A comparison of underwater visual distance estimates made by scuba divers and a stereo-video system: implications for underwater visual census of reef fish abundance
Euan Harvey A D , David Fletcher B , Mark R. Shortis C and Gary A. Kendrick AA School of Plant Biology, Faculty of Natural and Agricultural Sciences, University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia.
B Department of Mathematics and Statistics, University of Otago, PO Box 56, Dunedin, New Zealand.
C Science, Engineering and Technology Portfolio, RMIT University, GPO Box 2476V, Vic. 3001, Australia.
D Corresponding author. Email: euanh@cyllene.uwa.edu.au
Marine and Freshwater Research 55(6) 573-580 https://doi.org/10.1071/MF03130
Submitted: 28 August 2003 Accepted: 28 June 2004 Published: 14 September 2004
Abstract
Underwater visual census of reef fish by scuba divers is a widely used and useful technique for assessing the composition and abundance of reef fish assemblages, but suffers from several biases and errors. We compare the accuracy of underwater visual estimates of distance made by novice and experienced scientific divers and an underwater stereo-video system. We demonstrate the potential implications that distance errors may have on underwater visual census assessments of reef fish abundance. We also investigate how the accuracy and precision of scuba diver length estimates of fish is affected as distance increases. Distance was underestimated by both experienced (mean relative error = −11.7%, s.d. = 21.4%) and novice scientific divers (mean relative error = −5.0%, s.d. = 17.9%). For experienced scientific divers this error may potentially result in an 82% underestimate or 194% overestimate of the actual area censused, which will affect estimates of fish density. The stereo-video system also underestimated distance but to a much lesser degree (mean relative error = −0.9%, s.d. = 2.6%) and with less variability than the divers. There was no correlation between the relative error of length estimates and the distance of the fish away from the observer.
Extra keywords: accuracy, bias, precision, sampling error.
Acknowledgments
The authors acknowledge grants and financial aid from the New Zealand Department of Conservation (Research Grant #1822), the University of Otago Research Committee, the Southland Regional Council (New Zealand) and the Division of Sciences at the University of Otago, Dunedin, New Zealand that made this research possible. Also acknowledged is support from Sony New Zealand Ltd, THC Milford Sound, Fiordland Lobster Co. and Mobil and Gore Services Ltd. This work was completed thanks to postdoctoral fellowship funding from the New Zealand Foundation for Research Science and Technology and the University of Western Australia to Euan Harvey. The authors are grateful to Chris Battershill, June Hill, Justin McDonald, Philip Mladenov and Di Watson and for comments and suggestions on this manuscript. Additionally, our thanks to all novice and experienced scientific observers who willingly gave their time to assist with this research.
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