Behavioural response thresholds for the assessment of noise impact on Antarctic marine mammal species

Increasing levels of anthropogenic noise in our oceans is concerning, especially in Antarctic waters where marine mammals are more naïve to sound sources, compared to those residing in industrialised ocean basins. Marine mammals are vulnerable to anthropogenic noise, due to their dependence on underwater sounds for their survival. The Protocol on Environmental Protection to the Antarctic Treaty stipulates that anthropogenic activities in Antarctic waters must be assessed in advance, for the comprehensive protection of the natural environment and its dependent ecosystems. There are few quantitative studies on the behavioural responses of Antarctic marine mammals. Furthermore, the absence of agreed upon thresholds for marine mammal behavioural responses has created difficulties in conducting impact assessments in a standardised manner. In instances where knowledge gaps persist amidst high priority management issues, the expert elicitation approach can be implemented to utilise an expert's qualitative knowledge. This study elicited probability distributions of noise levels which would lead to a significant behavioural response, when received under specific acoustic exposure scenarios. The sound sources examined include vessels, research seismic survey activities and hydroacoustic research equipment. The expert judgements exhibited substantial variability amongst thresholds due to limited data on marine mammals in Antarctic waters, with low frequency cetaceans eliciting a threshold of L_p = 137 dB re 1 µPa for vessel noise, compared to L_p = 145 dB re 1 µPa for hydroacoustic research equipment. These thresholds are an initial tool in the assessment process, whilst guiding priorities for future research to quantitatively address this pressing management issue.