Localising Cetacean Sounds for the Real-Time Mitigation and Long-Term Acoustic Monitoring of Noise

Noise can have a detrimental effect on cetaceans, as well as on other marine animal species. It can cause stress and increase risk of mortality by interfering with their use of sounds in communication (social behaviour and reproduction) and in navigation (echolocation or biosonar to orientate and look for food). Acoustic overexposure, e.g. in areas of heavy shipping, seismic surveys, military exercises, offshore windmills or gas/oil exploration, can lead to hearing loss. While temporary threshold shift (TTS) represents a reversible hearing loss over time, a permanent threshold shift (PTS) results in non-reversible lesions in mammal ears, i.e. a permanent hearing loss caused by long term and/or intense exposure. Although the impact of low to mid frequency (<5kHz) acoustic pollution from the above mentioned human marine activities with regard to cetacean disorientation and death remains poorly understood, available evidence is strongly suggestive of some negative direct or indirect effects: There is an increasing mortality rate from shipping collisions, and cetacean mass strandings after military maneuvers have also been recently related with the use of active sonar, both suggesting that some populations may already be suffering from acoustic impact (i.e. TTS, PTS or blast injuries). The control of noise impact on the marine environment constitutes a scientific challenge and requires a dynamic analysis of the situation based on the parallel development of applied solutions to balance human interests and the conservation of marine species. This objective implies the ambitious synthesis of many advanced acoustic technologies that must be designed to monitor the real-time presence of determined cetacean populations in conflictive areas. Many cetacean species can be identified by their specific calls. The recording of these signature acoustic signals can reveal their presence in monitored areas. Since sound propagates efficiently in water, the detection range of these signals can be quite large, exceeding 100 km in favourable conditions for low-frequency calls far above visual detection methods. This acoustic potential to non-intrusively detect and monitor cetacean species in their environment gave rise to Passive Acoustic Monitoring (PAM) techniques, for which research is very active. The localisation of whales from their sounds in their habitats was initiated in the 1970s. This was rapidly applied to tracking whales over large distances.