Giant manta rays are one of the world’s most iconic and best-loved marine animals. However, due to their elusive nature, surprisingly little is known about what they eat.
A new study shows that instead of the gentle-giant, plankton-eating fish scientists had previously thought they were, giant manta rays are actually deep-ocean predators. This new information could be critical in helping efforts to protect these incredible creatures in the future.
The study, a collaboration between the University of Queensland (UQ), Australia, and the Marine Megafauna Foundation, began back in 2010, and focused on Isla de la Plata, off the Ecuador mainland, which hosts the world’s largest aggregation of giant manta rays.
“The previous knowledge of giant manta ray diet was based on observations of feeding activity on surface water zooplankton at well-known aggregation sites,” lead researcher Katherine Burgess, a PhD student at UQ, said in a statement.
Normal procedure would be to study the contents of their stomachs, but such an invasive and potentially lethal procedure is dangerous to attempt with protected species. Giant manta rays were listed as "vulnerable" on the International Union for Conservation of Nature's Red List of Endangered Species back in 2011.
Instead, Burgess explained: “We studied the giant manta rays’ diet using biochemical tests, such as stable isotope analysis, which works on the ‘you are what you eat’ paradigm. These tests can determine what animals have been eating by examining a piece of tissue from a muscle biopsy from a free-swimming animal.”
The study found that even though rays are filter-feed animals, the majority of their diet actually comes from deep-sea sources rather than surface zooplankton. Their findings, published in the journal Royal Society Open Science, showed that 73 percent of their diet was made up of fish and other animals from as deep as between 200 and 1,000 meters (650 and 3,280 feet) under the surface.
The concern for conservationists now is that as fisheries exhaust the ocean surface, they will expand their fishing to deeper waters, which could lead to an increase in the number of rays either caught and killed accidentally or overfished.
"At present, fisheries are increasingly going for species found in deeper waters as surface stocks of other species deplete,” Burgess told the BBC. “As such, any sort of fishery pressure can have a severe impact on populations, whether it be incidental or targeted."
This could mean the giant manta rays' protected status is in serious need of upgrading. Watch this space.