Directional hearing of cod (Gadus morhua) under approximate free field conditions

Abstract

The cod,Gadus morhua L., can hear the direction of a sound source of 75 Hz in the transversal plane. At the experimental site in the middle of a fjord, local depth 35 m, the sound sources were situated at radial distances of 4 to 5.3 m from the subject in its netting cage. Both in two-alternative and four alternative choice experiments, in which an acoustic discrimination of separate similar sound sources was required, reward conditioning yielded positive results. The bearing of the sound source was decisive for the discrimination and not the identity of the sound sources used (intensity or timbre deviations). If the reverberation characteristics of the fjord at the receiving point were strongly dependent on the spatial position of the source, the results of the choice experiments might represent a demonstration of pseudo-directional hearing. However, from acoustic measurements of the time averages of the sound parameters such an anisotropy for sources with different bearing was not apparent (Fig. 5). Furthermore, in a well-trained cod, in which the pars inferior of only one single labyrinth was put out of function by surgery, the discrimination of the bearing of the sound sources was abolished but not the detection of sound as such (see Table 5). Detection of the direction of the sound therefore seems to be a function of the labyrinths and not of the lateral line system (directionalhearing). Estimates are given of the minimal angle that can be distinguished (about 22° for a 50% detectability likelihood; Fig. 6), and of the minimal acoustic level necessary for acoustic orientation (about — 13 dB re 1 μ bar; see Fig. 9). Finally an extension to an existing detection model of directional hearing is proposed. It receives empirical support in the next paper (Schuijf and Buwalda, 1975).