Abstract
The size and structure of a photographically identified population of reef manta ray, Manta alfredi, were examined at aggregation sites over a four-year period in southern Mozambique. The use and standardisation of photo-ID techniques was examined as a minimally-intrusive means to study this species. Using these techniques, we report on the size, structure and seasonality of this population of M. alfredi. In total, 449 individuals were identified during this time period, 40.5% of which were re-sighted on at least one occasion. The longest period between re-sighting events was 1,252 days. During the study period, annual population size estimates for M. alfredi ranged from 149 to 454 individuals. The superpopulation size estimate for the entire study period was 802 individuals, the first reported for M. alfredi at a monitored aggregation site. A highly significant sex bias was evident with a female:male ratio of 3.55:1. The majority of rays (89.9% males; 49.7% females) were considered mature, with most individuals between 3.0 and 4.9 m in disc width. Manta alfredi were observed at the study sites in each month of the calendar year. The maximum number of individual rays seen per dive was 30. Large numbers of rays (20 + per dive) were seen in the months of November, December and January, which coincide with the breeding season. Natural markings were unique to individuals and did not change substantially with time, which provided further support for their use in the identification of individual M. alfredi over multiple years. Multiple re-sightings of individual M. alfredi suggest that many individuals in this population exhibit site fidelity to the examined aggregation sites. As target subsistence fishing for M. alfredi exists along the Mozambican coastline, management efforts to monitor and prevent overexploitation at these critical habitats should be a priority.
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Acknowledgments
This research was conducted as a part of a research programme at the University of Queensland with funding provided by the Save Our Seas Foundation, the University of Queensland, the Project AWARE Foundation, Ocean Revolution, Idea Wild, and the Hillary Vider’s scholarship. We express our gratitude to Casa Barry Lodge in Tofo Beach Mozambique for their hospitality and cooperation with this project as well as their assistance with logistics. We thank Simon Pierce for his assistance in the field. Ken Pollock for advice relating to the mark-recapture analyses. All experiments carried out in this study comply with the laws of Mozambique and the research ethics of the University of Queensland.
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Marshall, A.D., Dudgeon, C.L. & Bennett, M.B. Size and structure of a photographically identified population of manta rays Manta alfredi in southern Mozambique. Mar Biol 158, 1111–1124 (2011). https://doi.org/10.1007/s00227-011-1634-6
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DOI: https://doi.org/10.1007/s00227-011-1634-6