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Open Access Publications from the University of California

Placebo Bait Uptake Trial to Test Feasibility of Polynesian Rat (Rattus exulans) Eradication on Wake Atoll


Rodent eradications have contributed to the recovery of many threatened species, but challenges often exist for campaigns that occur on tropical islands when compared to more temperate regions. A post-operational review of a rat eradication operation on Wake Atoll indicated that certain areas, such as those with high alternative food abundance, may have contributed to the failure to remove all Polynesian rats. We conducted a nontoxic bait uptake trial to evaluate whether the maximum prescribed bait application rate for Brodifacoum-25W rodenticide pellets was sufficient to expose all rats to a lethal dose at three sites on Wake Atoll, including around a solid waste aggregation area (SWAA), which was previously identified as “high risk.” We monitored bait persistence and condition throughout the treatment period as well as rat movement via radio tracking. Bait uptake by rats was also assessed by trapping and examination of rat orifices and gastrointestinal contents for pyranine biomarker incorporated into the bait pellets. The rate of bait disappearance differed by site, with bait disappearing the fastest in vicinity of the SWAA. Rat movement also varied by site, with rats observed traveling greater distances around the SWAA, sometimes exceeding 300 m. The SWAA was the only site at which we observed rats negative for biomarker exposure. We suggest that these negative observations resulted from lack of bait availability or movement of rats into the core trapping area from outside the treatment area. However, we cannot rule out preferential selection of alternative food sources over bait pellets and suggest that this possibility should receive further attention. Based on our results, we conclude that, of the three sites, the maximum bait application rate prescribed on the product label was not high enough to provide every rat an opportunity to encounter bait at and around the SWAA. Given the rapid disappearance of bait and the regular immigration of rats from distant habitat, we recommend that an even greater application rate be prescribed and that the heavier treatment be extended over a much larger area surrounding the SWAA.

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