Population estimation and trappability of the European badger (Meles meles): implications for tuberculosis management

Abstract

Estimates of population size and trappability inform vaccine efficacy modelling and are required for adaptive management during prolonged wildlife vaccination campaigns. We present an analysis of mark-recapture data from a badger vaccine (Bacille Calmette-Guérin) study in Ireland. This study is the largest scale (755 km(2)) mark-recapture study ever undertaken with this species. The study area was divided into three approximately equal-sized zones, each with similar survey and capture effort. A mean badger population size of 671 (SD: 76) was estimated using a closed-subpopulation model (CSpM) based on data from capturing sessions of the entire area and was consistent with a separate multiplicative model. Minimum number alive estimates calculated from the same data were on average 49-51% smaller than the CSpM estimates, but these are considered severely negatively biased when trappability is low. Population densities derived from the CSpM estimates were 0.82-1.06 badgers km(-2), and broadly consistent with previous reports for an adjacent area. Mean trappability was estimated to be 34-35% per session across the population. By the fifth capture session, 79% of the adult badgers caught had been marked previously. Multivariable modelling suggested significant differences in badger trappability depending on zone, season and age-class. There were more putatively trap-wary badgers identified in the population than trap-happy badgers, but wariness was not related to individual's sex, zone or season of capture. Live-trapping efficacy can vary significantly amongst sites, seasons, age, or personality, hence monitoring of trappability is recommended as part of an adaptive management regime during large-scale wildlife vaccination programs to counter biases and to improve efficiencies.

Figure 1. Badger population size and trappability estimates.

A. Estimated badger population size for each full session (1–5) within the Kilkenny study area during the study period. Solid-line is the closed-subpopulation derived population estimate, the dotted line is the minimum number alive (MNA) population estimate, and the dashed line is the number of badgers trapped per session. B. The solid line is the estimated trappability using the closed-subpopulation model during each session with associated exact 95% confidence interval. Dotted line represents the MNA-derived trappability.

Figure 2. Percentage of unmarked badgers caught in a sequence of capture sessions in the Kilkenny study area.

Solid line represents all badgers trapped; dashed line represents adult badgers only.

Figure 3. Map of the study area in Co. Kilkenny.

The area is divided into three zones, A, B and C. The ‘reference area’ from the Four Area Project is shaded. Dots represent all known setts (both active and inactive) within the trial area. Black dots are main setts; hollow dots are non-main setts.