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


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).