Effects of naval sonar on seals

There is a growing concern that active military sonars might injure or harm marine mammals. We have therefore done controlled exposure experiments on captive hooded seals to investigate behavioral, physiological and possible neuropathological effects. The animals were instrumented with dataloggers recording heart rate, diving and swimming activity, before being released into a 1200m3 net cage in the ocean. The sonar exposure involved “soft start” and “slow start” procedures using simulated sonar signals between 1-7 kHz at source levels from 134-194dB (RMS, re1uPa@1m). The sound pressure level inside the net cage was 10-27dB below the source level. After completion of the experiments the animals were autopsied and the brains examined for neuropathological effects of the exposure. We found that the animals initially responded with avoidance to signals above 160-170dB (received levels). This involved reduced diving activity, commencement of rapid (exploratory) shallow swimming 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. We found no sign of traumatic brain injury upon post mortem examination.