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5. Stock Assessment - Size, Structure and Distribution of the Coconut Crab Population

Over the 71/2 month period of the coconut crab survey programme a total of 124 transect surveys were completed from a scheduled 198, with each transect being surveyed an average of 4 times and a total of 224 coconut crabs recorded. The relatively large number of cancelled surveys (74; 33% of the total scheduled) was attributable primarily to manpower problems and, to a lesser extent, inclement weather and transport problems. The sole on-island field-worker, Mr. Etuata, was also a quarantine officer and was required to conduct quarantine inspections associated with aircraft and shipping movements. On many occasions Mr Etuata's quarantine duties prevented him from completing scheduled surveys.

As indicated in section 4.2, transects were to have been surveyed every 4 weeks and a schedule of transect surveys was prepared accordingly. However due to the factors referred to above, approximately 33% of scheduled surveys per month were not completed. Rescheduling cancelled surveys to maintain appropriate frequency and intervals of transect surveys was impossible due to the unpredictability of survey cancellation. As a result surveys of most transects were irregular and less frequent than was originally deemed necessary to obtain data capable of reflecting temporal changes in coconut crab abundance and distribution.

5.1 Size of the Coconut Crab Population

In calculating mean CPUE values for each vegetation category (refer section 4.5) it was found that for the Coastal Forest category variance of transect CPUE values was unacceptably high. Consequently, for the purpose of analysis, this category was divided into 7 sub-categories, reflecting the number of transect groups around the coast and hence for which CPUE data would be available; each sub-category was centred about these coastal transect groups (ie. C1, C2, C3/C4, C5/C6, C7, C8/10, and 34/35). Calculated crab populations for each Coastal Forest sub-category were then summed to provide a total Coastal Forest coconut crab population.

5.1.1 Results

Mean CPUE, population size and population density of coconut crabs for each vegetation category are shown in table 2. Much variation was recorded in CPUE and density values for the different categories, with mean CPUE levels ranging from a maximum of 0.38 in Coastal Forest down to 0 in Fernland areas. This variation is not unexpected as it is a reflection of differences in habitat suitability and exploitation level amongst the different areas.

Coastal Forest Category Of the 181440 coconut crabs estimated to be on Niue, 137 560 (75.8%) occur in the Coastal Forest category, which comprises only 11.5% of the total land area. Two points concerning the validity of the population estimate for this vegetation category are pertinent:

  1. As indicated above, variation in CPUE levels for each Coastal Forest sub-category was high, ranging from a maximum of 0.78 to a minimum of 0.14. Delineation of where changes in Coastal Forest CPUE occurred was primarily qualitative, given that the number of Coastal Forest sampling areas was insufficient for a quantitative assessment. However much of the variation in CPUE values among the different sub-regions was attributable to variation in exploitation pressure. Sub-regions which were known to experience high levels of exploitation also had low CPUE values. Hence it was possible to amalgamate data on known exploitation areas with transect survey results to assist in assessing points of delineation for changes in sub-category CPUE levels.

  2. The actual area occupied by Coastal Forest is based on a 1990 Niue Forestry map which uses as its reference aerial photographs taken in 1981. There are 2 problems stemming from this. The first, and most obvious, is that in the 10 years since the aerial photographs were taken the extent of the various vegetation types would not have remained static. It is quite probable that the land area still covered with Coastal Forest will have contracted, however the extent of any contraction is impossible to estimate.

Secondly the area classed as Coastal Forest is not homogeneous with respect to suitability as coconut crab habitat. The area seaward and immediately landward of the coastal cliff line (where present), normally characterised by high levels of shelter in the form of abundant pinnacles/limestone formations (particularly the east and, to a lesser extent, the north and south regions of the island), is most suited to coconut crabs. No accurate data is available on the extent of this habitat type within the general classification of Coastal Forest, and therefore no allowance for habitat variation within the Coastal Forest category can be made. It is by necessity therefore that Coastal Forest has been considered as habitat homogeneous in calculation of a regional population from sub-regional CPUE data - this will inherently lead to some degree of over-estimation. The above two points combine to render the population estimate for the Coastal Forest category somewhat unreliable, tending to over-estimation.

Mean CPUE and density values for this vegetation category are comparable to, though lower than, the mean CPUE and density values recorded in the heavily exploited regions of Santo in Vanuatu (0.39 and 5100 resp.; Fletcher 1988).

Coastal High Forest and Coastal Light and Scattered Categories Coastal Primary Forest (3.8% of the total land area) was calculated to contain 10 820 (6% of the total) coconut crabs while Coastal Light and Scattered (21.8% of the total land area) was calculated to contain 25 840 (14.2% of the total) crabs. Mean CPUE and density values for Coastal Primary Forest (0.07, 707) and Coastal Light and Scattered (0.063, 612) are surprisingly similar, and very low. This similarity in CPUE values from two vegetation categories with quite different levels of coconut crab habitat suitability suggests that proximity to the coast is more important in determining coconut crab distribution than is habitat suitability taken on its own.

