Overcoming hermit-crab interference during rodent-baiting operations: a case study from Henderson Island, South Pacific

Abstract

Context Rat (Rattus spp.) eradication operations in the tropics are often put at risk by the presence of land crabs, in particular hermit crabs (Coenobitidae), which consume toxic bait and thereby render it unavailable to rats. Aims We estimated hermit-crab densities in areas of high abundance, assessed crab bait consumption rates and determined the required baiting application rate to ensure that 100% of Pacific rats (Rattus exulans) consumed bait in areas with high crab densities on Henderson Island, South Pacific. Methods Crab densities and rat densities were estimated through Distance sampling of crabs and mark–recapture of rats. Non-toxic rhodamine-dyed cereal bait pellets were spread by hand at varying densities within four study plots and rats were captured 12 days later and examined for the presence of rhodamine biomarker dye. Key results Crab densities (primarily Coenobita perlatus) varied between beaches from 130 crabs ha–1 to 1370 crabs ha–1, with rats occurring at respective densities of 28 and 7 rats ha–1 on these beaches. Estimated bait consumption rate of C. perlatus was 3.8 g crab–1 day–1. In total, 100% of rats were positive for rhodamine-dyed bait, despite the high densities of crabs present. Conclusions The results support the prospects for a successful rat eradication on Henderson Island, although careful consideration must be given to the potential impact of an eradication on non-target species. The estimated bait consumption rate by C. perlatus found in the present study may allow other eradication operations to make initial calculations of the required baiting densities to overcome interference from this species.