The oceanic manta ray population off the coast of Ecuador is now estimated to number 22,000-plus individuals – making it more than 10 times bigger than any other known sub-population of the species.
Mobula birostris, also known as the giant manta, is the largest type of ray in the world, with wingspans that can reach more than 6m. Populations are typically small and vulnerable to human impacts, but the Ecuador population has been pronounced not only massive but potentially healthy.
The just-published Proyecto Mantas Ecuador study was led over 14 years by Fundación Megafauna Marina del Ecuador in collaboration with the Manta Trust, the Marine Megafauna Foundation (MMF) and the Ocean Ecology Lab at Oregon State University (OSU)’s Marine Mammal Institute.
“It’s clear that something different is happening here,” said Joshua Stewart, an assistant professor who heads the institute and is a co-author of the study. “This is a rare story of ocean optimism.
“In other regions, we typically have population estimates of 1,000 to 2,000 animals, which makes this species very vulnerable. In this area, we’ve estimated that the population is more than 22,000 mantas, which is unprecedented.”
Stewart said that feeding conditions were particularly favourable for a large, healthy manta population around the border of southern Ecuador and Peru, where cold, nutrient-rich water upwells to the surface. Oceanic mantas can find large quantities of krill and other zooplankton in this part of the eastern Pacific, with a handful of the mantas also wandering out as far as Galapagos.
Mainly because of commercial fishing, both targeted and bycatch, in 2019 the IUCN Red List threat category for oceanic manta rays moved up from Vulnerable to Endangered.
Mantas spend much of their time in open ocean and move around unpredictably, so can be difficult to study. But in the late 1990s Proyecto Mantas Ecuador researchers discovered that an oceanic manta population aggregated each August and September around Ecuador’s Isla de la Plata, making it relatively easy to locate and study.
It also helped that the island was a popular scuba-diving area. “Many of the photos used in our study were contributed by recreational divers who became citizen-scientists when they snapped photos of manta rays,” said lead author Kanina Harty of the UK-based Manta Trust. “We get a huge amount of information about each animal just from these photographs.”
Photographs showing the mantas’ unique spot patterns allowed individuals to be tracked over time, and injuries, evidence of mating and maturity to be documented. From 2005 to 2018 the researchers used their own and recreational scuba divers’ data to identify more than 2,800 individuals and estimate a total population of more than 22,000.
“That is significantly larger than what we’ve seen in oceanic manta ray populations elsewhere,” said Manta Trust chief executive Guy Stevens. “This is by far the largest population that we know of.”
Describing Isla de la Plata as a “globally important hotspot”, Michel Guerrero of Proyecto Mantas Ecuador said that “while this population may be healthy thanks in part to its large size, it is essential that we take the necessary steps to protect and prevent the declines that many other manta ray populations have faced”.
Fishing for mantas has been illegal in Ecuador since 2010, and 2016 in Peru, although line-entanglement, vessel strikes and bycatch remain hazards, said Guerrero. The study showed that 563 of the rays bore visible injuries or scars, and more than half of these individuals were either seen entangled in fishing gear or showed evidence of previous such encounters.
Stewart said that the mantas would also “likely be impacted by a warming climate if upwelling strength and the abundance of food changes alongside ocean temperatures”, adding that continued monitoring of the population was required to understand how human activity and climate change might affect food availability, distribution and overall population health.
Additional co-authors of the study just published in Marine Ecology Progress Series are Anna Knochel of Fundacion Megafauna Marina del Ecuador and the MFF’s Andrea Marshall and Katherine Burgess.
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