Located in the Gulf of Mexico off the coast of Texas at US National Oceanic and Atmospheric Administration (NOAA)’s Flower Garden Banks National Marine Sanctuary, the juvenile manta ray habitat is the first of its kind to be described in a scientific study. Joshua Stewart, a PhD candidate at Scripps Institution of Oceanography at the University of California San Diego in the US, observed the juvenile mantas while conducting research on manta population structure at Flower Garden Banks.
“The juvenile life stage for oceanic mantas has been a bit of a black box for us, since we’re so rarely able to observe them,” said Stewart, lead author of the study published in the journal Marine Biology. “Identifying this area as a nursery highlights its importance for conservation and management, but it also gives us the opportunity to focus on the juveniles and learn about them,” he said. “This discovery is a major advancement in our understanding of the species and the importance of different habitats throughout their lives,” he added. Known as the gentle giants of the sea, oceanic manta rays (Mobula Birostris) are large, plankton-eating rays that live in the open ocean and can reach sizes of up to 7 metres in wingspan as adults. Oceanic mantas are typically found in subtropical and tropical waters around the world with aggregation sites commonly found far from coastal areas, making their populations hard to access and study.
For this reason, major knowledge gaps remain in their basic biology, ecology, and life history. Baby mantas are virtually absent from nearly all manta populations around the world, so even less is known about the juvenile life stage. Stewart has spent the past seven years studying manta rays and encountered hundreds of adults in the wild, but his sighting of a juvenile at Flower Garden Banks in 2016 was a rare encounter for him. After noticing several other small mantas in the area, he talked with the marine sanctuary staff to see if this was a regular occurrence. Working with marine sanctuary staff, Stewart and colleagues looked through 25 years of dive log and photo identification data collected by sanctuary research divers. Mantas have unique spot patterns on their underside that can be used to identify individuals, much like a human fingerprint. Using the photo IDs and observational data, researchers determined that about 95 per cent of the mantas that visit Flower Garden Banks are juveniles, measuring an average of 2.25 metres in wingspan. The researchers also compared the habitat use of mantas at the banks to established criteria for defining shark and ray nursery habitats and determined that the sanctuary is the world’s first confirmed manta nursery ground. Recent genetic evidence indicates that oceanic mantas and a proposed third manta species (Mobula Birostris) are present at the banks.