Preliminary insights into the population characteristics and distribution of reef (Mobula alfredi) and oceanic (M. birostris) manta rays in French Polynesia

Abstract

In French Polynesia, both currently recognized manta ray species, Mobula alfredi and M. birostris, are observed. Despite being an important cultural asset and generating significant economic benefits through manta ray watching tourism, published data on the ecology and threats to these species in the region are scarce. Based on an 18-year dataset of sighting records collected by citizen scientists and during two scientific expeditions, this study provides the first insights into the population characteristics and regional distribution of the two manta ray species in French Polynesia. A total of 1347 manta ray photographs (1337 for M. alfredi and 10 for M. birostris) were examined for the period January 2001-December 2017, with photo-identification techniques leading to the successful identification of 317 individual M. alfredi and 10 individual M. birostris throughout the Society, Tuamotu and Marquesas Islands. We provide the first confirmation of sympatric distribution of both species in the Society Islands. Our results highlight strong and long-term site fidelity of M. alfredi individuals to certain aggregation sites (> 9 years for 16 individuals) and reveal some degree of connectivity between populations, with 10 individuals recorded moving between islands located up to 50 km apart. Analysis of photographs of individuals bearing sub-lethal injuries (n = 68) suggests that M. alfredi are more likely to be injured at inhabited islands (Maupiti or Bora Bora; 75% of all injured individuals) than at uninhabited islands, with 75% of injuries related to boat propeller strikes and fishing gear entanglements. Our findings emphasize the need for further research to allow for a comprehensive evaluation of population structure, size and threats to manta rays in this region.

Sublethal Injuries and Physical Abnormalities in Maldives Manta Rays, Mobula alfredi and Mobula birostris

“…Indeed, the highest percentage of boat strike injuries recorded occurred in some of the busiest tourism atolls (e.g., North Malé and South Malé) (Maldivian Manta Ray Project [MMRP], 2019b). These observations are consistent with a study in French Polynesia, which found M. alfredi were more likely to be injured around inhabited islands with more marine traffic than at remote uninhabited areas (Carpentier et al, 2019). Here, no boat strike injuries were observed for M. birostris, probably because M. birostris in the Maldives, unlike M. alfredi, rarely visit nearshore lagoonal or reef habitats, where heavy boat traffic occurs (Maldvian Manta Ray Project [MMRP], 2019a; Stevens and Froman, 2019).…”

“…Predatory pressure on the Mozambique population appears high, especially when compared with other documented M. alfredi populations in Maui, Hawaii, and eastern Australia, where 33% (Deakos et al, 2011) and 23% (Couturier et al, 2014) of individuals were observed to have shark-inflicted injuries. In Ningaloo, Western Australia, just 2.7% of the M. alfredi population had injuries which unambiguously originated from predation events (McGregor et al, 2019), while in French Polynesia, this was the case for just two individuals (Carpentier et al, 2019). Many of these study sites, including in the Maldives, are shallow, protected coastal reefs which favour resident manta ray behaviour (Stevens, 2016;Venables, 2020).…”

“…Unfortunately, throughout the Maldives, M. alfredi, especially juveniles, aggregate in shallow lagoons where increasing boat traffic is likely to lead to greater sublethal injuries and mortality events if protective management measures are not introduced (Stevens, 2016;Maldivian Manta Ray Project [MMRP], 2017;Harris et al, 2020). To reduce the impact of tourism and fishing activities on juvenile M. alfredi, these important nursery aggregation sites should be protected through speed limits, outboard engine restrictions, and no-take fishing zones (Carpentier et al, 2019;Germanov et al, 2019;Stevens and Froman, 2019).…”

“…Anthropogenic originated injuries are apparent in every monitored mobulid population across the world, predominantly resulting from interactions with fishing gear and vessel strikes (Deakos et al, 2011;Stewart et al, 2018b). Entanglement in fishing line, nets, and mooring ropes can cause serious injury and death (Couturier et al, 2012;Carpentier et al, 2019). Spending considerable time at the surface (Braun et al, 2014(Braun et al, , 2015, e.g., while feeding, manta rays are especially susceptible to severe injuries from boat strikes and from contact with propellers (McGregor et al, 2019;Stevens and Froman, 2019).…”

“…Assessment of the origins of sublethal injury to individuals provides a method for investigating the relative impact of different types of threats to a species (Archibald and James, 2018). Researchers have investigated sublethal injuries to M. alfredi in Mozambique, Hawaii, French Polynesia, Australia, and Indonesia (Marshall and Bennett, 2010;Deakos et al, 2011;Carpentier et al, 2019;Germanov et al, 2019;McGregor et al, 2019), and to M. birostris in southeast Florida, United States (Pate and Marshall, 2020). Previous studies have analysed only a single injury origin type in detail, e.g., predatory bites in Mozambique (Marshall and Bennett, 2010) and boat strikes in Australia (McGregor et al, 2019), or provided only a limited quantification of the injuries observed (e.g., Deakos et al, 2011 in Hawaii).…”