Sharks aren’t flying through tornados, but it appears some of them are weathering tropical hurricanes. Thanks to the surprise arrival of two tempests during two separate shark monitoring projects in 2016 and 2017, researchers were able to track four large shark species before, during, and after the storms. In a study that appeared online April 22 in Estuarine, Coastal and Shelf Science, researchers reported that while other species retreated from the hurricanes, tiger sharks held fast.
The team took advantage of an opportunity to monitor something that hadn’t been tracked before, says Marcus Drymon, a marine scientist from Mississippi State University who was not involved in the work, but has collaborated with two of the study’s authors in the past. “It’s a really interesting study.”
Hurricanes can devastate coastal communities, and their damage extends below the water’s surface too. Surging storms can destroy reefs, displace marine fauna, and upwell nutrients to spawn harmful algal blooms. To evade the turmoil, some animals will evacuate the shallows. For example, scientists detected fewer dolphins off the Maryland coast during and after intense storms. And when Tropical Storm Gabrielle hit the Florida Coast in 2001, researchers observed that juvenile blacktail sharks retreated into deeper waters.
But it wasn’t until 2016 and 2017, when Hurricane Irma swept past Miami and Hurricane Matthew slammed into the Bahamas, that scientists gleaned insights into the storm response of large sharks. Hammerschlag and his colleagues were conducting unrelated shark monitoring projects—on how urbanization influenced the movements of nurse sharks (Ginglymostoma cirratum), bull sharks (Carcharhinus leucas), and great hammerhead sharks (Sphyrna mokarran) in Miami, and how dive tourism is affecting tiger shark (Galeocerdo cuvier) movement in the Bahamas—when the storms arrived.
Having already tagged the sharks with acoustic signalers and deployed acoustic telemetry arrays—devices that pinged the team whenever a tagged shark was near—the researchers found themselves well prepared to track the large sharks during the hurricanes. “We looked at their space use and movement before, during, and after the storm,” says Neil Hammerschlag, a shark ecologist from the University of Miami and a coauthor on the study.
When Hurricane Irma arrived in Biscayne Bay just north of Key Largo, Hammerschlag and his colleagues were tracking nine nurse sharks, three bull sharks, and seven great hammerhead sharks. They found that during the hurricane, most of these sharks had left the study area. The sharks, they reasoned, must have fled for the refuge of deeper waters. “We found a similar pattern to what has been found for small sharks,” says Hammerschlag.
“But tiger sharks in the Bahamas didn’t behave that way at all,” he says. Even as the eye of the Category 5 hurricane barreled down on the study site there, the researchers continued to observe about one of the 12 tagged tiger sharks each day before and during the storm. And just after the hurricane passed, daily counts of tiger shark detections doubled and remained high for weeks.
As for why tiger sharks exhibited this behavior when the others did not, Hammerschlag speculates there could be a couple of reasons. Tiger sharks can weigh more than 1,900 pounds and grow to 18 feet long, making them the biggest species the researchers tracked; the next largest, the great hammerhead, reaches a similar body length but its weight tops out around 1,000 pounds. Tiger sharks’ robustness might help them endure rougher conditions, says Hammerschlag. And tiger sharks have “an incredibly diverse diet,” he adds. “From sea birds to sea turtles to dolphins to fish to other sharks, [they eat] almost anything in the water. [They] were probably taking advantage of all the new scavenging opportunities from dead animals that were churned up in the storm.”
Shark biologist Kim Holland of the University of Hawai‘i at Mānoa says more work will be needed to explain the sharks’ behavior. “You would think that if running away from a storm would be appropriate for one species, it would be good for all species, but here that isn’t the case,” he says. And scientists aren’t sure how sharks sense incoming storms—they could be responding to changes in atmospheric pressure or perhaps surf noise, but that hasn’t been established yet, he adds. “There are as many questions generated as questions answered.”
Looking toward the future, Hammerschlag says he believes it will become increasingly important to understand how shark populations react to hurricanes. “The number of major storms like hurricanes are increasing in frequency and ferocity, and large sharks help keep checks and balances in ecosystems,” he says. “How these animals respond to global change could have ecological or economic consequences.
L.F.G. Gutowsky et al., “Large sharks exhibit varying behavioral responses,” Estuar Coast Shelf Sci, 256:107373, 2021.