A drone has recorded footage of blacktip sharks fleeing to shallow waters away from a huge predator along the coast of southeast Florida. The footage provides the first evidence that adult blacktip sharks (Carcharhinus limbatus) use shallow waters as a refuge from the great hammerhead shark (Sphyrna mokarran).
Prior to a study by Florida Atlantic University, there is no documentation or evidence yet available that large adult sharks also swim in shallow waters to take refuge from its predators. Several juvenile shark species swim in shallow water nursery sites where they can grow to reduce the risk of getting eaten by larger species.
Using the unmanned aerial vehicles (UAV), students from FAU were able to unobtrusively observe and document natural behaviors of the sharks in the wild, providing insight into the seldom-seen interactions between the two species. They published their study in the Journal of Fish Biology.
Blacktip Shark: Can't Touch This
Blacktip sharks are both agile predators and prey. They eat teleost fishes, cephalopods and crustaceans; while also being a prey for larger sharks, such as the great hammerhead that can grow as big as 18 feet long.
Despite being big, hammerhead sharks are often found in relatively shallow waters where they usually feed. They typically prey on stingrays, bony fishes, and other sharks, so it is not a surprise to see them around the blacktip shark aggregations, which provide an abundance of possible prey.
A UAV was able to record on three separate occasions wherein a hammerhead shark was approaching an aggregation of blacktip sharks in Palm Beach County's nearshore waters.
According to the researchers, blacktip sharks in the area are under 6 feet, which they used to calibrate the scale in the video footage to get an estimate of the distance from the shore for these interactions. Based on their estimations, the interactions happened in water no more than waist deep, and in less than 150 feet offshore of the beach.
In all three video footages, the blacktip sharks are seen fleeing to shallow waters close to shore as a refuge from a great hammerhead, which is twice the former's size, making them approximately 12 feet long. Three separate footages were recorded during the day on February 28, 2018, and 2019, and also on March 3, 2019.
Stephen Kajiura, Ph.D., senior author, narrated that the hammerhead shark successfully chased the blacktip sharks toward the shore on two of the three footages, but has failed to capture its prey. The chases ended with the hammerhead turning away and back into the deep waters.
Hammerhead Shark Can't Swim on Shallow Waters
Hammerhead sharks have an exceptionally tall first dorsal fin that is longer than their pectoral fins. Their dorsal fin helps them generate lift when swimming on their side, unlike other shark species that use it to facilitate propulsion and precise turning.
Its caudal fin thrusts and propels the hammerhead forward, but both dorsal and upper lobe caudal fins are seen on the videos breaching the surface, so they are no longer generating lift, providing thrust nor helping to facilitate turning.
Thus, the shallow water constrains the shark's locomotion, which provides the blacktip shark with a functional refuge because of its smaller size that allows them to swim effectively away from the hammerhead.
