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Article << Previous     |     Next >>   Contents Vol 35(4)

The influence of animal mobility on the assumption of uniform distances in aerial line-transect surveys

Rachel M. Fewster A E, Colin Southwell B, David L. Borchers C, Stephen T. Buckland C, Anthony R. Pople D

A Department of Statistics,University of Auckland, Private Bag 92019, Auckland, New Zealand.
B Australian Antarctic Division, Channel Highway, Kingston, Tas. 7050, Australia.
C CREEM, University of St Andrews, The Observatory, Buchanan Gardens, St Andrews KY16 9 LZ, Scotland.
D Biosecurity Queensland, Department of Primary Industries and Fisheries, GPO Box 46, Brisbane, Qld 4001, Australia.
E Corresponding author. Email: r.fewster@auckland.ac.nz
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Line-transect distance sampling is a widely used method for estimating animal density from aerial surveys. Analysis of line-transect distance data usually relies on a requirement that the statistical distribution of distances of animal groups from the transect line is uniform. We show that this requirement is satisfied by the survey design if all other assumptions of distance sampling hold, but it can be violated by consistent survey problems such as responsive movement of the animals towards or away from the observer. We hypothesise that problems with the uniform requirement are unlikely to be encountered for immobile taxa, but might become substantial for species of high mobility. We test evidence for non-uniformity using double-observer distance data from two aerial surveys of five species with a spectrum of mobility capabilities and tendencies. No clear evidence against uniformity was found for crabeater seals or emperor penguins on the pack-ice in East Antarctica, while minor non-uniformity consistent with responsive movement up to 30 m was found for Adelie penguins. Strong evidence of either non-uniformity or a failure of the capture–recapture validating method was found for eastern grey kangaroos and red kangaroos in Queensland.

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