Scientist Stumbles On A Nursery for Manta Rays

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A young manta ray in the Flower Garden Banks National Marine Sanctuary/ NOAA

A scientist researching manta rays in a US marine park has discovered what is thought to be the first identified nursery for the giant pelagics.

Joshua Stewart, doing his PhD in the Flower Garden Banks National Marine Sanctuary in the Gulf of Mexico, was intrigued when he repeatedly saw rarely observed juvenile manta rays (Mobula birostris).

'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, who is also the executive director of the Manta Trust, a global manta conservation programme. '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. This discovery is a major advancement in our understanding of the species and the importance of different habitats throughout their lives.'

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, Stewart and the marine sanctuary staff determined that about 95 per cent of the mantas that visit Flower Garden Banks are juveniles, measuring an average of 2.25m  (7.38 feet) in wingspan.

Stewart and colleagues describe the manta nursery in the study, 'Important juvenile manta ray habitat at Flower Garden Banks National Marine Sanctuary in the northwestern Gulf of Mexico,' published in the journal Marine Biology.

'This is exciting news for the manta rays in the northwestern Gulf of Mexico,' said George P Schmahl, superintendent of Flower Garden Banks National Marine Sanctuary. 'The understanding that the mantas are utilizing the Flower Garden Banks National Marine Sanctuary, and possibly other reefs and banks in the region, as a nursery has increased the value of this habitat for their existence.'

Flower Garden Banks is a pristine sanctuary about 100 miles south of Texas with coral reef ecosystems that have remained far healthier than other reefs in the Caribbean and Gulf of Mexico. These coral communities have grown on top of hard substrate thrust out of the seafloor by the development of salt domes. The researchers suspect that the juvenile mantas are spending time at the relatively shallow banks to recover body temperature after accessing deep, cold waters off the continental slope.

Certain types of zooplankton are most abundant in these deeper habitats and are known to be a favourite prey item for mantas. The northern Gulf of Mexico is home to several species of large sharks, which may make basking at the surface a perilous way to warm up for small, juvenile mantas. Instead, the study's authors suspect that they retreat to the relative safety of the banks after diving into cold offshore waters. To confirm this, researchers at the sanctuary plan to begin tagging juvenile mantas at the banks to track their movements and diving behaviour.




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