Influence of size- and sex-biased harvesting on reproduction of the coconut crab Birgus latro

Larger individuals of many biological resources are more likely to be caught than smaller ones. However, larger individuals can contribute disproportionately to reproduction through sexual selection. Therefore, it is likely that size-selective harvesting of larger individuals can lead to a decline in the reproductive output. Populations of the coconut crab Birgus latro have been severely depleted in most habitats due to overharvesting. Thus, to understand the harvesting pattern and determine its impacts on coconut crab reproduction, we investigated the size and sex of crabs that are marketed for consumption. The harvested population structure and the relationship between male body size and mating success were determined using market research, field investigations, and laboratory experiments. The harvesting pattern was sex- and body size-biased, in which larger males (>40 mm thoracic length) were exclusively taken. Sex ratio was skewed significantly toward females and mean male size decreased in the harvested population. Most female coconut crabs were estimated to mate only with males that are larger than or approximately equal to themselves in body size, implying female preference for larger males as mates. The present large male-selective harvesting could lead to depletion of the resource through reduced reproductive output brought about by reduced probability of encountering potential mates. Therefore, we recommend male-only harvesting with slot size limits as an effective management measure for coconut crabs.