Drone footage captured off the coast of southeast Florida has recorded, for the first time, an evasive behavior practiced by blacktip sharks trying to escape huge predators. The adult sharks, Carcharhinus limbatus, flee for shallow waters when faced with hammerhead sharks, Sphyrna mokarran.
Many species of shark use shallow waters as a kind of underwater nursery where they can grow and perfect hunting strategies with a reduced risk of predation from bigger sharks. This method of seeking protection in the shallows hadn’t been recorded before in adult sharks until the emergence of this footage, which was captured as part of a study published in the Journal of Fish Biology.
By using the drone, researchers were able to observe the natural predator-prey behavior of these sharks without influencing their activity with the presence of boats. Agile blacktip sharks are efficient hunters themselves, feeding on fish, cephalopods, and crustaceans, but are predated on by bigger sharks including the great hammerhead. Hammerheads can be as big as 5.5 meters long (18 feet) and feed on stingrays, fish, and other sharks. They’re likely drawn by large groups of blacktips who present a good meal for the giant hammerheads.
The drone footage captured three events where a hammerhead approached an aggregation of blacktip sharks just off the coast of Palm Beach County. In all three predator-prey interactions, the blacktips were seen retreating to the shallower waters close to shore to avoid the incoming hammerhead. The events reveal that blacktip sharks will almost beach themselves in swimming right up to the shoreline as the massive hammerhead, which is almost double the size of the blacktips, struggles to swim in the increasingly shallow waters.
"In two of the three videos, the hammerhead shark actively chased one or more blacktips toward the shore but was unsuccessful at capturing its prey," said senior author Stephen Kajiura, a professor of biological sciences at Florida Atlantic University, in a statement. "The chases ended with the hammerhead making a sharp turn away from its intended prey and the shore, back into deeper waters. The chasing events showed the hammerhead struggling as it experienced difficulty following the blacktips into the shallow waters."
Hammerheads are known for their tall dorsal fins, which are actually longer than their pectoral fins and they use this to generate lift when swimming on their side. This fails to become an effective strategy for locomotion when the caudal fin is out of the water, meaning that very shallow waters limit the swimming ability of hammerheads as the caudal fin breaches the surface. This shortcoming enables the blacktips, who are better able to maneuver in shallow waters, to escape predatory hammerheads in the shallows.