Vulnerable sting rays usually find shallow waters safer than the depths, but during very low tides or in very high or low water temperatures they will shift into deeper lagoons – even though they are putting themselves in danger from predatory sharks.
That is the latest finding by scientists from the D’Arros Research Centre of the Save Our Seas Foundation (SOSF), working in D’Arros Island and St Joseph Atoll in Seychelles alongside the South African Institute for Aquatic Biodiversity.
Three species – cowtail sting rays and mangrove and porcupine whip rays – populate St Joseph Atoll year-round, and it is an important nursery area for them.
Usefully for researchers, the atoll consists of two major habitat types: shallow uninterrupted reef flat, with scattered seagrass beds that are sometimes exposed to air, and a deeper enclosed lagoon.
The team used passive acoustic telemetry to track 20 rays from each of the three species for an average of a year, at the same time logging environmental factors such as tides and temperatures. The rays were tagged with acoustic transmitters, and 40 underwater tracking stations spread across the reef-flat and lagoon were able to pick up the unique sound pulses emitted by each one.
Exposed to predators
Caught between the devil of tidal or temperature extremes and the deep blue sea, the rays found the more stable conditions at depth preferable, even if they were more exposed to predators there.
The researchers say that their preferred habitat is likely to go on altering as such extremes become more common with climate change.
Their study does however report that D’Arros Island and St Joseph Atoll reefs, a marine protected area (MPA) for the past two years, are showing signs of recovery following the severe coral bleaching that has affected the western Indian Ocean.
“Sting rays are really important for keeping oceans healthy, especially in tropical places like Seychelles,” says SOSF project leader Chantel Elston, the lead author of the study. “This research helps to present further evidence that the isolated St Joseph Atoll provides suitable habitat for threatened sting rays, and that the newly announced MPA will have real conservation benefits.”
“When you know what the priority habitats for vulnerable species are, and how and when they move around, management plans can be developed for their conservation,” adds SOSF Seychelles Ambassador Helena Sims.
“And when it comes to marine management, the Seychellois are ahead of the game. This last batch of MPA designations means that 30% of Seychelles’ waters is now protected – 10 years ahead of the international target.”
The sting ray study is published in the journal Frontiers in Marine Science.
Related story on Divernet recently: Sting Rays Can Do Sums
Steve has been a scuba diver for 30 years and became editor of Diver magazine in 1996, following 10 years with BBC World Service and the 10 before that in motoring journalism.