“While extreme noise events, such as seismic airgun surveys and Navy sonar exercises, are known to cause fishery depletions and catastrophic whale strandings, the ever increasing din of human industrial noise is perhaps more sinister. We know that these noises mask biologically significant sounds, but we don’t know when critical thresholds will be exceeded – when animals will not be able to find enough food, lose contact with their mates, fail to hear navigation and location cues, or not sense incoming predators.
Even as these thresholds are pushed, it will be hard to know the impacts, but even harder to persuade noisemakers to quiet their activities.”
How Fish Use Sound
All of the world’s approximately 21,000 species of fish rely on sound to hear, many to communicate and almost all to navigate through the sunless darkness of the sea. Species have adapted to use sound in the world’s many different underwater environments in different ways. Unlike mammals, fish don’t have vocal chords to produce sound. Instead many types of fish use an organ known as an air bladder. The air bladder is a gas-filled sac that is used to help a fish float and stabilize itself in the water at a certain depth. When the bladder is contracted, the gas inside is released and a wave of sound is shot into the water. The sounds vary from species to species and can range from whistles, booms, growls, thumps, and grunts to tinkling, chime-like sounds. Some species also grind their teeth to make noise, and other fish use a combination of these techniques.
For other species of fish, a sensitive organ known as the lateral line runs along the inside of a fish’s body and acts as the fish’s ears. It picks up and processes external vibrations that help fish determine the direction of water currents, sense the presence of nearby objects and remain with their schools. Ability to hear varies among fish. The American shad, for instance, has amazing hearing and can detect ultrasonic frequencies up to 180 kHz. Which means that these fish can detect the ultrasonic clicks of dolphins.
How Marine Mammals Use Sound
Marine mammals use sound to shape their world by relying on a process known as echolocation. Echolocation is natural sonar. The animals produce sounds that bounce off objects and return as echoes. This information is then processed to help them navigate through the darkness, avoid predators and find food. Sound is also used to identify each other, determine the social order and maintain the bonds of mothers and their young.