Monday 22 August 2005

PB1
1000-1230 hours

026
Factors affecting passive acoustic surveying of marine mammals
Cato, Doug¹, Rogers, Tracey², Noad, Michael³, McCauley, Robert⁴
1 Defence Science & Technology Organisation & University of Sydney Inst of Marine Science, Pyrmont, Australia
2 Aust Marine Mammal Research Centre, Zoological Parks Board & Vet Faculty Uni of Sydney, Mosman, Australia
3 School Of Veterinary Science, University Of Queensland, St Lucia, Australia
4 Centre For Marine Science & Technology, Curtin University, Perth, Australia
Author email: doug.cato@dsto.defence.gov.au
Passive acoustic monitoring of marine mammals shows much promise because their vocalisations are usually detectable over great distances, often much greater than they are visible. Transforming detections of vocalizations into estimates of animal abundance, however, is complicated and requires an estimate of the spatial density of individuals. Following the practice of visual surveying, this may be related to the probability of detection of an individual as a function of distance. Factors that affect this include the acoustic propagation and ambient noise, and the species vocal behaviour, which includes source level, vocalization rate and proportion of animals vocalizing. Variations in propagation and ambient noise cause variations in detection range of more than a factor of ten so have the potential to cause significant uncertainty in results if not adequately known. We discuss results of marine mammals surveys which illustrate these issues. Ship based visual and acoustic surveys of seals in Antarctic waters have found that there are many more acoustic than visual detections for some species but not for others. Using calling behaviour and source levels measured in near shore studies allowed an estimate of the number of animals detected for the more vocal species. Humpback whales migrating along the east coast of Australia pass so close to shore near Brisbane that all most all can be counted visually, and many more are seen than are heard. From combined visual and acoustic surveys, we have been able to ground truth acoustic estimates of population trends and abundance against visual estimates. This has shown the effectiveness of acoustic monitoring and provided the information required to use acoustics effectively well off shore for this species. Long term recordings using noise loggers in northern Australian waters have detected a wide range of biological sounds, and the use of will be extended other areas.

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