Manta Rays Hit by Boats More than Previously Thought
Watching one manta’s propeller wound heal has revealed a scarring pattern previously mistaken for a shark bite.
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Off the coast of Western Australia, a reef manta ray named Whoopi has helped researchers uncover a previously underreported threat to her species—boat strikes.
Scientists have studied the rays in Australia’s Ningaloo Reef, part of a UNESCO World Heritage Site, since 2004 and amassed a catalog of 1,071 individuals as of 2018. As part of that catalog, the biologists document and classify any wounds or scars they see on the animals. Some mantas sport semicircular missing chunks that clearly look like shark bites, while others have V-shaped cuts or thin slices that look like wounds from being entangled in fishing line or hit by boats. Others have old wounds that are harder to classify.
Individual manta rays are identifiable by the patterns of spots and coloration on their bellies, similar to how humans can be identified by unique fingerprints. Whoopi has been sighted every year since 2004, when the study began. On June 30, 2015, Whoopi showed up with a fresh injury on her right wing tip: a series of five sharp slices that clearly looked like a propeller strike.
Christine L. Dudgeon, a marine biologist at the University of Queensland in Australia, and a team of scientists decided to follow the ray and watch how her cuts healed over time. This has never been done before for rays, and only in two other studies for sharks.
“The wound healed quite rapidly,” says Dudgeon. After a month, the wounds had healed by about a third. After 295 days, they had closed by 95 percent. “It’s a really cool study,” says Joshua Stewart, a marine biologist with Manta Trust who was not involved with the work. “They were able to meticulously document something we had talked about anecdotally, but never quantified or extensively measured.”
Had the researchers only seen the healed scar tissue, they probably would have classified Whoopi’s wound as a shark bite or predation attempt, says Dudgeon. This inspired them to go back in their catalog and reexamine and reclassify other wounds.
It turns out mantas were probably getting hit by boats more than previously thought. Some jagged wing tips thought to have been caused by shark teeth were more likely propeller wounds, they write in their paper. After reassessing the catalog, the percentage of mantas with scars due to clear predation attempts dropped from about 12 percent to just 2.6 percent, or 29 individuals. About 13 percent of the mantas had scars similar to Whoopi’s, wounds that had been cataloged as unknown injuries.
“I don’t think I would have classified the healed scar pattern as a propeller injury if I had only seen the finished product,” says Stewart. “It’s definitely going to make us think twice as we go through our ID catalogs and classify injuries.”
Unlike marine mammals and sea turtles, manta rays are fish and do not need to come to the surface to breathe. Yet, they still spend a lot of time basking and feeding at the surface, which makes them vulnerable to boat strikes. These injuries can leave them at greater risk to predators and infection, and could impact their growth and ability to successfully reproduce. For the last decade, says Stewart, biologists have been most concerned about people fishing for mantas or catching them by accident when trawling for other species. “Those are without a doubt the acute threats to populations,” Stewart says. “But as those fishing impacts are being addressed more and more, it’s becoming clear to us as a community that we also have to take into consideration the effects of sublethal impacts—such as boat strikes.”
To Dudgeon, there’s a simple fix. “Let’s reduce boat speed in this area and give the animals a chance to get away.”