Movement Ecology of the Reef Manta Ray Manta alfredi in the Eastern Red Sea

Abstract

Many well-studied elasmobranch populations have recently exhibited significant decline. The limited data related to fisheries and sightings for many unstudied or poorly understood populations indicate that these are also suffering. Directed fisheries and more cryptic threats such as entanglement and vessel strike represent significant risk to mobulid rays, arguably one of the most vulnerable elasmobranch groups. Very little information currently exists describing the basic ecology of manta rays or quantifying anthropogenic threats and impacts; however, recent efforts have drastically improved the body of knowledge available for these species, including oceanographic influences on movement, seasonal migration, and mating behaviors. Nevertheless, Red Sea mantas remain completely enigmatic. In this thesis, Chapter 1 details results from tagging 18 reef manta rays Manta alfredi in the eastern Red Sea using satellite and acoustic tag technology and demonstrates that mantas occupy areas with high human traffic. The combined satellite and acoustic techniques define both regional movements and ‘hotspots’ of habitat use where there is significant potential for manta-human interaction. I also present opportunistic sighting data that corroborate anthropogenic impacts on this population. Chapter 2 explores the vertical component of the nine satellite tags that were deployed on Manta alfredi as described in the previous chapter. Seven tags returned adequate data for analysis. Three of the seven were physically recovered yielding full archival datasets of depth, temperature, and light levels every 10-15 seconds for over 2.6 5 million cumulative data points. Mantas frequented the upper 10 m during the day and occupied deeper water through nocturnal periods. Individuals also exhibited deep diving behavior as deep as 432 m, extending the known depth range of the species. An investigation of 76 high-resolution deep dives suggests gliding is a significant behavioral component of these dives and may provide an efficient mechanism for travel compared to continuous horizontal swimming. This study is the first to employ satellite telemetry techniques on Manta alfredi and is the only study directed at mobulids in the Red Sea. A holistic understanding of these behaviors is essential for developing and implementing appropriate management techniques, and this work is particularly timely in light of recent international trade regulation as mantas were listed on Appendix II of the Convention on International Trade in Endangered Species.

Date of Award	Jul 2013
Original language	English (US)
Awarding Institution	Biological, Environmental Sciences and Engineering
Supervisor	Michael Berumen (Supervisor)