The second Mokarran Protection Society campaign took place in Rangiroa, an atoll in French Polynesia, from December 2020 to February 2021. This year, the research areas and techniques carried out to understand the seasonal presence of the great hammerhead shark, Sphyrna mokarran, in the Tuamotu Archipelago, are now wider.
After having explored the Tiputa pass, the exchange interface between lagoon and open ocean, this year's focus was the lagoon itself. This vast inland sea remained a mystery since very few sightings of great hammerhead sharks have been reported so far. Yet, observations made in the pass prove that individuals enter the lagoon and return to the ocean frequently.
© Banner photo: Thomas Pavy/Mokarran Protection Society
This leads to the question – why do sharks enter the lagoon? Is it for food purposes or for other key needs? To answer this, a citizen survey has been launched in Rangiroa, involving local fishermen (spear fishermen, anglers, "fish park" fishermen) and excursionists. This aims to draw on the traditional knowledge of the atoll inhabitants to gain a deeper understanding of the distribution of S. mokarran in the lagoon of Rangiroa. Polynesian culture connects strongly with the ocean, since local communities depend on it for their livelihood. As such, they regularly travel around the lagoon and may know strategic observation sites for the great hammerhead shark such as feeding, breeding or reproduction grounds. To this day, the stakes are high as no reproduction or nursery sites have been identified in the world. This survey also offered an opportunity to assert the association within Rangiroa community by providing awareness about shark ecology and by ensuring feedback if, in the future, notable sightings are made. This season, around a hundred sea users were interviewed and new sites of interest for the study of the species have already emerged all around the atoll.
Another focus of the 2021 campaign was the development of an educational tool kit and a Mokarran board game for schoolchildren of French Polynesia. Building on a network of aware schoolteachers from different islands, those workshops aimed to educate schoolchildren, and by extension, their families back home, about the fragility of shark populations and the need to preserve its ecosystem.
To follow up on the work carried out last year, the photo-identification and laser photogrammetry techniques continued to complete and refine the results and hypothesis of the species' presence in both passes of Rangiroa, Tiputa and Avatoru. This was a decisive step towards highlighting possible inter annual fidelity to Rangiroa atoll, should the same individuals from the last campaign be sighted again this season. The attention also turned to Tikehau, another Tuamotu atoll located 15 km from Rangiroa. lts unique Tuheiava pass has seen its reputation grow for the past few years as a hotspot to observe the great hammerhead shark. Defining geographical connectivity in Rangiroa, between the Tiputa and Avatoru passes, and between Rangiroa and Tikehau if any, is crucial in order to sketch potential migration pattern.
This mission's first results were already promising. With more than 180 observations and 70 laser measurements for the two atolls combined, 23 different individuals have been identified on Rangiroa and 7 on Tikehau. Connectivity has not been established between the two atolls, but the data is still being analysed. One individual, recognisable by its curved dorsal fin, was observed in the area last year, thereby making him the first inter annual seasonal resident. The other individuals identified this season had not been observed previously, which means that Rangiroa's population of great hammerhead shark reaches at least 50 different specimens in two years of scientific campaign and probably many more. Again in 2021, 100% of the gendered individuals were identified as females with total lengths ranging from 2,14 m to 4,5 m, a measurement record for the association.
© Photo: Thomas Pavy/Mokarran Protection Society
These first results, yet to be completed, reaffirm the unique feature of Rangiroa, and now also Tikehau, amongst the world's main great hammerhead shark hotspots. Aware of the urgency of improving knowledge on this critically endangered species, significant technical and scientific resources have been mobilized - with the arrival of a forthcoming Gombessa expedition.