Inland High Forest Category This category comprises 7.9% of the total land area and was calculated to contain 4 910 (2.7% of total) coconut crabs. Mean CPUE and density values (0.014, 169) are very low and are at least an order of magnitude less than Coastal Forest values. Although remarkably similar in general topography and food availability to Coastal Primary Forest, the increased distance from the ocean renders this vegetation category much less suitable for crabs.

Inland Light and Scattered Category This category comprises 24.6% of the total land area and area and was calculated to contain 2 310 (1.3% of total) coconut crabs. In stark contrast to the Coastal Light and Scattered category, mean CPUE and density values are extremely low (0.004, 46), to the point where crabs can be considered as rare, if not effectively absent.

Coastal and Inland Fernland Categories These categories comprise 30.4% of the total land area and no crabs were recorded in either category. Both vegetation zones possess none of the habitat requirements of coconut crabs in that there is no form of suitable shelter, little suitable food and a hot, low humidity, environment.

5.2 Structure of the Coconut Crab Population

The structure of the coconut crab population was assessed using morphological data obtained from 272 individuals captured during transect surveys and through random encounters. It is assumed that morphometric measurements taken from crabs located in a particular vegetation category are representative of crabs in that category.

5.2.1 Sex Ratio

The size of the male and female coconut crab populations on Niue was calculated in the same manner as that used to estimate the total coconut crab population, however in this instance sex specific CPUE values were used in calculating the male and female population sizes for each vegetation category or sub-category (refer Table 3).

On this basis the ratio of males to females is 1:1.09. Given the migratory reproductive behaviour of female coconut crabs (Chapman 1948, Gray 1981, Schiller et. al. 1991) it would be fair to expect that the calculated sex ratio for a given area would undergo an annual cycle, particularly in those coastal areas favoured by females for brooding and subsequent release/hatching of their eggs. Due to a lack of sampling frequency (refer section 5.0) available data was insufficient to demonstrate any significant temporal change in the sex ratio at any survey site.

The M:F ratio was lowest in the Coastal Forest and tended to increase with increasing distance from the coast. In Coastal Forest the M:F ratio was approximately 1:1.4, while no females were recorded in Inland High Forest. This trend was most pronounced on the western side of the island with a M:F ratio of 0.4:1 recorded along the coastal zone from Alofi to Avatelle. This low M:F value may be attributable to sex selection by hunters for the larger male crabs. Market survey data (refer section 7) indicates that 93% of coconut crabs available at the Alofi market are male. In addition market records indicate that a large percentage of crabs available from the market are sourced from the Avatele/Tamakautoga area. It is likely therefore that the low M:F ratios between Alofi and Avatele are the result of selective hunting of male crabs for sale at the market.

The overall M:F ratio of 1:0.9 is near unity and is similar to that reported for Palau and Eniwetok (Helfman 1973) and Vanuatu (Fletcher 1988). During the 1988 pilot stock-survey of Niue (Schiller 1988b) the calculated ratio of males to females was 1:1.09. The 1988 study used data collected predominantly during the coconut crab reproductive period from surveys of near-ocean and foreshore transects. As such it would be expected that there would be an over-representation of females with respect to males. Consequently it is considered that no inference can be drawn from the theoretical change in calculated sex ratio from 1988 to 1990.

5.2.2 Size Structure

Analysis of male and female morphometric data indicates that the associated size frequency distributions (Fig 5A & 5B) are significantly different (T-test, p=0.0001) with males reaching a larger size than females (Table 3). This was also reported during the 1988 pilot stock survey and similar results have been reported for Palau (Helfman 1973), Guam (Amesbury 1980), Vanuatu (Fletcher 1988) and Christmas Island (Schiller 1988a). However the magnitude of the male-female size difference is less than that reported for coconut crabs on Christmas Island or Vanuatu (Schiller 1988b).

Mean thoracic length and mean weight of male and female coconut crabs on Niue are much less than that reported for coconut crabs on Christmas Island (Schiller 1988a), Vanuatu (Fletcher 1988) and The Cook Islands (Lavery, pers. comm.) For example mean TL of males on Christmas island is 46mm whereas mean TL of Niue males is only 32.5mm.

Growth in coconut crabs is not only extremely slow (k=0.05) but also variable (Fletcher et. al. 1991). As a result crabs from the same cohort may become appreciably different in size as they age. Consequently use of coconut crab size-frequency data to obtain reliable estimates of the age structure of the population is fraught with inaccuracy.

Examination of morphometric data for each transect site revealed that much variation existed both within and between habitat categories, particularly in respect of male coconut crab data. It was found both the maximum and the mean size of males recorded along transects near known hunting areas were significantly lower than those recorded from transects away from hunting areas. The degree to which mean and maximum size were affected appeared related to the proximity of the hunting area to the transect and the level of exploitation experienced by that hunting area. The proximity of hunting areas to transects had no discernible effect on female mean and maximum size, probably as a consequence of the lower hunting pressure on females.

