The waters off the Rangiroa Atoll in French Polynesia are home to an astonishing creature, a true survivor from an age when dinosaurs roamed the Earth around 400 million years ago: the great hammerhead shark. At up to 6 metres in length, they are imposing predators with an array of incredible sensory organs housed in their distinctive hammer-shaped heads. But there are still many unanswered questions about these mysterious creatures. Marine biologist Dr Johan Mourier of the University of Perpignan is dedicated to exploring the behaviour and genetic make-up of great hammerhead sharks to help preserve this endangered species in the wild. In the last 25 years alone, their world population has shrunk by about 80%. Yet very little is known about them. Dr Mourier wants to get to the bottom of the sharks’ migratory patterns, as well as their social behaviour. By tagging individual sharks with GPS locators, he hopes to establish where the animals go, what their migration routes are and where they are most threatened. It is becoming increasingly clear that sharks tend to move very quickly and deliberately between different fishing grounds. Is this also true for the Great Hammerheads? Are changes in these traditional fishing grounds threatening the hammerheads’ survival? The extreme shyness of these mighty predators makes them difficult research subjects. To make matters worse, great hammerheads are loners, unlike their common hammerhead cousins. And since tagging is usually done by using airtanks, it is very difficult to approach these shy individuals. This is where Frederic’s silent and calm approach underwater makes all the difference: Dr Mourier is working together with Fred to get closer to the animals than ever before. This gives him not only the chance to study their behaviour up close, he is even able to select individuals for tagging to reflect a cross-section of the population. The researchers are able to track the tagged sharks and plot their migratory patterns for the very first time. For Frederic, this undertaking is not only exciting, it is seriously dangerous and requires months of preparation and training. Approaching a predator like this one in open water is not for the faint-hearted, especially since it is thought that sharks are able to ‘feel fear’ and respond to it aggressively. On Frederic and Christian’s arrival in Papete, the capital of French Polynesia, Dr Mourier diligently highlights the risks and pitfalls of trying to dive with great hammerhead sharks: they are powerful predators, not to be underestimated. Frederic and Christian have their work cut out. They have to plan each dive meticulously – not least how Christian and his diving equipment can stay close enough to Frederic to document his findings without disturbing the sharks. Minimising potential risks is also top of their agenda. Test dives perfect their technique and illustrate just how abundant and stunning the local marine life is. They come across many shark species: Silvertips, oceanic white tips, grey reef sharks, lemmon sharks and many others. Dr. Mourir shows and explains their nursery: bays with hundreds of baby sharks. No matter which shark species Frederic wants to approach, he cannot afford to panic. He must remain calm and in control – instinctive reactions like sudden, hecticmovements could be detrimental. Loose control, and it could cost him his life. The tagging and sampling process in itself is even more dangerous than the initial approach: Frederic has to use a harpoon to dart the sharks from their immediate vicinity. The tagging could be interpreted as a threat and prompt an attack. But luck is on their side. After many days Fred finally can set the tags near the dorsal fin of two great hammerheads. This is the crucial moment Dr Mourier has been waiting for: the tags will give him a unique chance to track the sharks’ movements. A huge success for Frederic, Christian and the research team. The insights gained into the great hammerheads migration patterns and their genetic make-up will combine to paint a much more accurate picture of these elusive predators’ lives. For science today, this undertaking is truly uncharted territory.

• published: 13 Nov 2015
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