3S2–Behavioral Response Studies of Cetaceans to Naval Signals in Norwegian Waters

The long term goal of this international cooperative research program is to investigate behavioral reactions and the sound exposures required to elicit them of herring, killer whales, pilot whales and sperm whales to Low Frequency Active Sonar (LFAS) and Mid Frequency Active Sonar (MFAS) signals, in order to establish safety limits for sonar operations for these species. The hypothesis that we are exploring in our current research program is that the large difference in hearing sensitivity of killer whales at the two sonar frequencies influences their behavioral reactions. Killer whale hearing is thought to be >25dB less sensitive at 1-2 than at 6-7 kHz. Exposure levels analyzed relative to this curve in fact reveal that the “sensation levels” of the 6-7 kHz sonar at the time of the behavioral change in fact exceeded those of the total 1-2 kHz exposure. 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 probability of behavioral effects has never been directly assessed. Our preliminary results do not support the hypothesis that killer whales are more sensitive to the MFAS than the LFAS sonar, as predicted by sensation level.