Spatiotemporal variations in reef manta ray (Mobula alfredi) residency at a remote meso‐scale habitat and its importance in future spatial planning

Abstract

The Chagos Archipelago's vast no‐take marine protected area (MPA, 640,000 km2) provides refuge for elasmobranchs facing unsustainable depletion by fisheries. Nonetheless, illegal, unreported and unregulated (IUU) fishing poses a substantial threat, and potential future changes to the use of the MPA could render elasmobranchs increasingly vulnerable to exploitation, putting geographically isolated populations, such as reef manta rays (Mobula alfredi) at risk of local extinction. Therefore, the species' long‐term movements and habitat use must be identified to help prioritize current enforcement activity and inform future spatial planning. Passive acoustic telemetry and modelled environmental data were used to investigate variations in 42 tagged M. alfredi utilization of a meso‐scale aggregation hotspot, Egmont Atoll, between 2019 and 2022. Mobula alfredi displayed the highest levels of residency ever reported (77%), with prolonged absences (>2 months) limited to seven individuals. Egmont atoll was used year‐round, with activity peaks during the southeast monsoon (April – November), particularly at sites on the southwest, while sites on the northwest were predominately frequented in the northwest monsoon (December–March). Tags were most likely to be detected when the Indian Ocean Dipole (IOD) was in a positive phase with a greater mixed layer depth, associated with a depression of chlorophyll α levels in the Indian Ocean. Thus, M. alfredi may be particularly reliant on Egmont Atoll, where they are predominantly observed feeding, when prey resources are limited elsewhere. In a region where the threat of fisheries is of increasing concern, the identification of crucial M. alfredi habitats is essential for conservation management planning. Given the significant role of Egmont Atoll for the local population, regular IUU enforcement patrols are crucial, particularly during the southeast monsoon. Any future changes to the MPA should prioritize preserving and actively enforcing no‐take regulations at Egmont Atoll.

First records of the sicklefin (Mobula tarapacana), bentfin (Mobula thurstoni), and spinetail (Mobula mobular) devil rays in the Chagos Archipelago

“…Currently, the only mobulid-focused project is the Manta Trust's Chagos Manta Ray Project, which is working to protect the reef manta ray (Mobula alfredi) population (Harris, 2019;Harris et al, 2021Harris et al, , 2023Harris et al, , 2024. On May 12, 2023, during a scientific research expedition to Egmont Atoll in the Chagos Archipelago, a M. tarapacana was encountered by the Chagos Manta Ray Project team on the surface 16 km offshore, west of the Ile des Rats Island on the northwest tip of the atoll.…”

“…With the global population decline trends of M. tarapacana, M. thurstoni, and M. mobular (Fernando & Stewart, 2021;Haque et al, 2021;Lewis et al, 2015), the Chagos MPA offers the species crucial refuge from commercial fisheries and most other anthropogenic threats that they face elsewhere (Lawson et al, 2017;Stewart et al, 2018). These characteristics potentially make the mobulid populations of Chagos Archipelago strongholds for the species survival similar to the region's M. alfredi population (Harris et al, 2024). Nonetheless, illegal fishing is common within the MPA, and elasmobranchs are heavily targeted (Collins et al, 2021;Tickler et al, 2019).…”