Largest Manta Ray Population Discovered Off Coast of Ecuador

The largest known manta ray population has been identified off the coast of Ecuador.

This massive group is more than 10 times larger than any other subpopulation of the species, new research finds.

Oceanic manta rays are the largest ray species, with a wingspan or disc width of more than 20 feet. These slow-moving migratory fish are typically found in small, fragmented populations, making them vulnerable to the impact of human interference.

But the newly noted population is large and likely very healthy.

“It’s clear that something different is happening here,” said study co-author Joshua Stewart, an assistant professor with the Marine Mammal Institute in Oregon State University’s College of Agricultural Sciences. “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.”

Oceanic manta rays are difficult to study because they spend so much of their time in remote offshore areas that are hard for researchers to reach.

The research was led by Proyecto Mantas Ecuador of Fundación Megafauna Marina del Ecuador, a conservation organization based in Ecuador. The group worked with the Manta Trust, the Marine Megafauna Foundation, and the Ocean Ecology Lab at Oregon State University’s Marine Mammal Institute.

Large, Annual Gathering

In the late 1990s, researchers noticed that a population of manta rays gathered annually in August and September near Isla de la Plata, a small island off Ecuador’s central coast. The area was accessible, so the animals were easy to find and study. In addition, they congregated in a popular diving area. Because visitors took so many photos of the ray, that also gave researchers more images and data to study.

“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 Manta Trust. “We get a huge amount of information about each animal just from these photographs.”

Manta rays have bodies that are shaped like diamonds, with long pectoral fins that seem like wings. They are black and white and each has a distinctive pattern of spots on its underside, which is unique from all other rays. This specific pattern helps researchers track individual animals and follow their movements and locations.

For this study, researchers collected data between 2005 and 2018 from their own observations and from scuba diver photos. They were able to identify more than 2,800 different rays and estimated a total population of more than 22,000 animals.

“That is significantly larger than what we’ve seen in oceanic manta ray populations elsewhere,” said Guy Stevens, chief executive and founder of the Manta Trust. “This is by far the largest population that we know of.”

Cautionary Tale

The rays likely are drawn to that location for food because of its nutrient-filled water.

Oceanic manta rays (Mobula birostris) are listed as endangered by the International Union for Conservation of Nature (IUCN) Red List of Threatened Species. Their main threat is fishing, particularly as unintentional bycatch as well as targeting for fishing.

Ecuador banned manta ray fishing in 2010, but the animals are still threatened by bycatch, vessel strikes, and entanglement in fishing lines.

Researchers are happy to have discovered a large number of rays, but they are still concerned about how climate change and human activity can affect their stability.

“While there is good news about this population, it is a cautionary tale,” said Stewart. “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.”