The majority of fish species are known to detect sounds from below 50 Hz up to 500 or even 1,500 Hz. A smaller number of species can detect sounds to over 3,000 Hz, while a very few can detect sounds to well over 100 kHz.
In a somewhat vague and unclear manner, fish are termed as either hearing generalists or hearing specialists. Hearing generalists are fish with the narrower bandwidth of hearing - typically able to detect sounds up to 1 or 1.5 kHz. Specialists have a broader hearing range, detecting sounds above 1.5 kHz. Furthermore, where specialists and generalists overlap in frequency range of hearing, the specialists generally have lower hearing thresholds than the generalists, meaning that they can detect quieter sounds.
Fish species with either no swim bladder (e.g. elasmobranchs, the collective name for sharks, skates and rays) or a much reduced one (many benthic species living on, in, or near the seabed like flatfish) tend to have relatively low auditory sensitivity, and generally cannot hear sounds at frequencies above 1 kHz. The sound pressure threshold can be as high as 120 dB re 1 μPa (hereafter dB) at the best frequency. Such fishes are therefore “hearing generalist” species. Fish without swim bladders are only sensitive to the particle motion component of the sound field.
Fish having a fully functional swim bladder have increased hearing sensitivity, especially when there is some form of close coupling between the swim bladder and the inner ear. These transmit oscillations of the swimming bladder wall in the pressure field to the inner ear. With the ability to perceive also the pressure component of sound these fish are referred to as “hearing specialists”.
In the clupeids, (common food fish like herrings, shads, sardines and anchovies), the coupling takes the form of a gas-containing sphere (prootic bulla) connecting the swim bladder to the hearing system. This considerably lowers their hearing thresholds and extends the hearing bandwidth to higher frequencies up to several kHz.
In otophysan fish (e.g., the carps, minnows, channel catfishes, and characins; the majority of freshwater fish worldwide), a bony coupling is formed by the Weberian ossicles. These bones allow them to use their swim bladder as a sort of drum to detect a greater range of sounds, and create a super-league of hearing-sensitive fish.
Fish Sensory Systems
Fish have evolved two sensory systems to detect acoustic signals: the ear and the lateral line.
Fish do not need an outer or middle ear, since the role of these structures in terrestrial vertebrates is to funnel sound to the ear and overcome the impedance difference between air and the fluids of the inner ear. Since fish live in water, and have the same density as water, there is no impedance difference to overcome. Fish do have an inner ear which is similar in structure and function to the inner ear of terrestrial vertebrates. The most important similarity between ears of all vertebrates is that sound is converted from mechanical to electrical signals by the sensory hair cells that are common in all vertebrates. Extreme high intensity sounds are able to fatigue or damage these cells, resulting in temporary or permanent hearing loss. However, fish continue to add sensory hair cells throughout their lives. In addition, there is evidence that fishes can repair sensory cells that have been fatigued due to sound exposures that cause a shift in auditory thresholds.
Fish will move along with the sound field from any source. While this might result in the fish not detecting the sound, the inner ear also contains dense calcium carbonate structures - the otoliths - which move at a different amplitude and phase from the rest of the body while stimulating sensory hair cells. This system with relative motion between the otoliths and the sensory hair cells acts as a biological accelerometer and provides the mechanism by which all fish hear.
However, in fish with a swim bladder the acoustic sound pressure can indirectly stimulate the fish's inner ear via the bladder. For the stimulation to be efficient, the swim bladder either must be close to or have a specific connection to the inner ear. In one form, a gas bubble makes the mechanical coupling; in another the inner ear is directly connected to it by a set of small bones called the Weberian ossicles. Since the air in the swim bladder is of a very different density to that of the rest of the fish, in the presence of sound the air starts to vibrate. This vibration stimulates the inner ear by moving the otolith relative to the sensory epithelium. In these cases the fishes are sensitive to both particle motion and pressure modes of sound, leading to enhanced pressure detection and a broadened frequency response range.
The lateral line consists of a series of receptors along the body of the fish enabling detection of hydrodynamic signals (water motion) relative to the fish. It is involved with schooling behaviour, where fish swim in a cohesive formation with many other fish and for detection of near-by moving objects, such as food.
For fish to hear a sound source, the generated sound pressure level should be higher than its auditory threshold and background noise levels from natural sources and anthropogenic sound.
Traditionally, studies of hearing have used behavioural or electrophysiological methods. The qualitative behavioural methods are based on conditioning the fish with acoustic signals in conjunction with either reward (food) or punishment (electric shocks). The electrophysiological methods insert electrodes into either the midbrain or auditory end organs of the test fish to record neuronal activities in response to acoustic signals. This requires invasive surgery. A decade ago, a non-invasive auditory brainstem response (ABR) method where electrodes are placed on the skin of the fish’s head was developed to obtain audiograms of fish, similar to those we described in GeoExPro Vol. 7, No. 6 to test marine mammals. It has now become a standard method for fish auditory physiology research.
Fish lacking the swim bladder or having a swim bladder that is not in close proximity or mechanically connected to the inner ear are sensitive mainly to sound acceleration. Their audiogram shows a sharp upper frequency limit for hearing at 200-300 Hz. This response is typical for fish species like flounders, flatfish, demersal fish (such as bullheads and sculpins), wolfish, mackerel, salmonids, redfish and eels. The European plaice is known to be significantly sound sensitive into the infrasonic band (less than 20 Hz) and this probably holds for many of these fish species.
Cod, haddock and pollack have similar hearing capabilities and sense sound in the frequency range 0.1-450 Hz. For sound intensities close to threshold cod fish are sensitive to sound pressure in the frequency range 100-450 Hz and to sound acceleration for frequencies below 100 Hz. For sound intensities above the threshold value cod fish will detect both sound acceleration and sound pressure over a substantial frequency range, 20-150 KHz. Sound pressure thresholds in cod fish in the frequency range 60-300 Hz lie in the range 80-90 dB.
The herring family has a upper hearing frequency limit of 1-8 kHz, with an optimum range of 0.6-2 kHz. These hearing specialists are also sensitive to sound pressure towards lower frequencies. Sound induced escape responses can be triggered from sound pressure by infrasonic sound down to 5 Hz.
Recent studies have demonstrated that the inner ear of the herring subfamily shad is specialized to sense ultrasound in the range 20-120 Hz, with threshold values in the range 150-160 dB for frequencies at 80-100 kHz. The sensitivity is sufficient for the shads to sense attacking dolphins’ ultrasonic clicks, which can have sound pressure up to 220 dB @ 1 m. The other subfamilies of herring do not have ultrasound hearing.