Cetaceans and Naval Sonar: Behavioral Response as a Function of Sonar Frequency

摘要

Data on the responsiveness of free-ranging cetaceans to mid- frequency sonar signals are lacking, with only a few species having been studied in relation to a few types of sonar signals, mostly SURTASSLFA (Nowacek et al., 2007). This specific project was initially motivated by observations of possible killer whale (Orcinus orca) reactions to sonars, in the Vestfjord basin of Norway and the USS Shoup incident in Haro Strait in Washington State (WWF-Norway, 2001; NMFS, 2005). While those incidents have not led to observation of strandings or direct mortality, the perceived behavioral changes in response to sonar have negatively impacted the public image of the Navies involved, and may have harmed the stakeholder community that works with killer whales. The hypothesis explored in this research program is that strong difference in hearing sensitivity of killer whales at the two sonar frequencies influences their behavioral reactions. Using all available hearing data from captive animals, our research team produced a composite killer hearing curve (Fig 1). It can be clearly seen that killer whale hearing seems to be 20-25dB less sensitive at 1-2 kHz than at 6-7 kHz. The term sensation level refers not to absolute intensity of a sound, but intensity relative to the hearing threshold for that sound for a given individual. Acoustic criteria recommend use of sensation level to estimate physiological impacts on hearing (Southall et al., 2007), but the specific influence of hearing sensitivity on the risk of behavioral effects has never been directly assessed.