BIOT Science Expedition 2014 - Pete Carr BIOT Science Expedition 2014 Initial Report Coconut Crab Surveys

Population Assessment of the Coconut Crab, Birgus latro

Peter Carr, Permian Global and Heather Koldewey, ZSL

A base-line study of a non-harvested population of coconut crab, Birgus latro, in a native, mature, mixed forest was undertaken on Diego Garcia in 2011 by a US scientist.  It is thought this base-line represents “the ideal population” only achieved in conditions of plentiful food, adequate recruitment and where there is no harvesting. For the first time, coconut crab surveys were conducted in the northern atolls to determine population dynamics for this IUCN Red-Listed Data Deficient species.

On this expedition, three islands each were surveyed (Vache Marine, Middle Brother, Eagle) using the same methodology as that used in previous Diego Garcia surveys. After dark, starting at 1930 hours, 25 x 30m transects were conducted using DISTANCE methodology, involving remaining on islands overnight. Data were collected on the size, sex and weight of crabs along the transects. The islands were selected for their size, vegetation composition and rat presence/absence.  The results are still being analysed, however, there appears to be a possible correlation between island size, vegetation composition and rats.  The hypothesis being tested is that rats on small, vegetation impoverished islands predate adult coconut crabs.