Masking of southern resident killer whale signals by commercial ship noise.

Abstract

The endangered southern resident killer whales (SRKWs) emit sound to communicate with each other and to hunt fish. Communication or fishing are possible only within a distance (R) at which a signal can be detected. We determine detection distance by comparing the power spectra of the ambient noise and the received signal, with attention to the auditory response curve of the receiver. In Haro Strait, the center of the SRKW critical habitat, about 21 commercial ships per day, increases the ambient noise level by about 20 dB. To assess how ship noise may affect the SRKW communication and hunting, we define the fractional reduction in the zone of audibility at any location and time as the ratio of the area where signal detection is expected to occur in the increased noise regime to the maximum detection area expected under ideal conditions R2/Rmax2. We map the decreased zones of audibility in Haro Strait during average and extreme ship noise by combining field measurements of spreading rates with source power spectra (1–100 kHz) of common SRKW signals and typical ships.