Acoustic monitoring of marine mammals
Amongst marine mammals especially members of the two orders Pinnipedia (seals) and Cetacea (whales) have evolved an acute sense of hearing and also use sound for communication purposes. Moreover, numerous toothed whale species are capable of using echolocation to detect and characterise their food, navigate underwater and to avoid obstacles. The transmission of sound is the most efficient form of information transfer underwater not only for the animals to communicate and orient in their underwater environment but also for us to detect and localise the animals themselves by using active and passive acoustic detection methods. The acoustic characteristics of sound emissions of marine mammals can differ considerably, ranging from very short pulsed echolocation clicks to long lasting frequency modulated songs (e.g. [1]). Due to their mainly submerged lifestyle it is difficult to assess the movements and habitat use of whales and seals. To monitor their movements as well as their abundance and density in a selected area over time it is important to chose the appropriate methods.
The abundance and distribution of marine mammals can best be assessed by line transect distance sampling methods and analysis techniques (e.g. [2]). Data can be gathered from either visual observation above the surface (land-based from an elevated point on the shore, ship-based or from an airplane) or by using acoustic detectors underwater.
The detection of sounds emitted underwater by the marine mammals is done by using hydrophones (e.g. [3]). The sensitivity of these acoustic receivers and the width of the hydrophones frequency spectrum in relation to the ambient noise define the detection range of the hydrophones and the variety of marine mammals that can be detected with it. The receiving hydrophones are either be towed behind a vessel ([4]) or deployed as stationary hydrophones (SOSUS: [5] [6], T-PODs: [7][8]). When towed, usually two or more hydrophones are installed in an array, as the time difference of arrival of the individual sounds at the spatially separated hydrophones allows to triangulate and analyse for bearing of the received sound and thereby to locate the sound source. Usually a specialised sound recognition software is needed to identify the species which the recorded sounds can be attributed to.
With decreasing size of the technical acoustic components it is possible to deploy sound recording systems on marine mammals to monitor their acoustic emissions simultaneously to the sound immissions received by them. In combination with satellite tags and data loggers (D-tag: [9], A-tag: [10]) this new approach allows to follow the animals' movements and to analyse its behaviour in relation to environmental parameters in retrospect using dead reckoning methods ([11]).
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