Many people once believed the underwater world was silent. However, fish do make sounds, and the aquatic environment is often filled with diverse acoustic signals. These sounds are typically imperceptible to human ears without specialized listening equipment, revealing a hidden layer of communication beneath the surface.
How Fish Produce Sounds
Fish create sounds using various physiological mechanisms. A primary method involves the rapid vibration of the swim bladder, a gas-filled organ. Fish like drums and croakers have specialized sonic muscles that contract rapidly, causing the swim bladder to pulsate and generate drumming or thumping sounds. These muscles are among the fastest contracting muscles known in vertebrates.
Another significant sound-producing mechanism is stridulation, which involves rubbing together hard, bony body parts. This can occur when fish grind their teeth, rub fin rays against their skeletal structure, or articulate skull bones. Marine catfishes, for instance, produce squeaking sounds by moving pectoral fin spines against their shoulder girdle, and seahorses make clicking noises by rubbing parts of their skull. Fish can also produce hydrodynamic sounds as a byproduct of rapid movements through water, such as sudden changes in speed or direction, creating pressure waves.
Why Fish Make Sounds
Fish vocalizations serve various communicative purposes in their social interactions and survival. Sounds are frequently used during courtship and mating rituals. Male fish may produce specific calls to attract females, such as the plainfin midshipman humming to draw mates to its nest. The loud, low-frequency drumming sounds of black drum fish during spawning can even be heard by humans on land, illustrating the intensity of these reproductive signals.
Sounds also function in territorial defense and aggressive displays, allowing fish to warn off rivals or deter threats without direct physical confrontation. Some species use vocalizations as warning signals to alert other fish to danger or as a fright response when startled. Within groups, sounds contribute to schooling cohesion and navigation, helping fish maintain position and coordinate movements. Pacific herring, for example, produce high-pitched “Fast Repetitive Tick” (FRT) sounds by expelling gas from their swim bladders, thought to help them stay together in protective shoals, especially at night.
The Diverse Sounds of Fish
The underwater soundscape created by fish is remarkably varied, encompassing a wide spectrum of acoustic characteristics. Fish produce percussive sounds like clicks, pops, and snaps, often resulting from stridulation or quick movements. Other common vocalizations include grunts, growls, and croaks, which are typically low-frequency sounds produced by the swim bladder.
Some fish generate more sustained sounds, such as whistles and hums. The oyster toadfish, for instance, is known for its distinct “boat whistle” hum, a continuous sound produced by rapid swim bladder vibrations. The variety extends to thumps, purrs, knocks, rattles, barks, and hoots. These diverse acoustic signals allow for species-specific communication, much like bird songs in terrestrial environments.
Uncovering Underwater Acoustics
Scientists study and record fish sounds using specialized equipment, primarily hydrophones. These devices are deployed in marine environments to capture the subtle and sometimes loud vocalizations of fish. Acoustic research faces challenges, such as distinguishing fish sounds from other underwater noises and overcoming the masking effect of human-generated sounds like those from scuba equipment.
Understanding fish soundscapes is becoming increasingly important. Passive acoustic monitoring (PAM) allows researchers to identify fish species, track their movements, and monitor population health and behavior without direct intervention. This information is valuable for conservation efforts, helping to assess the impact of human activities like noise pollution on marine ecosystems and to develop more effective management strategies for fish populations.