Estimating Demographic Parameters for
Capture-recapture Data in the Presence of Multiple Mark Types
Sophie Smout, Ruth King and Paddy Pomeroy
University of St. Andrews
Summary
In mark-recapture studies, various techniques can be used to
uniquely identify individual animals, such as ringing, tagging or
photo-identification using natural markings. In some long-term
studies more than one type of marking procedure may be implemented
during the study period. In these circumstances, ignoring the
different mark types can produce biased survival estimates, since
this assumes that the different mark types are equally catchable
(i.e. a homogeneous capture probability across mark types). We
consider an integrated approach where we simultaneously analyse data
relating to different marking techniques, using the AIC statistic to
discriminate between competing models. This technique also allows us
to estimate both relative mark-loss probabilities and recapture
efficiency rates of the different marking methods. We initially
perform a simulation study to explore the different biases that can
be introduced if we assume a homogeneous recapture probability over
mark type, before applying the method to a real dataset. We consider
an intensive observational study of UK female grey seals
(\emph{Halichoerus grypus}) at a single breeding colony, where there
are three different methods used to identify individual within a
single study (i.e. branding, tagging and photo-identification based
on seal coat pattern or pelage).