This constitutes the basis for an acute directional hearing, which in teleosts occurs in both azimuth and elevation ( Hawkins & Sand, 1977 Buwalda, Schuijf & Hawkins, 1983). These organs are acceleration detectors, showing highly directional responses to vibration ( Sand, 1974 Hawkins & Horner, 1981 Fay, 1984) in addition to detecting gravitation or acceleration due to movements of the animal. The otolith organs of the inner ear are responsible for sound detection in teleosts. The sensitivity of cod is thus sufficient to detect the highest levels of ambient infrasound, and we put forward the hypothesis that fish may utilize information about the infrasound pattern in the sea for orientation during migration, probably in addition to an array of other sensory inputs. The spectrum level at 0♱ Hz in the sea ranges between 120 and 180 dB (re 1 μPa), with corresponding particle accelerations from less than 10 −6 to more than 10 −4ms −2. The threshold values, measured as particle acceleration, showed a steady decline towards lower frequencies below 10 Hz, reaching a value close to 10 −5 ms −2 at 0♱ Hz. Using a standing-wave acoustic tube, we have studied the sensitivity of cod to infrasound down to 0♱ Hz by means of the cardiac conditioning technique. This low-frequency noise may propagate long distances with little attenuation, causing a directional pattern of infrasound in the sea.
In the infrasound range the spectral slope is particularly steep. Below about 50 kHz the level of ambient noise in the sea increases continuously towards lower frequencies.
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