Traditionally, studies of hearing have used behavioural or
electrophysiological methods. Te qualitative behavioural methods are based on conditioning the fish with acoustic signals in conjunction with either reward (food) or punishment (electric shocks). Te electrophysiological methods used to involve inserting electrodes into either the midbrain or auditory end organs of the test fish to record neuronal activities in response to acoustic signals. Tis requires invasive surgery. In the late 1990s, 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 Section 3.9 to test marine mammals. It has now become a standard method for research into fish auditory physiology. 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. Teir audiogram shows a sharp upper frequency limit for hearing at 200–300 Hz. Tis response is typical for fish species like flounders, flatfish, demersal fish (such as bullheads and sculpins), wolf fish, mackerel, salmonids, redfish and eels. Te European plaice is known to be significantly sound sensitive into the infrasonic band (less than 20 Hz) and this probably
Sound in Water Any sound source in water produces both oscillations of water molecules – particle motion – and pressure variations – sound pressure. Particle motion can be either described as acoustic displacement, particle velocity, or particle acceleration. Sound pressure is the parameter with which most are
familiar, since it determines the ‘loudness’ of a sound to humans and mammals. Far away from a sound source, in the far-field of the source, the ratio of sound particle
Figure 3.86: The cod has been shown to detect sound pressure at the higher frequencies of its hearing range. At low frequencies, below about 100 Hz, however, the cod is particle motion sensitive. Its swim bladder appears to serve as an accessory hearing structure: oscillations are transmitted through the surrounding tissue to the inner ear, even though there is no apparent specialised anatomical link to the inner ear. The cod possibly detects ultrasound. Astrup and Møhl (1993) indicate that cod has ultrasound thresholds of 185 to 200 dB @ 38 kHz, which probably allows for detection of odontocetes’ clicks at distances up to 10 to 30m (Astrup, 1999).
holds for many of these fish species. Cod, haddock and pollock have similar hearing capabilities
and sense sound in the frequency range 0.1–450 Hz. For sound intensities close to the threshold, cod 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 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 between 80 and 90 dB. Te herring family has an upper hearing frequency limit of
1–8 kHz, with an optimum range of 0.6–2 kHz. Tese 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 specialised to sense ultrasound in the range of 20–120 Hz, with threshold values in the range of 150–160 dB for frequencies at 80–100 kHz. Te sensitivity is sufficient for the shad to sense attacking dolphins’ ultrasonic clicks, which can have sound pressure up to 220 dB @ 1 m. Te other subfamilies of herring do not have ultrasound hearing.
velocity (V) and sound pressure (P) is constant, equal to the water impedance. But moving closer than around ¹⁄₆ of the wavelength, into the sound’s near field, this simple relationship breaks down. Te V/P ratio strongly increases with decreasing distance to the source, thus inducing high levels of particle accelerations. However, when a fish is free to swim away from the sound source, it will likely be in the far-field of the source. For a frequency of 1 Hz, the far-field distance is around 250m.
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Per Eide Studio/Norwegian Seafood Export Council
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