Behavioural and Physiological Responses of Hooded Seals (Cystophora cristata) to 1 to 7 kHz Sonar Signals

Controlled exposure experiments on captive hooded seals (Cystophora cristata) were made to examine behavioural and physiological effects of sonar signals. The animals were instrumented with data loggers recording heart rate, dive depth, and swimming activity, and then released into a 1,200 m 3 net-cage in the ocean. The exposure consisted of three different 1-s sonar signals covering the 1 to 7 kHz band transmitted either by using 10-s inter-ping intervals and gradually increasing source level from 134 to 194 dBRMS (re 1 µPa @1 m) within 6 min, or using the maximum source level of 194 dBRMS from the first ping but gradually decreasing the inter-ping intervals from 100 s to 10 s within 10 min (duty cycle increasing from 1 to 10%). Transmission loss from the source to the animal varied from 10 to 27 dB, depending on the exact location within the net-cage and the transmitted frequency. The animals responded to the initial (10% duty cycle) exposure with avoidance to signals above 160 to 170 dBRMS (re 1 µPa) received levels. This involved reduced diving activity, commencement of rapid exploratory swimming at surface, and eventually displacement to areas of least sound pressure level. However, already upon the second exposure, the initial rapid swimming activity was absent, while the reduction in diving activity became even more pronounced. No differences were found in behavioural response to different transmitted frequencies. Increased heart rate at the surface indicates emotional activation during sonar exposure, but lack of effect of sonar exposure on heart rate during diving indicates that physiological responses to diving remain intact.