ORCID

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.

Publication Date

2024-02-01

Publication Title

Aquatic Conservation: Marine and Freshwater Ecosystems

Volume

34

Issue

2

ISSN

1052-7613

Acceptance Date

2023-12-30

Deposit Date

2024-02-02

Embargo Period

2024-02-03

Funding

We express our sincerest gratitude to the Bertarelli Foundation and the Garfield Weston Foundation, whose generous support made this research possible. This study contributes to the Bertarelli Programme in Marine Science. We thank the British Indian Ocean Territory Administration (BIOTA) for granting us permission to undertake the research. Thank you to the Manta Trust team and everyone else who assisted in the field and beyond: Rebecca Carter, Simon Hilbourne and Annie Murray, Rhys Diplock, Claire Collins, Anna Patal, Marleen Stuhr, Craig and Mickael and all the research vessel crew. Thank you to the two anonymous reviewers for offering constructive feedback, which improved the manuscript. This work was supported by the Bertarelli Foundation and the Garfield Weston Foundation Funding information

Keywords

IUU fishing, Mobula alfredi, acoustic telemetry, elasmobranch, marine protected area, residency

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