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Studying, Understanding and Protecting the Great Hammerhead in French Polynesia

 

The great hammerhead shark, Sphyrna mokarran, is an understudied shark species mainly due to its cryptic and migratory nature. However, studies suggest that the species is globally subject to a variety of anthropogenic stressors. It is both a targeted and a by-catch species in both inshore and offshore fisheries, by industrial and artisanal fisheries. Due to their large size and high cartilage fiber content, common to all hammerhead sharks, Great Hammerhead fins are highly valued on shark fin markets, contributing significantly to the decline of the species. This high mortality rate, combined with their late sexual maturity and the fact that females breed only once every two years, increases the risk of extinction of the species which has no time to regenerate. It's thus no surprise that its proven vulnerability to human activities and the alarming evidence of the global decline of the great hammerhead populations, has led to its official listing on the IUCN Red List of Threatened Species. Since 2018, the species has been assessed as 'Critically Endangered'.

The great hammerhead shark is present in Polynesian waters and particularly in the Tuamotu Archipelago. Despite being a protected species in French Polynesia, S. mokarran remains vulnerable in international and foreign waters. Determining the origin of these individuals and their migratory routes will eventually make it possible to initiate protocols to prevent the extinction of this animal.

In an effort to contribute to our knowledge about this majestic species, and above all to its protection, Blancpain supports the Mokarran Protection Society, a newly established Polynesian environmental association dedicated to the study of the great hammerhead shark. The inaugural mission took place in January 2020 and was intended to observe the behaviour of the large predator in the wild in order to identify and count its population. On this occasion, Blancpain President & CEO Marc A. Hayek travelled to Rangiroa to participate as a volunteer underwater videographer. 

© Banner photo: Thomas Pavy

 

In Rangiroa, the second largest atoll in the world with a lagoon surface area of 1'450 km², S. mokarran observations are frequent while more occasional in neighboring atolls. Rangiroa has two passes which regulate the flows entering and exiting the lagoon under the influence of the tides: Tiputa to the east, and Avatoru to the west. The Tiputa Pass presents characteristics particularly favorable for the observation of the still poorly known S. Mokarran in the wild and without feeding. During the four months of more intense observations of the great hammerhead shark, from December to March, divers from all over the world travel to this site to try to catch a glimpse of this fascinating animal, so original by the typical shape of its cephalofoil and by its imposing size.

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The laser photogrammetry technique provides more accurate information on the size of the individual than simple observation, avoiding approximation errors. Two lasers mounted in parallel are fixed on a plate with known dimensions. A camera is fixed equidistant from the lasers and allows the capture of an image with projection of this distance on the animal, thus giving a scale to make different measurements of the individual – fork length, pre-caudal length, width of the cephalofoil, height of the 1st dorsal fin, total length.

The Mokarran Protection Society is a non-profit association under the 1901 law, created in 2019 to study and protect the great hammerhead shark population of French Polynesia. It aims to bring together human and material resources necessary for a better knowledge and preservation of this species critically threatened by extinction. Its vision is participatory science that allows volunteer scientists to exploit the observations of contributing divers. It therefore seeks to establish and maintain a network of dive centers and diver partners to collect data and information throughout the year. 

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