To integrated models ... and beyond …
Ruth King (University of Edinburgh)
Friday 4th December, 2020 15:00-16:00 Zoom
Meeting ID: 810 5220 4802
Capture-recapture studies are often conducted on wildlife populations in order to improve our understanding of the given species and/or for conservation and management purposes. Such studies involve the repeated observation of individuals over a period of time, and in many cases, over years and even decades. At initial capture each individual is uniquely marked (e.g. using a tag/ring/using natural markings) and released. The data then correspond to the set of observed individual capture histories, detailing at which capture occasions each observed individual is recorded at. For standard capture-recapture models, it is well known that the survival probabilities correspond to apparent survival, with permanent migration and death confounded. We describe an integrated modelling approach, where in addition to the live resighting of individuals, we are also able to recover dead individuals, typically referred to as capture-recapture-recovery data. We show how such data permits the estimation of the dispersal (or migration) parameter and hence true survival probabilities. We describe the general modelling approach and, using sufficient statistics, calculate a corresponding efficient likelihood expression, which makes use of hidden Markov model-type ideas.
We apply the approach to data from a colony of guillemots (Uria aalge) collected over the period 1992-2018. For these data there are physical challenges with the data collection process, with the aim of minimising the disturbance of the population in their natural habitat. Individuals are ringed as chicks on the beach below the breeding ledges, but future sightings are then based on visual sightings using long-range telescopes, which are only able to survey a small proportion of the colony, resulting in a partially monitored capture-recapture study. Thus, individuals who locate away from the monitored areas to breed become unobservable (guillemots are philopatric to their breeding location). We show that applying the standard capture-recapture (Cormack-Jolly-Seber) models leads to biased estimates of survival parameter, which can be corrected when using the additional ring-recovery data. Finally we discuss some further ongoing challenges for these data.