Nearfield detection of dipole sources by the goldfish (Carassius auratus) and the mottled sculpin (Cottus bairdi).

Surprisingly few behavioral data exist on dipole source detection by fish, despite the fact that dipole sources more closely approximate biologically relevant signals than do more nearly monopole sources such as loudspeakers, the stimulus used in nearly all fish auditory studies. In this study, dipole source detection is investigated for two fish species that differ in both their auditory and lateral line systems, the two systems capable of detecting dipole sources. Conditioned suppression of respiration in the goldfish and an unconditioned orienting response in the mottled sculpin were used to measure detection of a 6 mm diameter, sinusoidally vibrating sphere as a function of vibration frequency and source distance. Sound pressure thresholds for the goldfish were nearly independent of distance (15-60 mm) at 800 Hz, but increased with distance at 50 Hz, as they did for the mottled sculpin. The slopes of 50 Hz source level-distance functions, however, differed between the two species. Slopes for the goldfish were independent of distance, remaining at around 8 dB per distance doubling, which is near the 6 dB per distance doubling measured for sound pressure attenuation away from the source, but less than the 18 dB per distance doubling for incompressible flow, measured with an anemometer. Those for the mottled sculpin increased with increasing distance, approaching 18 dB per distance doubling. The nonlinear increase in source level necessary to reach threshold detection was quite similar to the nonlinear decrease in incompressible flow levels measured with the anemometer. Nonlinear increases with distance for 50 Hz sources near the trunk of the mottled sculpin were also similar to those near the head of the fish, where changes in source frequency had little effect on source level-distance functions. These results indicate that sound pressure detection by the ear is important for dipole detection by the goldfish, but that incompressible flow detection by the lateral line is more important for the mottled sculpin. They also indicate that fish such as the goldfish, with a pressure-sensitive swimbladder, are capable of detecting dipole sources at greater distances than are fish without such structures.