Sharks Can Detect and Avoid Sounds Nearly 80 Meters Away, Study Finds

The ocean is filled with sound, and sharks may use those sounds to gather information about their surroundings long before they can see what is happening. Sounds from prey, predators, and other animals can reveal activity far beyond a shark’s immediate field of vision.

A new study from Florida Atlantic University provides some of the first quantified evidence that free-swimming sharks can detect distant sounds and determine where those sounds are coming from. Researchers found that wild blacktip sharks detected low-frequency underwater sounds from as far as 74 meters (243 feet) away and often turned rapidly away from the sound source.

Testing shark hearing in the wild

Blacktip sharks were well suited to the experiment because they predictably gather off the coast of southeastern Florida. Large groups congregate along the shores of Palm Beach County each winter, while smaller groups can be found there year-round.

The area’s clear, shallow water also enabled researchers to monitor individual sharks from above while limiting interference with their natural behavior.

“Their abundance and ease of access have allowed us to observe them from above, without interfering with their natural behavior and presenting controlled underwater sounds,” said senior author Stephen Kajiura, Ph.D., professor of biological sciences in Florida Atlantic University’s Charles E. Schmidt College of Science.

Researchers conducted the experiments in shallow coastal waters throughout Palm Beach County. After anchoring their boat, they placed underwater speakers in the water and allowed them to float with the current until they were 19 meters from the boat. This setup helped reduce the possibility that the boat itself would influence the sharks’ behavior.

The team played three groups of low-frequency sounds: 100–200 hertz, 200–400 hertz, and 400–800 hertz. They also played a 10-kilohertz control tone, which is beyond the known hearing range of sharks.

Rather than trying to lure the sharks toward the speaker, the researchers played sounds loud enough to trigger a startle response.

Sharks responded to sounds from 74 meters away

Researchers used calibrated hydrophones to measure sound levels at different locations. This allowed them to determine what each shark was hearing when it responded.

At the same time, a drone flying 40 to 50 meters above the water recorded the movements of the free-swimming sharks. The researchers later analyzed the footage frame by frame to measure each shark’s distance from the speaker and how sharply its swimming direction changed.

The sharks responded to all three low-frequency sound ranges but did not respond to the 10-kilohertz control tone. They detected the low-frequency sounds from distances of up to 74 meters (243 feet), substantially farther than had previously been demonstrated in free-swimming sharks.

The sharks also often turned quickly away from the speaker. This response suggests that they were not merely detecting the presence of sound; they could also determine the direction from which it was coming.

More than 70% of the recorded responses occurred in the acoustic far field, where sound behaves differently than it does close to its source. The sharks were most sensitive to lower-frequency sounds, detecting them from greater distances and at lower sound levels.

“What makes this discovery particularly interesting is that the sharks were responding to sounds beyond the acoustic near field, where sounds behave differently than when they are close to the source,” Kajiura said. “This suggests that we are detecting the movement of particles associated with sound even at significant distances from the sound source, something we have not been able to demonstrate before in free-swimming sharks.”

How do sharks hear without swim bladders?

The findings are especially notable because sharks lack the gas-filled swim bladders found in many teleost fish. In those fish, the swim bladder may contribute to the detection of sound pressure.

Sharks are thought to rely heavily on their inner ears. These organs contain a sensory structure called the macula nigra, which may help detect the movements and vibrations produced as sound travels through water.

Why wild shark hearing experiments matter

Studying shark hearing in the ocean can reveal information that is difficult to obtain in a laboratory aquarium. Sound repeatedly bounces off aquarium walls, producing complex acoustic patterns that can make the animals’ responses difficult to interpret.

“When you try to perform an auditory experiment in an aquarium, the sound bounces off the walls, causing complex and confusing signals. It’s like being in a house of mirrors. This is why it’s so important to perform this type of experiment with wild sharks in the ocean, to get a natural response,” said lead author Caroline Sullivan, who conducted the study as part of her master’s degree in biological sciences.

A hidden world of underwater sensations

The study suggests that sharks can receive information about events hundreds of feet away before animals or objects come into view. Researchers now want to learn more about how shark sensory systems detect and process distant acoustic signals.

“The ocean is an acoustic environment, and sharks are clearly attuned to that environment, but in ways that we’re only just beginning to understand,” Kajiura said. “Being able to sense and react to sounds from hundreds of feet away provides these predators with an important source of information about their surroundings. The next question is how their sensory systems are able to pick up and interpret these distant sounds.”

This research was supported by the Colgan Foundation, awarded to Mr. Kajiura, and the National Rescue Sea Turtle Foundation, awarded to Mr. Sullivan.