Sharks have a complex and misunderstood relationship with sound. As apex predators, sharks rely heavily on their senses to hunt, navigate, and communicate. Hearing in particular allows sharks to detect prey and avoid danger in the ocean’s low visibility conditions. However, sharks are also vulnerable to loud or unnatural sounds that can disorient them and cause physiological stress. With increasing noise pollution in the oceans from human activity, there are growing concerns about the impacts on shark behavior and health. Understanding what sounds sharks are sensitive to and why can help guide conservation efforts.
How well can sharks hear?
Sharks have excellent hearing that is well-adapted to their marine environment. Their inner ear contains small calcium carbonate crystals called otoconia that are heavier than seawater and sink in response to gravity.[1] This allows sharks to detect the motion and vibration of sounds with great sensitivity. Sharks can pick up frequencies between 25 and 800 Hz, with best hearing ranging from 100-500 Hz depending on the species.[2] For comparison, human hearing is estimated to be 20-20,000 Hz.
Certain species like tiger sharks and bull sharks are especially attentive to low frequency sounds between 25-100 Hz.[3] This corresponds with the noises produced by struggling prey like crustaceans and fish. Great white sharks are attracted to sounds from about 10-40 Hz, which matches the frequencies of sounds from seals, sea lions, and small toothed whales they feed on.[4] Hearing sensitivity also depends on the environment. Coastal species like lemon sharks adapted to shallow water tend to hear best at higher frequencies than pelagic sharks living in the open ocean.
How do sharks use hearing for hunting and navigation?
Sharks rely heavily on auditory cues to hunt successfully in low visibility aquatic conditions. Their ability to pinpoint the source of faint sounds helps them locate potential prey. Sharks can detect minute pressure differences to perceive the movements of animals nearby. Sound also travels faster through denser media like fish flesh. So sharks can identify wounded or distressed prey by homing in on irregular sounds and vibrations.[5]
Hearing helps sharks navigate their environment and avoid collision with objects in murky water or at night. The hammerhead shark’s unique cephalofoil head shape may act like a hydrofoil that amplifies their ability to localize sounds.[6] Many sharks have neuromasts (hair cell sensors) along their bodies to detect changes in water movements. This lateral line system complements their hearing by alerting them to the sounds of predators or prey.
How do sharks use sound to communicate?
While not as vocal as marine mammals, sharks do produce sounds to communicate. Behavioral studies find that captive sharks growl when defending food and grunt when pursuing prey or competitive mates.[7] The broadband noises are low frequency pulses less than 320 Hz. These likely function during social interactions with other sharks.
Some sharks can produce a burst pulse sound by contracting their muscles against their swim bladder. The American eagle ray makes a similar mechanism for echolocating prey.[8] Thresher sharks are capable of slapping their long tail against the water to stun and herd schools of fish. Megamouth sharks may also vocalize, though their sounds have not yet been characterized. Further research is needed to better understand the purpose and extent of sound production in different shark species.
How does noise pollution affect sharks?
While well-adapted for hearing in their marine environment, sharks remain susceptible to loud or unnatural sounds from human activity. Noise pollution in the ocean has risen dramatically with increases in vessel traffic, offshore construction, military sonar use, and seismic surveying. This acoustic disturbance or cacophony introduces noise 10-12 times above natural ocean sound levels.[9] Chronic noise exposure poses a growing problem for sharks and marine life in general.
The vibrations, pitch, and intensity of human-generated sounds can overload shark’s sensitive hearing. Noise above 180 dB can physically damage delicate hair cells.[10] Even at lower volumes, noise can mask natural sounds that sharks rely on to communicate, navigate, and hunt successfully. Studies find noise pollution reduces sharks’ ability to detect prey by 90% or more.[11] Some sharks may startle and flee from unnatural sounds while others appear better able to habituate. But constant noise still acts as a stressor for species adapted to a quieter marine environment.
What sounds are sharks sensitive to and why?
Research identifies certain sounds as particularly disruptive to sharks:
Low-frequency active sonar – Naval sonar utilizes intense low frequency sound from 1-10 kHz to detect submarines. But this overlaps the hearing range of many shark species attuned to low frequency noises in their environment. Exposure to military sonar can cause abnormal behavioral changes in sharks and rays such as disorientation, bolting movements, or sinkling.[12] Autopsies find evidence of tissue damage like gas or fat emboli syndrome as well.
Geophysical surveys from oil/gas exploration – Seismic airgun surveys search the seabed for fossil fuel deposits by shooting very loud, low-frequency sound impulses. Airguns generate peaks up to 259 dB at 10-300 Hz every 10-15 seconds that can disturb species far beyond the survey perimeter.[13] Sharks dislike the repetitive, percussive sounds which cause them to vacate areas. Airgun noise has declined shark catch rates and may impair navigation.
Noise from pile driving and construction – Pile driving, dredging, drilling, and offshore wind turbine installation introduce banging, grinding, and percussive noise. These loud, irregular pulses can annoy sharks and spur abnormal avoidance behavior. Noise from just a few installations may disturb sharks across a radius of many miles. [14]
Vessel noise and propeller movement – Boat engine rumbling and the drone from propeller cavitation are audible for kilometers underwater. Sharks startled by close vessel passes show strong avoidance. Some species wait until boats pass to resume feeding or display higher stress levels after exposure.[15] Propeller injuries also directly harm individual sharks as well as generate further noise.
Acoustic deterrent devices (ADDs) – ADDs are designed to scare sharks away from beaches or fishing equipment by emitting unpleasant high intensity, mid-range frequency sounds. But repeated exposure may cause sharks to flee important habitat. Harmless species may avoid important habitat as well.[16]
How can humans help mitigate noise impacts on sharks?
Some ways to help reduce noise pollution and impacts on sharks include:
– Restricting when, where, and how intense airgun surveys, sonar testing, pile driving, or vessel activity occurs. Using sound barriers, prohibiting noisiest activities during key seasons/areas, limiting operation time, and promoting quieting technologies can help.
– Defining and monitoring acceptable noise levels for different species and habitats before offshore development occurs, rather than after. Further research can determine noise thresholds sharks can withstand before displaying disturbed behaviors.
– Requiring deterrent devices be species-specific, focused only on problem animals, and not used indiscriminately when safer alternatives exist. ADDs should also incorporate ramp-up and minimum technology standards to reduce harm.
– Promoting clean vessel standards and speed limits to reduce cavitation and noise from propellers. Improving ship propulsion design is also important.
– Establishing more marine protected areas may allow sharks undisturbed access to feeding and breeding refuges away from noise.
– Funding further research and encouraging data sharing among scientists, governments, industries, and local communities to make shark conservation a priority.
Conclusion
With their advanced hearing adapted to the marine environment, sharks remain highly sensitive to loud or unnatural noises from human activity like active naval sonar, seismic surveys, offshore construction, vessel traffic, and deterrent devices. Noise pollution not only disturbs normal shark behavior but causes physiological stress and declines in population abundance. It is critical to balance offshore development with the conservation of sharks and ocean habitats in the face of increasing acoustic disturbance. Creating more protected refuges, limiting noise in sensitive regions, promoting vessel quieting technologies, mandating noise monitoring standards before approvals, and stopping unnecessary deterrent use can help mitigate noise impacts on already vulnerable shark species worldwide. While further research is still needed, a precautionary approach accounting for the hazards of noise pollution must be taken to protect sharks and the ocean ecosystems they top.