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Variation in harbour seal counts obtained using aerial surveys

Published online by Cambridge University Press:  12 February 2010

Louise Cunningham ,
John M. Baxter  and
Ian L. Boyd
Louise Cunningham*
Affiliation:
Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland, UK The Scottish Government, Victoria Quay, Edinburgh, EH6 6QQ, Scotland, UK
John M. Baxter
Affiliation:
Scottish Natural Heritage, Silvan House, 3rd Floor East, 231 Corstorphine Road, Edinburgh, EH12 7AT
Ian L. Boyd
Affiliation:
Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland, UK
*
Correspondence should be addressed to: L. Cunningham, Sea Mammal Research Unit, Gatty Marine Laboratory, University of St Andrews, St Andrews, Fife, KY16 8LB, Scotland email: lc66smru@gmail.com
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Abstract

Aerial surveys of harbour seals (Phoca vitulina) are usually carried out to provide an index of population size. This can be normalized, either by design or by post-hoc analysis to reduce the effects that date, time of day, tide and weather might have on the number of seals counted. In order for long-term trends to be determined from these counts it is assumed that the mean number of seals at a particular site does not vary during the survey period, and that the start and duration of the survey window does not vary with location or between years. This study used a combination of repeat land-based and aerial surveys to test the assumption for constancy of counts during the survey period. The study focused on harbour seal abundance at haul-out sites around the Isle of Skye in north-west Scotland. The coefficient of variation in these counts was estimated to be 15%, based on repeat aerial surveys using thermal imaging. Land-based counts were used to examine the effect of covariates on seal numbers using generalized additive modelling. This site-specific model predicted that the current aerial survey window for harbour seals in the UK, which is a three-week period during the moult, is about a week too early and that count variation could be reduced by surveying 1 1/2 hours earlier in the tidal cycle. Furthermore, the pupping period showed even higher (though more variable) abundance of hauled out seals than during the moult.

Keywords

abundancecoefficient of variationmonitoringPhoca vitulinaScotland
Type
Research Article
Information
Journal of the Marine Biological Association of the United Kingdom , Volume 90 , Special Issue 8: Marine Mammals , December 2010 , pp. 1659 - 1666
Copyright
Copyright © Marine Biological Association of the United Kingdom 2010

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