Coconut crabs, Birgus latro (Anomura: Coenobitidae), of Sorol Atoll, Yap, with remarks on the status of B. latro in the Federated States of Micronesia

摘要

A preliminary survey of coconut crabs, Birgus latro (L.), on Sorol Atoll, Yap, during 3 weeks in June and July 2011 is only the second quantitative assessment of B. latro for any island or island group in the Federated States of Micronesia (FSM). A population density of at least 169 crabs/ha (based on 10 m and 2 m fixed-width transects combined) and as high as 354/ha (2 m fixed-width transects only) was estimated for Pigelmol Island in the atoll. The largest male measured 71.4 mm in thoracic length (TL) and weighed 2,750 g, and the largest female was 44.0 mm TL and weighed 720 g; 24 males (26% of the 91 crabs from Pigelmol) were larger than the largest female. The sex ratio did not differ significantly from 1:1. Crabs were active during the day as well as at night. Population density of B. latro on Pigelmol probably is among the highest of any island in the FSM, but a dearth of similar studies in the FSM precludes more-definitive interisland comparisons. Coconut crabs were not recorded on the two Sorol Atoll islands where suitable habitat was lacking, or marginal at best, and they were scarce on the three others. Overexploitation by residents of the former settlement; predation by rats, pigs, and monitor lizards; and competition from hermit crabs (Coenobita spp.) are suggested as possible reasons for the scarcity of crabs, at least on the main island. Government legislation regulating harvest of coconut crabs in the FSM is mainly at the state level and varies greatly across the FSM. Although the most effective conservation practice appears to be traditional leadership at the community level, poaching remains frequent, enforcement is difficult, and the few existing scientific management strategies are applied without baseline knowledge of stock populations.