Seen or heard? A comparison of visual and acoustic autonomous monitoring methods for investigating temporal variation in occurrence of southern right whales
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Passive acoustic detectors are widely used for monitoring distribution of cetaceans. Autonomous visual methods are less frequently employed; they are limited to detections during daylight and good weather, but offer potential advantages due to certainty of species’ identification and longevity of deployment. To compare performance of acoustic and visual methods, temporal changes in distribution of southern right whales, Eubalaena australis, were monitored in the remote sub-Antarctic Auckland Islands (50°31′S, 166°16′E). A time-lapse camera was deployed for 2 years from August 2010, taking an image every 70 min. The presence of whales was scored for each image taken during daylight (n = 8295). A passive acoustic recorder was deployed in August 2011, recording for 3.75 min every hour until battery life expired after 10 months. Each recording (n = 6978) was aurally reviewed. Both methods revealed a similar seasonal distribution; maximum detection rates were in the austral winter and no whales were detected in January or February. However, at the peak of right whale occurrence, the proportion of samples with whales detected was much higher for the acoustic recorder (93.9% in August) than the time-lapse camera (14.8%). A generalised additive model fitted to the visual data revealed significant effects of sea state and visibility. Acoustic detection rates were higher, probably because detection range is greater, and less affected by weather. The solar-powered time-lapse camera system, however, functioned effectively for much longer. We discuss the relative merits of visual and acoustic detectors and attempt to draw conclusions about their efficacy for different focal species and monitoring locations.
We would like to thank the Foundation for Research Science and Technology (FRST), the New Zealand Whale and Dolphin Trust, and the Marine Science Department at the University of Otago for funding this work. Sincere thanks also to the crew of RV Polaris II, Phil Heseltine, Anthony Davidson, Tim Cole for assistance in the field, Marta Guerra for assistance with image analysis, and New Zealand’s Department of Conservation for permitting. WR was supported by a FRST post-doctoral fellowship, and TW by a University of Otago Ph.D. scholarship. The manuscript was improved thanks to comments from two anonymous reviewers and the assistance of the editors.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflicts of interest.
Research was carried out under Marine Mammal Research Permit Per/NO/2010/05 issued by the Department of Conservation to Will Rayment and Steve Dawson. The time-lapse camera was deployed on Enderby Island in compliance with High Impact, Research and Collection Permit SO-28239-RES issued by the Department of Conservation to Will Rayment. The acoustic recorder was deployed in Port Ross in compliance with Permit to Undertake Specified Study within a Marine Reserve SO-31108-MAR issued by the Department of Conservation to Trudi Webster. No ethical approvals were required.
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