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The ecology and conservation of manta and mobula rays in the Indo-Pacific

  • Author(s): Stewart, Joshua David
  • Advisor(s): Semmens, Brice
  • et al.
Abstract

Manta and mobula rays (collectively mobulids) are planktivorous pelagic rays that have received little scientific attention in comparison to many other marine megafauna species. Major knowledge gaps remain in the biology and ecology of mobulid rays, often hindering effective management of these species. In the past decade, substantial declines in mobulid populations have been recorded in response to targeted fisheries and bycatch of these species in both commercial and artisanal fisheries. This dissertation seeks to fill several critical knowledge gaps in the spatial ecology and foraging behavior of mobulid rays in the Indo-Pacific. Chapter 1 uses a combination of satellite archival tagging, stable isotope analysis, and high-throughput genetic sequencing to examine the horizontal movements and population structure of oceanic manta rays (Manta birostris) in the Indo-Pacific. We find that oceanic manta populations exhibit philopatry within relatively restricted home ranges, and demonstrate little connectivity between both neighboring and distant populations. These findings suggest that management and conservation action can be applied effectively at local scales in addition to the international agreements that are typically relied upon for marine megafauna species. Chapter 2 examines the diving behavior of oceanic manta rays at a remote archipelago off of Mexico’s Pacific coast using satellite archival tagging and opportunistic submersible observations. We found evidence of multiple feeding modes with different vertical distributions, seasonal changes in vertical habitat use, deep-water foraging behavior, and association with both the thermocline and the deep scattering layer. Taken together, these findings suggest a high degree of behavioral plasticity in oceanic manta rays that may explain their ability to remain resident by shifting their prey targets vertically rather than over large horizontal distances. Chapter 3 uses stable isotope analysis and Bayesian mixing models to assess the trophic overlap in five species of sympatric mobulid rays at three sites across the Indo-Pacific. We find that the degree of trophic overlap is inversely related to regional primary productivity characteristics, with greater overlap in lower-productivity regions. We posit that this is a result of prey density thresholds required by mobulids to make foraging energetically profitable. The trophic relationships we observe help explain the patterns of multispecies mobulid bycatch in regions such as Sri Lanka and the Philippines, and interspecies seasonal and spatial differences in bycatch risk of mobulid rays in Peru. This dissertation contributes substantially to our understanding of mobulid ray spatial ecology and foraging dynamics, and appropriate management and conservation approaches for these threatened species.

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