Sharks detect low-frequency sounds from 250 feet away, study finds

Off the coast of Palm Beach County, Florida, researchers from Florida Atlantic University placed an underwater speaker in shallow water and waited to see what the sharks would do. What they discovered was a sensory capability that had never been clearly demonstrated in wild, freely swimming sharks: the ability to hear and locate low-frequency sounds from nearly 250 feet away.

The study, published in Integrative Organismal Biology, focused on blacktip sharks, animals that gather predictably in Southeast Florida's coastal waters. The researchers chose this location and species deliberately. Blacktip sharks congregate along the Palm Beach County coast in large numbers each winter, with smaller populations present year-round. The shallow, clear water allowed scientists to observe individual animals from above using a drone while keeping the sharks undisturbed in their natural habitat. Stephen Kajiura, a professor of biological sciences at FAU and senior author of the work, noted that the sharks' abundance and accessibility made it possible to conduct controlled acoustic tests without disrupting their normal behavior.

The experimental setup was designed to isolate the sharks' hearing from other variables. Researchers anchored a boat, then positioned an underwater speaker and allowed it to drift with the current as far as 19 meters away from the vessel, reducing the chance that the boat itself would influence the animals' responses. They played three groups of low-frequency sounds—100 to 200 Hertz, 200 to 400 Hertz, and 400 to 800 Hertz—at volumes loud enough to provoke a startle response. They also used a 10 kiloHertz control sound, which falls outside the known hearing range of sharks. Rather than trying to attract the animals toward the speaker, the researchers wanted to see if the sharks would react by turning away.

Calibrated hydrophones measured sound levels at different distances, allowing researchers to determine exactly what acoustic pressure each shark experienced at the moment it reacted. A drone flying 40 to 50 meters above the water recorded the movements of free-swimming sharks during both the experimental and control sounds. Scientists then analyzed the footage frame by frame, measuring how far each shark was from the speaker and how sharply its swimming direction changed in response.

The results were striking. Sharks detected low-frequency sounds from distances of up to 74 meters—243 feet—substantially farther than had previously been demonstrated in freely swimming animals. They showed no response to the 10 kiloHertz control sound, confirming that their reactions were genuine acoustic responses. More than 70 percent of the recorded responses occurred in what researchers call the acoustic far field, the region where sound behaves differently than it does near the source. Critically, the sharks did not simply detect the presence of sound. They made rapid, sharp turns away from the speaker, indicating they could determine the direction from which the sound was coming.

The sharks proved most sensitive to lower frequencies, detecting them from greater distances and at lower sound levels. This finding carries particular significance because sharks lack the gas-filled swim bladder found in many bony fish—an organ that can assist in sound pressure detection. Instead, sharks rely largely on their inner ears, which include a specialized sensory structure called the macula neglecta, believed to help them detect motion and vibrations as sound travels through water. The fact that sharks can detect particle motion from hundreds of feet away, something not previously demonstrated in free-swimming animals, suggests their sensory systems are far more sophisticated than laboratory studies had indicated.

Caroline Sullivan, the lead author who conducted this work as part of her master's degree, emphasized why ocean-based research proved essential. In laboratory tanks, sound bounces repeatedly off the walls, creating complex and confusing acoustic patterns that distort what researchers can actually measure. "It is like being in a house of mirrors," Sullivan explained. The open ocean provided the clarity needed to observe genuine shark responses.

The implications extend beyond academic curiosity. The ocean is fundamentally an acoustic environment, and these findings suggest that sound provides sharks with useful information about events occurring hundreds of feet away, long before prey or other objects enter visual range. Kajiura noted that being able to detect and respond to sounds from such distances gives these predators an important source of information about their surroundings. The next phase of research will focus on understanding precisely how the shark sensory system detects and processes these distant acoustic signals—a question that could reshape how scientists understand predator behavior and marine ecology.