The ocean is filled with sound, and for sharks, those sounds can reveal what is happening well beyond their field of view. Noise produced by prey, predators, and other animals may provide important information about the surrounding environment. Yet scientists have had little direct evidence showing just how far a shark swimming freely in the ocean can hear, or whether it can tell where a distant sound originated.
A new study from Florida Atlantic University is beginning to answer those questions. Researchers studying blacktip sharks (Carcharhinus limbatus) used an underwater speaker, a drone, and wild sharks swimming naturally in coastal waters. They found that the animals could detect sounds from substantial distances and abruptly turn away from the source, even when it was nearly 250 feet away.
The findings, published in Integrative Organismal Biology, provide the first quantified evidence that free swimming sharks can detect and orient away from sounds in the acoustic far field. The results offer a rare look at a basic shark sense that remains surprisingly poorly understood. Blacktip sharks were especially well suited for the experiment because they gather predictably off Southeast Florida.
Large numbers congregate along the Palm Beach County coast each winter, while smaller groups can be found there throughout the year. Clear, shallow water also allowed researchers to monitor individual sharks from above while minimizing interference with their natural behavior. "Their abundance and accessibility made it possible for us to observe them from above without disturbing their natural behavior, while also presenting controlled underwater sounds," said Stephen Kajiura, Ph.D., senior author and a professor of biological sciences in FAU's Charles E.
Schmidt College of Science. Researchers carried out the tests in shallow coastal waters across Palm Beach County. After anchoring a boat, they placed an underwater speaker in the water and allowed it to drift with the current as far as 19 meters from the vessel.
This setup helped reduce the possibility that the boat itself would influence the sharks. The team played three groups of low-frequency sounds, covering 100 to 200 Hertz, 200 to 400 Hertz, and 400 to 800 Hertz. They also used a 10 kiloHertz control sound, which lies beyond the known hearing range of sharks.
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