Fish And Sound
There are more than 30,000 species of fish, ranging in length from 10 m long whale sharks to 1 cm carps. There is a huge range in specializations and morphological adaptations. There are fish living in almost all of the world’s marine and aquatic habitats. Some environments are constantly pitch dark, such as the deep-sea and caves, some of them have extremely clear waters (such as tropical coral reefs) whereas others are murkier than coffee (such as some rivers). There are fish species that have no eyes and others with reduced abilities to see, as they live in waters where there is no or very limited amounts of light. However, up to date there has been no fish that has been found lacking ears. Thus, hearing is important to all fish. But, why do fish have ears, and what do they use them for? These are important questions that need to be answered to be able to understand how fish may be affected by man-made sounds in the oceans.
Sounds in the ocean – according to fish
Acoustic waves have to faces. Sound is both a pressure disturbance and a motion of particles, generated by a sound source (basically a structure moving back and forth). These two features of sound, pressure and motion, goes hand in hand: it is the movement of particles towards and away from each other that generate the increased and decreased pressure. When we talk about human hearing, we usually think of the pressure component. It is the sound pressure fluctuations that put the tympanic membrane into motion, driving our middle-ear ossicles and eventually pumping the fluid in the cochlea, where the hair cells are located that detect sound. Like all other mammals, we are thus primarily sensitive to the pressure component of the sound field.
For fish, the situation is radically different. The inner ear of a fish consists of calciferous structures, called otoliths, situated close to hair cells that can feel the movement of the otolith relative the surrounding fish body. When the fish is rocked back and forth in a sound field the heavier otolith lags behind – and the hair cells are bent and register the motion. Thus, the fish ear is primarily sensitive to the particle motion of the sound field. It is possible to create a sound field in the laboratory which only consists of sound pressure, with very little particle motion. In such as sound field, a mammalian ear is still registering the sound very well, whereas many fish cannot detect any signal at all.
The fish ear is fundamentally a motion detector. The back and forth motion of the particles in the sound field can be described as displacement (the distance the particles move), velocity (the speed of movement) or acceleration (the rate of change of speed). At low frequencies, below some 100 Hz, the fish ear is mainly sensitive to acceleration. The hearing abilities in this ‘acceleration mode’ are restricted to about 200 Hz. The fish can hear extremely low frequencies, in some cases below 0.1 Hz. The sensitivity is generally not that great, but the extension of the frequency range to low frequencies is phenomenal when compared to mammals.
Some fish species improve their hearing abilities by different anatomical structures. Fish with a swim bladder may not only detect the particle motion component of the sound field but also the pressure fluctuations. The swim bladder starts to pulsate because of the increased and decreased pressure in the sound field. This motion is propagating through the flesh of the fish to the inner ear. In some fish species there are specialized structures such as bones or air canals mediating the transfer of the signal from the swim bladder to the inner ear, at others have additional air-filled structures close to the inner ear apparently improving the hearing abilities of the fish even further. Hearing specialists such as carps and cat fish have sensitivity not much different from the one of mammals, and the frequency range of hearing extends to some 2-4 kHz. Only one small group of fish species is known to detect ultrasound (Alosinae, an anadromous Clupeid subfamily).
In the free acoustic field, far from the sound generating structure and far from any reflecting surfaces, the pressure and particle motion components of the sound field are proportional in magnitude. Also, when the pressure is at its maximum, so is the velocity of particle motions. Close to the sound source, the particle motion becomes much larger than the pressure component, and the velocity of particles is not peaking at the same time as the sound pressure. The acoustic world experienced by fish may therefore be very different than the acoustic world registered by mammals.
Sound used for communication
Many fish species communicate with the help of sound. In Scandinavian waters, gadoids develop muscles in the spring that are drumming on the swim bladder, creating a very low-pitch sound that is used during mating. Herring and sprat has a canal from the swim bladder to the anal opening where air can be released, generating a more high-pitched sound. It has been speculated that these sounds are used for some kind of communication between the herring within a school. Other fish, such as gobids, generate transients, presumably by grinding their teeth, when threatened or scared away from their territory.
The exact function of fish sounds has only been worked out in rare cases. Sounds are used by males during mating to attract females and chase off other males. Sound is also heard in other interactions, especially aggressive interactions, in many fish species. However, the exact function of such sounds is still mainly unexplored.
Sound used for orientation
The ‘laboratory rat’ of fish hearing is the gold fish. This is the fish whose hearing abilities have been studied the most, and they turn out the have some of the best hearing abilities of any fish. Still, not a single sound has ever been recorded that for sure was produced by a goldfish! So, apparently gold fish is not using their ears for communicate. Why then, do gold fish have such great ears?
You may ask yourself the same question for humans. Are our hearing abilities only tuned for communication, that is for listening to human speech? No, not at all. The frequency of best hearing in humans is about 5-10 times higher than the fundamental frequency of humans talking. We use our broad frequency range not only for communication, but also detect other important sound sources. A crying baby is often emitting frequencies closer to our best frequency of hearing than what adult humans do. So, our hearing may actually be more tuned to respond to infants needing our attention than for communication with other adults. In addition, we use our extended frequency range of hearing to orient ourselves, to figure out what kind of room we are in, or if we are outdoors or indoors.
In the same way, gold fish and other fish species use sounds to find out where they are and what is going on in their surroundings. An approaching predator will generate low frequency sounds that fish can detect and thereby avoid being eaten. This may be the most important reason why all fish have ears.
Noise which is dangerous to fish
Extremely loud sounds can cause tissue damage, temporary or permanent hearing loss and disorientation in fish. Explosives can stun fish at close range. Few sound sources naturally occurring underwater are sufficient intense to damage fish, even at close range. However, many man-made sounds, such as explosives, pile driving and air guns, can affect fish in many different ways. Due to the incredible variation in fish anatomy, physiology and, not the least, hearing abilities, it is difficult to make any general statements about how fish are affected by intense sound. It seems that fish respond differently, depending on the nature of the sound source and the species in question.
Noise which can disturb the natural behaviour of fish
Fish is extremely sensitive to low-frequency vibrations, below some 10s of Herz. If the sound source is sufficiently intense, fish usually respond by swimming away from the source. The reason for this is probably that low frequency sounds usually indicates an approaching predator. Higher frequency and weaker sounds can sometimes attract fish instead, as the source may stem from prey or from the sounds produced by feeding conspecifics. Some sounds can induce stress in fish, as they resemble the sounds of communicating predators, such as dolphins. Intense ultrasonic pulses create very strong avoidance responses in Alosinae, as such sounds can stem from an approaching echolocating toothed whale.
Human sound sources can disturb fish in many ways. Low frequency sounds from shipping and construction work may cause avoidance reactions, whereas higher frequency sounds from pumps et cetera may attract fish. Echosounders and sonars that operate at ultrasonic frequencies may disturb the behaviour of the few species which are able to detect ultrasound.
Noise which can interfere with communication – masking
Loud noise may interfere with acoustic communication in fish. However, little is known over which ranges fish are naturally communicating with sound, so it is difficult to assess how large the effect of masking is from human-induced sound sources such as shipping.
W. W. L. Au and M. C. Hastings (2008). Principles of Marine Bioacoustics, New York:Springer. General introduction to marine bioacoustics. First half of the book has emphasis on the physics of measuring and describing sound, the second half concerns the biological use of sound.
Popper, A. N., A. Hawkins (2012). The Effects of Noise on Aquatic Life. Springer-Verlag. Contain shorter chapters describing the most recent studies made on how fish respond to sound.