The world’s largest population of giant oceanic manta rays (Mobula birostris) has been formally identified off the coast of Ecuador, according to new collaborative research by the Marine Megafauna Foundation, Fundación Megafauna Marina del Ecuador, the Manta Trust and the Ocean Ecology Lab at Oregon State Univerity’s Marine Mammal Institute.
According to the study, recently published in the journal Marine Ecology Progress Series, more than 2,800 individual animals were identified between 2005-2018. Data modelling estimates the overall size of the ‘super-population’ to be in excess of 22,000 oceanic manta rays – more than ten times the size of any other population so far known to science.
Giant – or oceanic – manta rays are the largest known species of ray, with a disc width (wingspan) that can measure up to 7m across, although the average disc width of an adult is approximately 4.5m. They are distributed circumglobally in tropical and temperate waters, but, unlike their near-shore resident cousins, the reef manta ray (Mobula alfredi), giant mantas spend most of their time in the open ocean, congregating around sea mounts and oceanic islands, which makes them especially difficult to study.
Seasonal, predictable aggregations of giant mantas were discovered at Isla de la Plata in the Machalilla National Park, and the Bajo Copé
Marine Reserve off the coast of Ecuador Ecuador in 1996, and research has been focused on these areas since 2010.
Identification of giant manta rays for the study was provided by photographs taken by scuba divers, guides and instructors, from as early as 2005 – before dedicated research expeditions to the region had begun – using the spot markings present on the mantas’ ventral side, which are unique to each animal.
The population is driven to the region by the convergence of equatorial currents and strong upwellings which drive nutrient-rich water to the surface between June and September each year, providing sustenance for the krill and plankton which in turn become food sources for the filter-feeding manta rays.
Unfortunately, the same conditions which attract giant oceanic manta rays to the area also create perfect conditions for industrial-scale fishing. While it has been illegal to catch the rays in Ecuadorean waters since 2010, their presence and tendency to swim near the surface of the water for long stretches make them vulnerable to being caught by abandoned, lost or discarded fishing gear, accidental boat strikes and bycatch.
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A total of 563 animals were found to have injuries, 348 – one in eight of all the manta rays identified – were either entangled or ‘showed evidence of previous line scarring’. MMF co-founder and the world’s principal giant manta ray researcher, Dr Andrea Marshall, found herself having to regularly free entangled manta rays during her research dives.
‘I was shocked to see how prevalent entanglements were in the region,’ said Dr Marshall. ‘Every year we would spend dive after dive removing fishing lines, hooks, and, worst case, large fishing nets from the mantas.
‘Some of the mantas were so badly entangled with heavily fouled nets that the lines would embed into their skin, making rescue extremely difficult and dangerous,’ added Dr Marshall. ‘There were times I would find myself hooked and entangled myself as I tried to rescue the mantas, and I constantly worried about getting pulled into the depths once connected to the mantas and the lines.
‘Our team members could spend entire dives cutting away at the nets and lines- the mantas patiently cooperating, remaining calm, barely swimming, the whole time. These encounters always reminded us how intelligent these animals truly are. Some entangled mantas would swim directly to us as if to solicit assistance.’
Although the size of Ecuador’s giant oceanic manta ray population is relatively good news for a species that is currently classed as Endangered by the IUCN Red List for Endangered Species, the report concludes that continued observation is necessary to ensure the survival of the species.
Aside from the immediate threat to manta rays from fishing activities, the authors note that ‘the uncertainty of how the future effects of climate change stand to impact the intensity of upwelling and productivity in the region,’ requires further study of the population to better understand how changes in the ocean may impact their future.
‘While there is good news about this population, it is a cautionary tale,’ said Joshua Stewart, assistant professor with the Marine Mammal Institute at Oregon State University, and one of the paper’s co-authors. ‘Manta rays appear to be sensitive to environmental disruptions such as changes to ocean temperatures and food availability. They will likely be impacted by a warming climate if upwelling strength and the abundance of food changes alongside ocean temperatures.’
The complete paper, ‘Demographics and dynamics of the world’s largest known population of oceanic manta rays Mobula birostris in coastal Ecuador’, by Kanina Harty, Michel Guerrero, Anna M. Knochel, Guy M. W. Stevens, Andrea Marshall, Katherine Burgess and Joshua D. Stewart is available (open access) in the online journal Marine Ecology Progress Series
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