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3. Introduction

3.1 Background

Concerned at the possibility that the coconut crab on Niue was being over-exploited, the Niuean Department of Agriculture, Forestry, and Fisheries (DAFF) contacted the Food and Agricultural Organisation of the United Nations (FAO) requesting that a study of its coconut crab resource be conducted. In 1988 FAO commissioned a 4 week preliminary stock survey to ascertain the magnitude of any over-exploitation and to make recommendations in regards the necessity for future study to enable development of a suitable resource management plan. One of the major findings to emerge from that study was that the Niue coconut crab population was indeed suffering from over-exploitation and that a coconut crab resource management plan need be developed as a matter of some urgency (Schiller 1988b). Accordingly in 1990 FAO commissioned a comprehensive study of the coconut crab population on Niue to provide data pertinent to the formulation of a suitable resource management plan.

The coconut, or robber, crab Birgus latro (L.) is a terrestrial hermit crab belonging to a monospecific genus within the family Coenobitidae and is the largest of the land crabs. It grows very slowly (K= 0.05, Fletcher et. al. 1991) but is long-lived (30–40 years) and can attain weights in the vicinity of 4 kg. Although a close relative of land hermit crabs, coconut crabs only occupy discarded mollusc shells during the post-larval and early juvenile phases of their life history (Harms 1932, Reese and Kinzie 1968, Reese 1969) and during this period tend to remain in coastal areas in close proximity to the ocean. After approximately 1 year the juvenile coconut crabs discard their mollusc shells and move further inland; from this point the crabs have little need to return to the ocean apart to hatch their eggs and to occasionally drink salt water. This 'independence' from the ocean is only possible as a consequence of coconut crabs possessing well-developed thoracic lungs plus an efficient mechanism for recovery of ions excreted via the urine (Greenaway et. al. 1990). As such B. latro is the most terrestrial of the decapod crustaceans.

Coconut crabs occur over a broad geographical range in the tropical Indo-Pacific region, extending from the Aldabras Islands in the Indian ocean to Easter Island in the Pacific ocean (Reyne 1939) (Figure 1). However significant populations of B. latro are restricted to insular habitats and continental populations of coconut crabs are both extremely small and few in number. Coconut crabs have been known to man for over 300 years and since the first scientific description by Rumphius in 1705 have engendered much interest in both the general and scientific world communities. Prior to the early 1900's various studies of the coconut crab indicated nothing untoward was occurring in respect of the size and geographical distribution of crab populations. However a detailed study by Reyne in 1939 found that both the size and geographical distribution of B. latro populations had declined. Subsequent studies (Holthuis 1959, Taylor 1973, Helfman 1973, Horstman 1976, Storch et. al. 1979, Amesbury 1980, Wells et. al. 1983) have indicated that this trend has continued, accelerating over time so that now both the size and geographical distribution of coconut crab populations are significantly reduced and that localised extinction has occurred in areas where coconut crabs were sympatric with humans.

Prolific throughout its geographical range only 50 years ago, B. latro is now classed as ‘endangered’ by the International Union for the Conservation of Nature (Wells et. al. 1983). The extreme fragility of coconut crab populations is demonstrated by Fletcher (1988) who found that in Vanuatu extinction of island coconut crab populations through over-exploitation had occurred in less than 10 years.

Increasing human populations resulting in increasing unmanaged subsistence exploitation of B. latro, changing land use patterns resulting in degradation of coconut crab habitat, and the introduction of domestic animals are almost certainly causes of the observed decline in global coconut crab distribution and abundance. However it is the combination of increasing commercial exploitation plus the lack of any formal protective legislation (ie. minimum legal size, catch limits etc.) in most Indo-Pacific countries that is most to blame for the decline, and which poses the greatest threat to the future of the crab.

The flesh of the coconut crab is a prized food amongst the people of the Indo-Pacific region and on many islands the crab has great socio-cultural significance. Commercial exploitation of the coconut crab, via the tourist market, is becoming increasingly significant economically for many countries and may represent the only cash crop. In light of this an increasing number of these island nations are becoming concerned at the apparent decline in abundance of their coconut crab resource and are desirous of rectifying the situation through initiation of resource management plans/protective legislation to ensure perpetuation of that resource. While it is possible to formulate a general plan of coconut crab management and protection, recruitment rates and population size and structure, on which the specifics of such management plans are based, will vary between different regions. As a consequence it is necessary that development of a viable and acceptable coconut crab management plan for a particular region be pre-empted by a comprehensive coconut crab population survey of that region.

