Detecting, assessing, and mitigating the effects of naval sonar on cetaceans

Abstract

Effective management of the potential environmental impacts of naval sonar requires quantitative data on the behaviour and hearing physiology of cetaceans. Here, novel experimental and analytical methods were used to obtain such information and to test the effectiveness of an operational mitigation method for naval sonar. A Bayesian method was developed to estimate whale locations through time, integrating visual observations with measurements from on-animal inertial, acoustic, depth, and Fastloc-GPS sensors. The track reconstruction method was applied to 13 humpback whale (Megaptera novaeangliae) data sets collected during a multi-disciplinary behavioural response study in Norwegian waters. Thirty-one controlled exposure experiments with and without active transmissions of 1.3-2 kHz sounds were conducted using a moving vessel that towed a sonar source. Dose-response functions, representing the relationships between measured sonar dose and behavioural responses identified from the reconstructed tracks, predicted that 50% of the humpbacks would initiate avoidance at a relatively high received sound pressure level of 166 dB re 1 µPa. Very similar dose-response functions were obtained for cessation of feeding. In a laboratory study, behavioural reaction times of a harbour porpoise (Phocoena phocoena) to sonar-like sounds were measured using operant conditioning and a psychoacoustic method. Auditory weighting functions, which can be used to improve dose-response functions, were obtained for the porpoise based on the assumption that sounds of equal loudness elicit equal reaction time. Additional analyses of the humpback whale data set provided evidence that ramp-up of naval sonar mitigates harmful sound levels in responsive cetaceans located directly in the path of the source, and suggested that a subset of the humpback whale population, such as mother-calf pairs, and more responsive species would benefit from the use of sonar ramp-up. The findings in this thesis are intended to inform sound exposure criteria and mitigation guidelines for anthropogenic noise exposure to cetaceans.