Usefully Persistent? Anticoagulants as Quantitative Markers of Bait Uptake in Brushtail Possums

Prolonged persistence of second-generation anticoagulant rodenticides (SGARs) in animal tissues facilitates trophic transfer of residues, with exposure of predatory and scavenging non-target wildlife now widely reported. In many instances, antico-agulant residue levels measured in wildlife are apparently sublethal, although longer-term effects of such exposures are currently not well understood. Conversely, prolonged metabolic persistence is of practical utility when compounds are used as biological markers to determine food uptake by animals. We required two effective and distinct marker compounds to progress field-based research on optimal baiting strategies to manage introduced brushtail possums in New Zealand. Two SGARs (flocoumafen and bromadiolone) were evaluated, as neither are currently registered for application as rodenticides in areas typically subject to possum management. All captive possums ingesting small, sublethal (2-6 g) quantities of food containing 0.0005% (by weight) of either SGAR were reliably marked by the presence of residual concentrations in liver as measured by HPLC analysis. This marking persisted for at least six weeks, and liver concentrations in marked possums did not decline between three and six weeks after the marker was ingested. Marking was also quantitative for both SGARs with a correlation between liver residue concentration and the amount of marker ingested. Using such marker baits would provide at least a six-week period in which to recover possums in field research. By recording bodyweight and testing liver samples from such possums for both markers, regression equations generated from the work reported here will enable back-estimation, with confidence intervals, of the amounts of marker bait eaten by each possum