It is important to note that perpetuation of terrestrial species with a planktonic dispersal phase is in the most part dependent on the success of recruitment of offspring back into the land-based population. The success or otherwise of recruitment is dependent upon many factors-larval survival, the vagaries of oceanic currents, predation rates, presence of an appropriate emergence substrate etc., all of which are beyond human control. Consequently management strategies for this resource type can only address the anthropogenic (human intervention) factors and as such success of the managed species will ultimately depend on natural stochastic processes.

Coconut crab larvae are relatively intolerant (Schiller 1988, Schiller et. al. 1991) and successful recruitment of post-larvae back into the adult population irregular (Fletcher et. al. 1991). These factors, together with slow growth and substantial longevity, make the coconut crab an extinction-prone species in that it is extremely susceptible to even low levels of exploitation or habitat degradation. If coconut crab populations are to be maintained exploitation must be in accordance with a suitable resource management strategy.

3.2 Project Description

The coconut crab population study on Niue was conducted under the following terms of reference (TOR):

  1. To make a detailed study of the coconut crab resource.

  2. To prepare an appropriate resource management plan for the coconut crab, including draft legislation with respect to capture/trade restrictions.

  3. To prepare draft manuals or texts on maintenance of the coconut crab resource and the provision of educational materials such as video films, slides, posters, pamphlets etc.

  4. To produce an overall report.

Field-work extended over a 71/2 month period from May through to December 1990 and was conducted primarily by local staff. The author visited the Island on 3 occasions, 18-4-90 to 07-5-90, 05-09-90 to 25-9-90 and 17-07-91 to 24-7-91.

The primary objective of the current study was to obtain reliable estimates of the size, structure and distribution of the coconut crab population on Niue to enable formulation of a viable resource management strategy.

Niue Island

Niue island is situated in the South Pacific Ocean approximately 480 km east of Tonga, 550 km south-east of Samoa and 900 km west of Rarotonga. The island is isolated and does not form part of any recognised group.

Niue has an area of approximately 259 km2. It is 21 km long and 18 km wide, and the main road, which roughly follows the coastline, is approximately 64 km in length. The island is an elevated coral outcrop with a coral reef fringing a precipitous and broken coastline. In general formation it takes the shape of two terraces, the lower coastal terrace being approximately 28m above sea level; the upper terrace, which forms the bulk of the island, is approximately 69m above sea level. Apart from the rise from the lower to the upper terrace, there are no hills. The island has no running streams or surface water. [Text on Niue Island reproduced from 1:50 000 map of Niue (NZMS 250 Niue) published by the Department of Lands and Survey New Zealand, 1985]

3.3 Current Exploitation Patterns

The coconut crab is an integral component of the socio-cultural heritage of Niueans and is important as both a celebration and a staple food. Consequently demand for, and therefore exploitation of, coconut crabs remains high. A survey conducted during September 1989 (Census 1989) indicated that of the 522 households 173 (33%) had hunted crabs during that month, with a mean catch of 24 crabs. On a per capita basis this is high when compared to estimated per capita consumption in Vanuatu. However it is possible that the mean catch of crabs per household is an overestimate (refer section 3.3.1).

The coconut crab is not included in any Niuean wildlife or fisheries act and as a result the catch, sale and export of coconut crabs on Niue is completely unregulated. This situation is not unusual among those pacific Island nations with coconut crabs, only Vanuatu and, very recently, The Solomon Islands have regulations pertaining to the capture, consumption and export of the coconut crab.

There is no evidence of direct self-regulation by Niueans with respect to the size or sexual status of coconut crabs captured/consumed. Niue does however have a small (2km2) (Carew-Reid 1990) section of Huvalu Forest protected as a sacred area (tapu area) into which no person may enter for whatever purpose. This wildlife sanctuary was established by the elders of Hakupu Village and relies on tradition as opposed to legislation for its existence. Unfortunately other closures of land have not endured; Taputapu and Hikulagi were once tapu areas but are now regularly hunted. The limited size and lack of coastal frontage of the Huvalu Forest tapu area may however severely limit the effectiveness of the region as a coconut crab conservation area (refer section 8).

