Global Population Trends and Human Use Patterns of Manta and Mobula Rays

Abstract

Despite being the world’s largest rays and providing significant revenue through dive tourism, little is known about the population status, exploitation and trade volume of the Mobulidae (mobulids; Manta and Mobula spp.). There is anecdotal evidence, however, that mobulid populations are declining, largely due to the recent emergence of a widespread trade for their gill rakers, which is reflected in increasing Food and Agriculture Organization landings trends. Here, we present results from two dedicated diver surveys, one from the eManta project, which includes summary observations from ninety 10°x10° regions with ∼200–62,000 dives per region, and the other from the Reef Environmental Education Foundation, which includes spatially more detailed observations from 3 regions with ∼4,000–118,000 dives per region. We show that mobulids as a group, which includes eleven species, have globally and regionally restricted distributions, typically have low sighting frequency (<1% of dives) and aggregate in only a few locations. Of the regions surveyed by divers, almost half (47%) report declining mobulid sightings over the last decade. Divers indicate that although mobulid ecotourism occurs in many regions (45% of those reported, n = 41) they are considered protected in only 32% of the regions. Mobulids being fished or sold in local markets were reported from 16% and 12% of regions, respectively, with most being adjacent to mobulid abundance hotspot and ecotourism regions (e.g. Sri Lanka, Indonesia, east Africa). Identification of regions where ecotourism and exploitation are at odds could help prioritize conservation efforts. Vulnerability analysis, using life history characteristics, indicates that Manta spp. are vulnerable to exploitation, tolerating only low fishing mortality rates; data limitations prohibited such analysis for Mobula spp. Our analyses support previous studies in showing the need for improved conservation and monitoring efforts, and suggest that international and enforceable management policies are required to prevent further population decline.

Trophic overlap in mobulid rays: insights from stable isotope analysis

“…Both bycatch and targeted fisheries appear to contribute to global declines in mobulid abundance (Ward-Paige et al 2013, White et al 2015, Croll et al 2016. Targeted fisheries can be managed with legislation banning the capture of mobulids, but bycatch remains a more challenging and persistent threat due to the ubiquity of mobulid bycatch in artisanal and commercial fisheries of all types (Croll et al 2016).…”

“…While these growing targeted fisheries have catalyzed focused conservation and scientific attention for mobulids (Ward-Paige et al 2013, White et al 2015, bycatch of these species likely impacted populations long before large-scale targeted fisheries began. Mobulid rays are vulnerable to incidental capture in gill nets, purse seines, trawls, and even long lines (Croll et al 2016).…”

Spatial ecology and conservation of Manta birostris in the Indo-Pacific

“…Low fecundity and small population sizes make mantas highly susceptible to fisheries impacts (Dulvy et al, 2014). Targeted fisheries and bycatch are driving family-wide declines of mobulids (Ward-Paige et al, 2013;Croll et al, 2015) and long-term monitoring efforts have recorded local declines in manta and mobula sighting frequency .…”

Manta ray tourism management, precautionary strategies for a growing industry: a case study from the Ningaloo Marine Park, Western Australia

“…Localised population declines have been reported, generally from regions in which targeted manta ray fisheries operate. Decreased sighting rates and elevated anthropogenic mortality indicate population decline in the Philippines, Indonesia, India, Sri Lanka and Mexico (Marshall et al 2011a(Marshall et al , 2011cWard-Paige et al 2013). Most notably, in Mozambique an 88% decline in sightings was recorded over an eight-year period (Rohner et al 2013).…”