Sound source localization of a dipole by the plainfin midshipman fish (Porichthys notatus)

Localization of a dipole sound source was studied in female plainfin midshipman fish (Porichthys notatus). Experiments were conducted and videotaped in a 3.65‐m‐diameter tank using a dipole underwater speaker system placed near the center of the tank. The sound was a 90‐Hz tone, approximately the fundamental frequency of the male’s advertisement call. Pressure and particle motion components of the sound field were mapped with 9‐cm resolution. Pressure was measured using an eight‐hydrophone array, and particle motion vectors calculated from the pressure gradients. Mapping confirmed that the projector was operating as a dipole. Gravid fish were released 70 cm from the sound source at two different positions relative to the dipole axis: one near the dipole axis and one near the pressure null axis. Twenty‐five positive responses were recorded from each release site. The phonotactic response pathways along the dipole axis consisted of slightly curved tracks to the sound source, whereas pathways from the null axis consisted of greatly curved tracks to the source that followed the particle motion vectors. Results confirm that fish can locate a dipole sound source and are sensitive to the direction of acoustic particle motion. [Work supported by NSF.]