The net effect is that hunting of coconut crabs on Niue is completely exploitation orientated with no consideration of sustainable yield or resource conservation/management requirements. Such an attitude is understandable given that although Niue has been inhabited for over 1 000 years and coconut crabs hunted for generations, it is only in recent years that hunting has resulted in large reductions in crab numbers. Local crab hunters intimated that in the last 10 – 15 years coconut crabs had become progressively more difficult to catch, and were smaller in size (pers. comm.). There is little doubt that for many years the rate of coconut crab capture/consumption has exceeded the maximum sustainable yield of the island's crab population. As will be outlined in this report, if the observed decline in coconut crab abundance is to be reversed, traditional hunting practices and attitudes must be quickly changed.

It was postulated by Schiller (1988b) that the present problem facing the coconut crab on Niue is directly attributable to the commencement of coconut crab exports to New Zealand with the opening of Hanan International Airport in 1970. The introduction of regular air services between Niue and New Zealand enabled coconut crabs to be readily exported to expatriate Niueans resident in New Zealand. Niue Government statistics for the period 1970 to 1987 indicate that 5 000 Niueans emigrated to New Zealand (Abstract of Statistics 1984; Quarterly Abstract of Statistics 1987). Among the expatriate population the coconut crab remains a traditional/desirable food. Consequently, as numbers of New Zealand Niueans increased, so the demand for crabs would have increased. The current Niuean expatriate population in New Zealand is unofficially estimated at 10 000 (pers. comm.). Department of Agriculture and Fisheries' records indicate that for the period 1.06.87 to 18.03.88, from 3 200 to 5 900 coconut crabs were exported to those expatriates (Table 1). New Zealand imposes few restrictions on the import of dead coconut crabs. It is estimated that exporting of crabs has increased exploitation rates 200–350% over the 'normal' levels required to meet local demand.

3.3.1 Estimate of current levels of annual coconut crab exploitation

Extrapolation of the above mentioned DAFF export data for “medium” (≈ 26mm thoracic length) crabs indicated that in the 12 months from 01.06.87 approximately 5 800 individuals were exported to New Zealand. In comparison, it was recommended that the maximum sustainable yield for an area in Vanuatu of similar size to Niue, with abundant coconut crabs, was approximately 7 000 crabs per year (calculation based on data from Fletcher, 1988 unpublished ACIAR report). Export data pertaining to coconut crabs for the period from 18.03.88 to the present is unavailable and no estimates of coconut crab exports are possible. However sporadic air services over the period in question would have resulted in an overall reduction in numbers exported.

As indicated above, data from the 1989 census indicate that for the month of September 1989 173 households hunted crabs and had a mean catch of 24 crabs. Although the average monthly catch would be lower during the late autumn and winter months when the crabs are underground moulting, the September value of 24 crabs caught per household is at a time when the crabs are emerging from the ground but have not yet reached 'peak' abundance, and can probably be considered as an indication of the mean monthly catch rate for a year. If hunting patterns in general changed little over the year this would equate to an annual harvest of 49824 individuals (includes crabs for local consumption and export). This figure represents 27% of the coconut crab stock calculated to be on the island. Furthermore, assuming a male:female ratio of 1 for crab captures, such a harvest rate after only 2 years would effectively remove the female breeding component of the population. Hence there is a serious problem in reconciling census-based estimates of coconut crab harvest rate with the population estimate arising from this study. As such I consider use of census data to estimate annual coconut crab harvest levels as specious. Unfortunately population data generated from this study is also unsuited to estimates of harvest levels (such a requirement was not part of the project TOR) and consequently no estimate of present harvest levels can be made.

The 1988 pilot coconut crab stock-survey identified 3 major areas of concern in relation to the established coconut crab hunting practices on Niue.

  1. Females with eggs are taken and are in fact actively sought out as they are considered a delicacy.

  2. Sexually immature crabs are taken. This practice stems primarily from the custom belief that release of any crab, even very small ones, will result in those crabs never being caught again. In many cases the small crabs kept are too small for eating and end up dying for no purpose.

  3. Many of the crabs caught were destined for export to expatriate Niueans resident in New Zealand. As a result the coconut crab resource of Niue is being harvested to supply a demand that is much larger than has traditionally been the case.

While ‘normal’ Niuean coconut crab hunting practices are not conducive to conservation, it was suggested in the report from the 1988 pilot stock-survey that the current decline in coconut crab numbers is directly linked to the commencement in 1970 of coconut crab exports to expatriate Niueans resident in New Zealand.

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