A CAUTIONARY NOTE ON THE USE OF MNA-DERIVED TRAPPABILITY METRICS IN WILDLIFE PROGRAMMES, AS EXEMPLIFIED BY THE CASE OF THE EUROPEAN BADGER (MELES MELES)

Minimum Number Alive (MNA) is a widely used index of abundance and trappability in mark-recapture programmes. MNA is a negatively-biased abundance index, and is sensitive to capture probability and the capture session number. This suggests that MNA-trappability will be positively-biased, and that the extent of the bias will vary with true trappability and trapping regime. Inappropriate use of the index may therefore lead to over-optimistic estimates of trappability. In this note, we highlight the dangers of using MNA-trappability for wildlife practitioners. Using closed-population stochastic simulation, we verify that MNA-trappability indeed provides over-optimistic estimates, especially when true trappability is <70% and the number of trapping occasions is small (<7). Using severely-biased trappability estimates could have serious consequences for wildlife conservation, management or vaccination studies. We specifically discuss the implications of this finding in relation to control programmes of the European badger Meles meles and recommend that future studies, where possible, use multiple metrics of trappability to assess the consistency of results across estimators.