Growth and recruitment study on coconut crab (Birgus Latro) populations in Vanuatu

• Ministry of Agriculture, Livestock, Forests and Fisheries, Vanuatu
• Fisheries Department, Vanuatu
• University of Queensland, Australia

Coconut crabs are prized items, both in the diet of island people of the Indo-Pacific region and as restaurant delicacies. They are the largest of all terrestrial crabs, weighing as much as 4 kg, and the cause of their declining numbers may be exploitation by ever - increasing human populations.

A management regime is needed to establish balance between natural population size and desirable harvesting procedures. A record of growth rates, and a comprehensive understanding of the larval stages of the crab's life cycle are essential for devising an efficient management program. Any attempts to farm the crab will rely on this information.

The project is a response to a request from the Ministry of Agriculture, Forestry and Fisheries, Vanuatu. The results may have relevance for other small Indo-Pacific island nations whose populations of coconut crabs face similar threat.

Based at Luganville, Espiritu Santo, Vanuatu, the research program will centre of field operations. The Australian team, with local field assistants, will attempt to measure growth rates by a system of "mark and recapture" using transmitter or freeze branded tags which are designed to survive repeated moultings.

Vanuatu, a nation of scattered islands, has a range of human population densities. Stocks of coconut crabs will, therefore, be subject of differing exploitation levels, depending on their location. A survey carried out by the Port Vila-based staff of the Fisheries Department will gather information from the local fish market. Knowing the numbers, size and sex of the crabs on sale, and the details of their place of capture, will provide a picture of stock patterns in different parts of Vanuatu.

Project staff will observe crab populations is several locations, assessing population size and structure. The sex, length and weight of each crab encountered will be recorded, along with appropriate environmental data and habitat characteristics. Village people will be questioned to get some idea of the harvesting pressure in each region.

Attention will be given to stages in the life cycle of the crab. Microscopic examination of the 'glaucothoe" larva and observations of its behaviour will attempt to establish differences between it and the larvae of closely related species, especially Coenebita sp. Like the hermit crab, the glaucothoe larva of the coconut crab enters a small snail shell before coming ashore to complete its life cycle.

A concurrent rearing program is also proposed. By cultivating larvae in an aquarium situation, patterns of shell and microhabitat selection, and changes of behaviour during the transition from water to land can be conveniently observed.

Juvenile coconut crabs are easily distinguishable from other species, and observations of the ecology and general behaviour will be an integral part of other field work in the program. Records of individual moultings and growth rates must rely on the tagging system working as predicted. Juvenile crabs are too small for radio transmitter tags, and should the technique of freeze branding prove inadequate, research workers will resort to maintaining juvenile specimens in individual pens, where the identity of each occupant can be assured.

A study of ways to improve marketing of the crabs will test the preservation and food quality of crabs which have been cooked and frozen prior to shipment to the markets. The research team also hopes to improve the survival rate of crabs caught by traditional methods and take live to market.

A taxonomic survey of coconut crabs in other areas, using preserved material forwarded to the University of Queensland, will correlate and link biological data collected at Vanuatu with data about the coconut crab populations of other Indo-Pacific countries. The studies in Vanuatu will thus be relevant to the whole region.