This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Sound travels through the ocean carrying clues about potential prey, predators and other animals, giving sharks an important way to detect what they cannot yet see. But exactly how far a free-swimming shark can hear a sound—and whether it can determine where that sound is coming from—has remained largely unknown.
Now, a new Florida Atlantic University study of blacktip sharks (Carcharhinus limbatus) is helping close that gap. Using an underwater speaker, an aerial drone and free-swimming sharks, researchers found that sharks can detect sounds from considerable distances and respond by dramatically changing direction away from the source—even when the sound source is nearly 250 feet (76 meters) away. Results of the study, published in the journal Integrative Organismal Biology, provide the first quantified evidence that free-swimming sharks can detect and orient away from sound in the acoustic far field, offering new insight into one of the most fundamental yet least understood senses in these ancient marine predators.
Researchers chose blacktip sharks because their predictable seasonal aggregations off Southeast Florida provided a rare opportunity to observe free-swimming sharks in clear, shallow water. Each winter, large numbers of blacktip sharks gather along the Palm Beach County coast, while smaller groups remain in the area year-round. "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. The study was conducted in shallow nearshore waters throughout Palm Beach County. Researchers anchored a boat and deployed an underwater speaker that drifted with the current up to 19 meters (62 feet) from the boat, minimizing its influence on the sharks.
They tested three ranges of low-frequency sounds—100 to 200 Hertz, 200 to 400 Hertz and 400 to 800 Hertz—along with a 10-kiloHertz control sound outside the known hearing range of sharks. They played the sounds at a high intensity to startle the sharks rather than try to attract them. Researchers used calibrated hydrophones to map sound levels at different distances so they could calculate exactly what the sharks heard when they responded.
An aerial drone tracked free-swimming sharks from 40 to 50 meters (130 to 165 feet) above the water as researchers presented control and experimental sounds. Frame-by-frame video analysis was used to measure the sharks' response distance and changes in swimming direction. The sharks responded to all three experimental sound ranges but not to the 10-kiloHertz control.
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