A recent Florida Atlantic University study shows why hammerheads swim so close to us. It isn’t us they’re after, it’s blacktip sharks, which head nearshore to avoid being prey to the much larger hammerhead.
The furtive shadows of blacktip sharks are well known along Palm Beach County’s shoreline during their annual migration, but a new study found a novel reason why they hug the shallows so closely.
Wading-deep water just off the sandy coast provides refuge for blacktips from the less agile hammerhead shark — a predator that can’t maneuver as well nearshore, according to a Florida Atlantic University study published last month in Journal Fish Biology.
Blacktips are predator and prey, eating fish, octopus and crustaceans, but also being eaten by larger sharks such as hammerheads.
Study authors, FAU biological sciences professor Stephen Kajiura, and student Melanie D. Doan, have evidence of the blacktips eluding hammerheads in clear nearshore waters off Palm Beach County in three videos taken in 2018 and 2019.
The hammerhead sharks in the videos were at least twice the size of the blacktip sharks making them approximately 12 feet long.
"The chases ended with the hammerhead making a sharp turn away from its intended prey and the shore, back into deeper waters," said Kajiura, director of FAU’s Elasmobranch Laboratory in a press release. "The chasing events showed the hammerhead struggling as it experienced difficulty following the blacktips into the shallow waters."
Kajiura began tracking the blacktips’ yearly sojourn along South Florida’s coastline a decade ago by meticulously counting each shark in photos taken during flights from Boca Raton to Jupiter.
While previous studies have shown that young sharks use shallow water to avoid being eaten, "no documentation was available to show that large adult sharks actively swim into shallower waters to avoid predation," the FAU study says.
Despite the thousands of blacktips that cruise the coast each year, just 19 unprovoked shark bites were recorded in Palm Beach County between 2009 and 2019, according to the International Shark Attack file.
Tyler Bowling, manager of the Florida Program for Shark Research at the Florida Museum of Natural History, noted last year that blacktip sharks also hunt in shallow water for bait fish, which causes them to be in the same water as bathers.
"Most of the bites we see along the southeast U.S. are from these animals and are usually minor nibbles comparatively speaking," Bowling said last year.
Still, FAU’s study proves they are also heading into shore to escape hammerheads.
Kajiura said an unusual feature of hammerheads — an exceptionally tall dorsal fin — is one characteristic that prevents them from following blacktips into the shallows.
The large dorsal fin is used to generate lift when swimming on their side, instead of aiding propulsion and precise turning as seen in other shark species.
While the tail fin helps push the hammerhead forward, the dorsal fin and upper lobe of the tail fin are seen breaching the surface in each of the videos studied. The blacktip shark is able to remain completely submerged, making it more nimble.
"When the dorsal and caudal fins of hammerhead breach the surface, they are neither generating lift, providing thrust, nor helping to facilitate turning as efficiently as when they are completely submerged," Kajiura said in a press release. "The shallow water thus constrains the locomotion of the hammerhead, which provides the blacktip shark with a functional refuge because their smaller size allows them to continue to swim and maneuver effectively away from their larger predator."