Abstract
The time of year and day, the state of the tide and prevailing environmental conditions significantly influence seal haulout behaviour. Understanding these effects is fundamentally important in deriving accurate estimates of harbour seal abundance from haulout data. We present a modelling approach to assess the influence of these variables on seals’ haulout behaviour and, by identifying the combination of covariates during which seal abundance is highest, predict the optimal time and conditions for future surveys. Count data of harbour seals at haulouts in southwest Ireland collected during 2003-2005 were included in mixed additive models together with environmental covariates, including season, time of day and weather conditions. The models show maximum abundance at haulout sites occurred during midday periods during August and in late afternoon/early evening during September. Accurate national and local population estimates are essential for the effective monitoring of the conservation status of the species and for the identification, management and monitoring of Special Areas of Conservation (SAC) in accordance with the EU Habitats Directive. Our model based approach provides a useful tool for optimising the timing of harbourseal surveys in Ireland and the modelling framework is useful for predicting optimal survey periods for other protected, endangered or significant species worldwide.
Highlights
Deciding on the optimal time to conduct a survey of an animal population to determine the size of the population is a challenging task
Two seasonal peaks in abundance at haulout sites have been identified across the harbour seals’ geographical range, a peak during May/June associated with pupping and during August/September associated with moulting (Thompson and Harwood 1990, Thompson et al 1997, Jemison and Kelly 2001, Huber et al 2001, Reijnders et al 2003)
The objectives of this study were to investigate the effects of covariates such as month, time of day and weather on harbour seal abundance at haulout sites in Special Areas of Conservation (SAC) in southwest Ireland, to identify the peak in haulout counts and to determine the optimal timing and environmental conditions under which to conduct haulout counts in order to increase the accuracy of population estimates for the species in Ireland
Summary
Deciding on the optimal time to conduct a survey of an animal population to determine the size of the population is a challenging task Such surveys are necessary for reasons such as basic ecological audits for conservation management and planning decisions or population estimate and trend analyses for assessing the conservation status of endangered or protected species. Various approaches have been used to overcome this problem including mark-recapture models, distance sampling techniques and NAMMCO Scientific Publications, Volume 8 derived population estimates based on known births and deaths. It is especially difficult with semi-aquatic or aquatic and/or migratory species that are inaccessible or unavailable for counting during a significant part of their life histories. Identifying such periods is a challenge and requires survey effort and statistical manipulation of the resulting data
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