Determining effective acoustic array design for monitoring presence of white sharks Carcharodon carcharias in nearshore habitats

Inferences regarding animal presence from passive acoustic receiver arrays are driven by the spatial confguration of receivers. Large, dense arrays provide more information, but maintenance of multiple receivers is costly. Confguring acoustic receiver arrays to maximise coverage while minimising cost is therefore paramount. This study used data from a dense acoustic receiver array within a white shark Carcharodon carcharias nursery area on the east coast of Australia to assess how detection data of tagged white sharks in the area was afected by reducing the array size. Receivers were sub-sampled post hoc by simple random sampling, clustered random sampling, and sampling of the top performing receivers. Using the top performing receivers, array size could be reduced by 60% (10 out of 25 receivers) while still detecting a median of 100% of white sharks detected with the full array. With random and clustered sampling methods, a 40% reduction in array size (15 out of 25 receivers) detected a median of 100% of sharks. Reducing the array size by 60% using the top performing receivers resulted in a 35% decrease in the median number of detections per visit of the tagged sharks (67 out of 102.5 detections). In comparison, reducing the array by the same amount with random and clustered sampling methods resulted in a 57% decrease (44 out of 102.5 detections). The post hoc sampling methods used in this study are an empirical approach for optimising placement of limited receiver resources with broad application for establishing cost-efective monitoring.

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