Acoustic Properties of the Cod Swimbladder

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Abstract
1. The resonance frequency and damping of the swimbladder were measured for intact living cod at different depths. 2. At adaptation depth, the resonance frequency (fχ) of the organ was much higher than that predicted for an unrestrained gas bubble of similar volume. However, at much greater depths (where the hydrostatic pressure was 2 or more times greater than the adaptation pressure) fχ was only higher than expected by a factor of 1·25, and changed with depth in the manner of a free gas bubble. 3. The elevated values at greater depths can be explained in terms of the difference in shape between the swimbladder and an ideal spherical bubble, together with a clight effect from the surrounding tissues. We have calculated that the shear modulus of these tissues (µ1) has a value of approximately 1·7 x 106 dyne/cm2. However, we have suggested that the much higher elevation of fχ found at the adaptation depth results from a large increase in µ1 (by a factor of between 6 and 120). This increase may be the result of a maintained muscle tonus. 4. There is an immediate drop in fχ as a fish is moved to a new depth, but the elevated values are rapidly restored with time. This process of adjustment is too rapid to be accounted for by the restoration of the swimbladder to its former volume by the secretion or absorption of gas. It is possible that it is achieved by a muscular mechanism which restores µ1 to the former high value. 5. Though the maintenance of fχ at a high value may reflect mechanical processes concerned with the hydrostatic function of the swimbladder, or with the detection of static pressure changes, we suggest that it may also be of some value if the swimbladder is used as an accessory hearing organ. The maintenance of an fχ well above the hearing range of the fish ensures that the relative sensitivity of the animal to different frequencies does not alter with changes in depth.

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