How Does A Fish Listen?

Sounds are distortions in the flow of a fluid (whether the fluid be air or water) and are produced when an organism, or any pat of it, moves relative to the flow. These sounds are of varying frequency; swimming and feeding noises range upto 50 Hz, vocalization 50 - 400 Hz, and echolocation by marine animals upto 150 kHz.

Sound waves travel about 4.3 times faster in seawater than in air. The wavelength of a given sound frequency is thus 4.3 times longer in seawater, which has consequences for both echolocation and hearing, as we shall see below. At the same time the motion (vibration) of the sound source produces a to-and-fro movement of the fluid particles around it.

Theoritical analysis of the transmission of sounds produced by aquatic animals usually treats them in terms of two components, the far-field and the near-field, and a source such as the beating tail of a fish is usually modelled as a small vibrating sphere.

The far-field component comprises the acoustic pressure waves that propagate from the sound source at a velocity of about 1500 m/s, with little loss of energy. The energy in an acoustic wave is divided equally between the potential energy stored in the compressions and decompressions in the medium, and the kinetic energy in the increased velocities of particles in the medium.

If the wavelength is very long relative to the length of fish (a 300 Hz sound has a wavelength of 5 m) the pressure gradients between points on the fish's body will be very small. How then can an animal detect a distant sound or vibration?

Most tissues are close to acoustic transparency when compared with seawater (ie, have a similar acoustic impedance). Tissues of very high, or very low density (eg, bone of gas) have markedly different acoustic impedances, and consequently different accelerations to those of the rest of the animal.

An animal containing a gas bubble or gas bladder can also indirectly sense the far-field acoustic wave because it will induce the bubble to vibrate. When the sound frequency is close to the resonant frequency of the bubble the induced vibration is enhanced 3-10 times. as it vibrates, the bubble re-radiates the acoustic energy and generates its own near-field and far-field effects.

The bubble's resonant frequency increases with both an increase in ambient pressure, and a decrease in size. Even if the bubble (or swimbladder) size is actively maintained, its resonant frequency will be greatly affected by changes in the animal's depth. For example, if a bubble of diameter 0.2mm has a resonant frequency of 40kHz at 50m depth this will increase to 90kHz if the animal descends to 200m (typical of some diel vertical migrations).
--Taken from the book The biology of the deep ocean by Peter J.Herring