds of recorded sightings, but still the team has yet to ID any sperm whales.

DSCF6341The handheld hydrophone looks a bit like a satellite dish attached to the end of a broom handle. Image: Tommy Trenchard

That finally changes on a sizzling hot Wednesday morning as the vessel conducts a routine survey transect through a channel of deeper water that cuts across the northern end of the bank. It starts with a distant puff of white that materialises suddenly against the cobalt blue of the ocean. Within seconds, it has dissipated, but not before a lookout has spotted it. A shout goes up: ‘Blow at two o’clock!’ Before long, a RHIB is hoisted over the side of the ship by a hydraulic crane and Thompson and Lewis climb in. Once it has dived, a sperm whale can spend as long as 50 minutes underwater before returning to the surface, often miles away from where it last appeared, so speed is of the essence. The RHIB zips across the water, skipping lightly over the waves. Thompson stands in the bow, one hand clutching a metal handrail, the other gripping a DSLR camera with a telephoto lens. The pilot steers the boat towards a point behind the whale that will offer a clear view of the trailing edge of its fluke when it dives. The RHIB is within a few hundred yards, tantalisingly close, yet not quite close enough, when the whale’s back rolls smoothly out of the water and its tail raises skywards before dropping out of sight beneath the waves.

DSCF9844A sperm whale dives on the edge of the Saya de Malha Bank in the Indian Ocean. Image: Tommy Trenchard

Using a handheld directional hydrophone, which looks something like a satellite dish attached to the end of a broom handle, Lewis listens to the clicks made by the diving whale. He also hears others feeding in the depths and directs the boat towards the nearest of these. Around 20 minutes later, the noises floating up into his headphones abruptly stop, indicating that the whale is on its way to the surface. All eyes scan the waters around the boat until an audible hiss breaks the silence and a plume of white spray shoots up into the air just a few hundred metres away.

The RHIB slides into position and by the time the whale’s back starts to arch for another dive there are three cameras trained on the spot. As if in slow motion its gargantuan tail lifts up above the surface, water cascading off it in a translucent curtain as it flips towards vertical. Cameras fire off shots in machine-gun bursts as the fluke sinks gracefully and silently back into the blue.

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Subscribe to Geographical today for just £38 a year. Our monthly print magazine is packed full of cutting-edge stories and stunning photography, perfect for anyone fascinated by the world, its landscapes, people and cultures. From climate change and the environment, to scientific developments and global health, we cover a huge range of topics that span the globe. Plus, every issue includes book recommendations, infographics, maps and more!

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