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The world below the ocean surface is often a noisy place, researchers say. At least one thousand types of fish - and probably many more - produce sounds. These vocalizations can take a wide variety of forms, including pops, clicks, whistles, purrs, grunts, groans, barks, hums, hoots, rattles and tinkles. Fish produce sounds to attract mates, warn of danger, scare competitors and predators and maintain social cohesion. Some also create sound as a distress call. Although people have known for a long time that certain fish can vocalize, scientists have only recently realized how widespread this ability is.

Researchers record fish vocalizations with the aid of an underwater microphone called a hydrophone. Until recently divers were unaware of the wide variety of fish sounds in the ocean, because the sound of bubbles being released from scuba equipment masked the sounds produced by the fish. In addition, the bubbles often disturbed fish.

Fish sound researchers are now using rebreathers instead of conventional scuba gear. A rebreather is a self-contained system in which the diver repeatedly breathes in his or her exhaled air, so no gas bubbles enter the water. Carbon dioxide is removed from the exhaled air inside the rebreather. An oxygen sensor monitors the level of oxygen in the rebreathed air and a microprocessor controls the delivery of fresh oxygen into the air when it's needed.

Inside the abdominal cavity of most types of fish is a gas-filled sac called a swim bladder. The fish uses the swim bladder to control its buoyancy. When gas is added to the swim bladder the fish is more buoyant and can swim higher in the water. When gas is removed from the swim bladder the fish sinks in the water.

The swim bladder is filled in one of two ways. Some fish gulp air from the water surface. The air then passes through a duct connecting the esophagus to the swim bladder. Other fish have a gas gland, which extracts gas from the blood and sends it into the swim bladder.

In some fish the swim bladder is used as a sound-producing organ. A muscle attached to the swim bladder, called the sonic muscle, contracts and relaxes in a rapid sequence, causing the swim bladder to vibrate and produce a low-pitched drumming sound. The sonic muscle of the oyster toadfish is able to contract at a rate of 200 times a second.

Another way in which fish may produce sounds is by stridulation, a process in which hard body parts like teeth or bones hit each other. Body movements that create water currents or splashes are also used to create sounds for communication.

The three-spined toadfish, which lives in the western region of the Pacific Ocean, is the only fish so far known to produce non-linear sound. A linear sound wave travels as a single waveform. A non-linear sound is more complex and consists of overlapping waveforms that interfere with each other. Crying babies and animals such as marmots and songbirds emit non-linear sounds as alarm calls and to communicate distress. Film makers use music with non-linear sounds in movies to evoke emotions like fear and tension in viewers.

Three-spined toadfish have a swim bladder that is divided into two sections, forming two functionally separate areas. Each half of the swim bladder is controlled by its own sonic muscle and can produce its own sound. When each section of the swim bladder emits a different sound at the same time the sound is said to be biphonic. Scientists are trying to discover what role the non-linear sounds play in the three-spined toadfish community.

The midshipman is another type of toadfish. Plainfin midshipman fish off the coast of California are sometimes called "California singing fish". During the mating season the male midshipman hums - sometimes for long periods - by hitting his swim bladder with his sonic muscle. His humming is designed to attract a female. Once she deposits her eggs, the midshipman resumes humming to attract another female to his nest. The male guards the eggs until they hatch.

Researchers have discovered that there are two types of midshipman males - Type 1 and Type 2. Type 2 males are sometimes called "sneaker males". They are smaller than Type 1 males and do not hum, although they can make other vocalizations. Instead, they try to sneak into the nest and quickly fertilize the eggs before the Type I males notice what they are doing.

The black drum is a black or grey fish that can survive in brackish water. Black drums become very noisy during the mating season. The low pitched sounds that they produce travel long distances. In 2005, residents in Cape Coral on the Florida Gulf Coast complained that they were being kept awake at night by low-pitched throbbing sounds in their homes, which they believed were being created by an engineering fault in the buildings. It was eventually discovered that the sounds were being created by black drum fish, which at first the residents found hard to believe. The fish had swum into the canals and estuaries in the area. Their mating calls were penetrating the ground and the seawalls and into nearby houses.

The "Discovery of Sound in the Sea" website created by the University of Rhode Island contains a collection of audio files which play the sounds created by the black drum and other fish.

Herring communicate with each other by forcibly expelling gas from the anal area, producing bubbles and a high-pitched sound. The researchers call this sound production an FRT (Fast Repetitive Tick).

The fish gulp air from the water surface and then store it in the swim bladder. During the night, in darkness and when surrounded by other herring, air is released through the anal duct. The gas that is emitted isn't made from the digestion of food, since captive herring produce the sounds whether or not they have been fed. Herring have a good sense of hearing. The purpose of the FRT sounds may be to ensure that the fish stay close together.

Sea horses produce clicking sounds by rubbing two parts of their skull together. Squeaker catfish rub the spines located in their pectoral fins into grooves on their shoulders. The weakfish produces a purr with its sonic muscle and swim bladder.

Research of sound production in fish is still in its infancy. As scientists continue their investigations they are likely to find even more fish species that make sound and even more methods of fish vocalizations.