Of the 31,900 species of fish living today we know that more than 1,000 make sounds, ranging in frequency from 50 to 8,000 Hz. For most fishes, the sonic mechanism is a muscle that vibrates its swim bladder not unlike our vocal cord. For many other species, the sonic mechanism remains a mystery.
Like us, fish produce sound both unintentionally and intentionally. Unintentional sounds from fish come all the time, resulting from hydrodynamic patterns during swimming and feeding. These sounds can provide information to other fishes. In fact, the fishing industry has recognized predatory fishes’ ability to take advantage of unintentional sounds to find prey and have designed fishing lures that emit low frequency sounds that mimic those produced by injured prey fish.
Intentional sounds are created to stay in touch with the shoal, to warn other fish of danger, to attract, communicate with and stimulate mates, and to scare intruders away from eggs and young. The sounds that fish make are usually simpler than the calls of marine mammals. Fish sounds have been describes as grunts, scrapes, knocks, clicks, squeaks, groans, rumbles and drumming. Nearly all the fish species produce their sounds using their teeth, their swim bladder or a combination of both.
Some fish are capable of making very loud sounds. One of the noisiest is the oyster toadfish - a bottom dwelling fish found along the east coast of North America from the West Indies to Cape Cod. The toadfish is thought to make two types of calls; a grunting sound when it is aggressive or frightened, and a loud foghorn-like call heard underwater for great distances to attract a female during the spawning season.
Many fish take advantage of the noises other species make. We know that some sharks use sound to help them locate prey, while some smaller fish can detect the sounds larger predators make in their hunting. Furthermore, it is believed that a few fish species, including herrings and American shads, can detect the ultrasonic echolocation sound produced by hunting dolphins from a distance of up to 200m.
Cod You Believe It?
The cod is believed to speak mostly on special occasions. We do not hear much from them, but if aggressive or while spawning they are very vocal, producing a number of sounds via the swim bladder. These are mainly short-duration low-frequency pulses, described as grunts or bops, with frequencies in the range 50-500 Hz. In the spawning season the male makes pulsed, low-frequency sounds to scare away other fish. In contrast the cod’s courting and spawning sounds consist of long series of pulses given with increasing frequency. During the spawning act the sound resembles continuous low-frequency sound – not unlike the sound from a Harley Davidson motorbike! Lately, it has been speculated that they use the continuous low-frequency sound to synchronize the fertilisation process.
Larger Arctic cod caught during the spawning season offshore Norway are commonly called ‘skrei’, probably from the norse word ’skrida’,to wander. The skrei undergo seasonal wanderings, inhabiting the Barents Sea in the summer and autumn and migrating every December to January southwards to their spawning grounds between Finnmark and western Norway. The most important of these grounds are in
Lofoten and Vesterålen.
Herring-made Bubble Screens The sound production of Pacific and Atlantic herring is poorly understood. Recently it has been shown that the herring produce distinctive bursts of pulses, termed fast repetitive tick (FRT) sounds. These trains of broadband pulses, 1.7–22 kHz, last for a period of 0.6 to 7.6s. The function of these Don’t tell fish stories where the people know you; but particularly, don’t tell them where they know the fish.
The oyster toadfish doesn’t need good looks to attract a mate – just a nice voice. To generate the foghorn sound, the toadfish contracts its sonic muscle against its swim bladder thousands of times a minute. At nearly three times the average wing beat of a hummingbird, toadfish have the fastest known muscle of any vertebrate. Volker Steger/Science Photo Library sounds is unknown, but social mediation appears likely. It is conceivable that killer whales use the distinctive herring sounds as foraging cues.
In Norwegian waters in the late autumn billions of herring migrate from open oceanic waters into deep fjords to spend the winter season waiting for spring to approach. In February, they migrate southwards to their spawning grounds, and then out to open waters again. Flocks of killer whales follow the herring to feed on their favourite prey. Often the whales dive to over a hundred meters to drive herring up to shallower waters, forcing the fish into tight groups while preventing them from escaping to deeper, darker and safer water. During this process the whales emit echolocation clicks, click bursts, and whistles, some of which may help to tighten the herring school or coordinate whale movements. At the right moment individual whales swim into the herd of fish and perform tail slaps that produce thud-like sounds. Apparently stunned by the tail slap, many fish turn belly-up, remain motionless, and become an easy catch.
Attacks from killer whales have been documented in Vestfjorden, in northern Norway, using multibeam sonar and echosounder. It was observed that herring schools were forced from large depths up to the surface by killer whales and saithe, after which the herrings expelled gas from their swim bladder via the anus as a consequence of the rapid change in depth, thereby producing a curtain of tiny bubbles around the school. The bubbles may confuse and deflect both visually and acoustically oriented killer whales due to increased scattering of light, reduced range of vision, and confounding effects of the reflection energy of bubbles and fish.
Under cover of their bubble curtain, the herring have a chance of escaping.
Killer Whale’s Tail Slap
The Norwegian killer whales debilitate prey by slapping their tails into herring schools. It has been suggested that the thud-like sound produced by the tail slaps is caused by cavitation. Single pulses measured from such tail slaps have waveforms and spectral characteristics very similar to those from the clicks of pistol shrimps, which are known to be produced by cavitations (see GEO Expro Vol. 8, No. 1). The pistol shrimps produce broadband sounds with frequencies beyond 200 kHz and with a peak frequency in the range of 2–5 kHz. The source level is between 183–191 dB (p-p) re 1μPa @ 1m. The sound of killer whale tail slaps contain frequencies beyond 150 kHz, with peak frequencies below 10 kHz. The fact that the source levels of tail slaps are 186 dB (p-p) suggests similarity between the sound production mechanism of killer whale tail slaps, snapping shrimp, and a cavitating propeller.
Man-made Bubble Screens
Underwater bubbles inhibit sound transmission through water due to density contrast and concomitant reflection and absorption of sound waves. Therefore, it is not only herrings that exploit bubble curtains. Man-made bubble curtain systems that produce bubbles in a deliberate arrangement in water to attenuate unwanted wave trains, like water layer reverberations, were tested in the 1950s. In the 1970s it was proposed that a bubble screen above the source towed behind a submarine acquiring seismic below ice would reduce the effect of seismic scattering from the ice layer that was disturbing reflections from the sub seabed. In the latev1990s, in the shallow waters of western Hong Kongva bubble screen was used to reduce the sound of pile driving in hump-backed dolphin habitat.
Although the bubble curtain did not eliminate all behavioral dolphin responses to the loud noise, the experiment and its application during construction represented a success as the broadband pulse levels were lowered by 3–5 dB at a range of a kilometer.
Fisheries and Seismic
The potential impacts of seismic activities on fisheries can be divided into two types: acoustic disturbance of fish, and conflicts of interest over use of the same areas. Therefore, seismic acquisition is regulated in a manner to cause as little inconvenience as possible to the fishing industry, and to avoid the spawning periods. In the next issue we will focus on the sensitivity of fish hearing.