Mitigating, monitoring and assessing the effects of anthropogenic sound on beaked whales

Certain anthropogenic sounds are widely believed to cause strandings of beaked whales, but their impacts on beaked whale popula tions are not known and methods for mitigating their effects are largely untested. The sound sources that have been coincident with beake d whale strandings are military, mid-frequency sonar (2-10kHz) and airgun arrays, both of which are used widely throughout the world fo r defence and geophysical exploration, respectively and for which alternative technologies are not readily available. Avoidance of beaked whale habitats is superficially a straightforward means of reducing the potential effects, but beaked whales are widely distributed a nd can be found in virtually all deep-water marine habitats that are free of ice. Some areas of high beaked whale abundance have been identifie d, but the geographic distribution is poorly known for most species. Beaked whales are both visually and acoustically difficult to detect. Commonly used mitigation measures (e.g. ‘ramp-up’ and ‘detection-modification-avoidance’) have not been assessed for their effectiveness. Surveys to detect population-level impacts would likely require many years of regular monitoring and for most areas where beaked whale strandings have occurred, there are no pre-exposure estimates of population sizes. Risk assessment models can be used to estimate the soun d levels to which beaked whales might be exposed under a variety of scenarios, however, the lack of information on the causal mechanism for soundrelated beaked whale strandings makes it difficult to identify exposure levels that would warrant mitigative actions. Controlle d exposure experiments, which measure the behavioural responses of animals to fully characterised sound sources, may hold the greatest pot ential for understanding the behavioural responses of beaked whales to sound and for designing mitigation methods to avoid future impacts.