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 away.
The results of the study were published in the journal Integrative Organismal Biology.
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 which drifted with the current up to 19 meters away, which minimized the boat’s 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 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. They detected low-frequency sounds from up to 74 meters (243 feet) away, considerably farther away than previously demonstrated under free-swimming conditions. Interestingly, the sharks rapidly changed course away from the source, indicating they could determine the direction of the sound. More than 70% of responses occurred in the acoustic far field, and the sharks were more sensitive to lower frequencies, detecting them from farther away and at lower sound levels.
This finding is intriguing because sharks do not have the gas-filled swim bladder found in many bony fish, an organ that can help detect sound pressure. Sharks are instead thought to rely on their inner ears, including a specialized structure called the macula neglecta, to detect movement and vibrations produced by sound traveling through the water.
The findings also highlight the value of studying shark hearing in the wild, where researchers can observe responses to sound without the reflections and other limitations of laboratory tanks.
“Trying to do hearing experiments in a tank results in the sound bouncing off the walls which causes complex and confusing signals—it is like being in a house of mirrors. This is why it is so important to do these types of experiments in the ocean with wild sharks to get a natural response,” said Caroline Sullivan, lead author who conducted this work as part of her master’s degree in biological sciences.
The work was supported by the Colgan Foundation awarded to Kajiura, and the National Save the Sea Turtle Foundation awarded to Sullivan.