Female Size Structure

The size-frequency data for females recorded during the 1990 survey (Fig 6A) is not normally distributed (p=0.0176) and is left skewed (-0.6) with prominent kurtosis (0.5). The kurtosis suggests that the ‘tail’ size groupings are abnormally truncated. Absence of females with a thoracic length greater than 35mm is attributable to sustained and high exploitation pressure, whereas absence of small sizes could indicate a lack of successful recruitment (refer section 6.1). However the fossorial nature of very small (ie thoracic length < 15mm) coconut crabs makes them extremely difficult to locate even if they are present. Consequently any suggestion of limited/no recruitment based solely on evidence from size-frequency histograms is somewhat specious.

Comparison of female size-frequency histograms from the 1988 and 1990 surveys (Figures 6A & 6B) reveals that the two distributions are significantly different (T-test, p=0.0067). The 1988 size-frequency data is normally distributed (P=0.4231) with a small right, as opposed to left, skewness. This has certain ramifications with respect to possible patterns of recruitment and exploitation during the period 1988–90 and will be addressed in the discussion (Section 5.4).

The mean size (thoracic length) of female coconut crabs is very low but has increased from 25.59mm in 1988 to 26.99mm in 1990, an increase of 5.5%. Modal size has increased by the same percentage, from 24.8mm to 26.2mm. Given the nature of the 1990 size-frequency distribution the observed increase of 5.5% most likely stems from poor recruitment resulting in fewer small crabs, as opposed to an increase in the number of larger crabs present.

Inspection of female size-class data (Table 4) reveals that a calculated 90% of females in the population have a TL of 32mm or less. As will be discussed in section 4.2, in the context of the requirements for a reproductively viable population this is an alarmingly high proportion of small individuals.

Male Size Structure

As with the females, the size frequency histogram of males recorded during the 1990 survey (Fig 5A) is not normally distributed (P=0.0001) and has a prominent right skew (1.02). The mean size of males was a low 32.5mm. Unfortunately the limited sampling of males in 1988 precludes comparison of 1988 and 1990 morphometric data.

Comparison of 1990 male and female size-frequency distributions suggests that the male population is slightly more robust with a larger range of size-classes present. However the absence of small size-classes is even more pronounced than for the females. Inspection of male size-class data (Table 5) reveals that a calculated 75% of males in the population have a TL of 36mm or less. If the Vanuatu requirement for legal size crabs (43mm TL) were to be applied on Niue only 13% of males would be ‘legal’. The size structure of male coconut crabs on Niue is not what would be expected from a ‘free running’ coconut crab population, but is indicative of a highly exploited species.

5.3 Distribution of the Coconut Crab Population

The distribution of coconut crabs on Niue is a function of the proximity of their habitat from the coast, the availability of suitable habitat and past/present levels of exploitation.

5.3.1 The Influence of Coastal Proximity

In general increasing distance from the coast is associated with decreasing coconut crab density. Fletcher et. al. (1991) estimated that in Vanuatu coconut crab distribution is limited to habitats within 2km of the coast and Helfman (1980) reported that on Christmas Island coconut crabs were absent from the centre (ie. 3–5 km from the coast) of Guam. However Schiller (1988a) indicated that coconut crabs were present in relatively large numbers 4km inland. Fletcher et. al. (1991) suggested that coconut crab distribution is limited partly by the availability of suitable sheltering habitat, and that the importance of such habitat increases with increasing distance from the coast. Unfortunately no data/information was provided in support of this hypothesis.

Coconut crabs are air breathers and require a high relative humidity for their branchial lungs to remain functional. Exposure of coconut crabs to low humidity results in rapid dehydration leading to death (Schiller 1987). It is possible that a major limiting factor in the distribution of coconut crabs is their requirement for a relatively high humidity (ie. > 80%). Coastal areas of tropical regions generally have a high ambient humidity, around 90%. Movement away from the coast usually results in a decrease in the ambient humidity, except where vegetation remains very dense with a closed canopy.

It is suggested therefore that distribution of coconut crabs inland from the coast may to a large extent be limited by their requirement for a high humidity. The actual distance coconut crabs will occur inland will therefore be influenced by ambient humidity levels and availability of vegetation which provides high humidity and/or abundant hiding areas. Hiding areas would enable coconut crabs to take refuge from low humidity levels during the day.

Placement of survey transects in this study was determined through use of stratified random selection which used Fletcher et. al.'s estimate of a 2km maximum inland incursion of coconut crabs as a strata to classify areas as either coastal (< 2 km from the coast) or inland (> 2 km from the coast) (refer section 4.2). This decision, based initially on intuition, is now supported by transect survey data. For example at Transect T24, which is approximately 1.5 km from the coast, coconut crabs were present whereas at transect T23, which is approximately 2.2 km from the coast, crabs were absent. Both transects are in the same vegetation zone (High Forest) and are of similar geomorphology, only the distance from the coast is different. Similar results were obtained for transects T18/T12 and transects T31/T32 (refer figure 3 for transect locations).

As a generalisation it would appear that the distribution of coconut crabs on Niue tends to be restricted to specific vegetation zones occurring within 2km of the coast.

5.3.2 Availability of Suitable Habitat

To survive coconut crabs require a high humidity to prevent dessication, numerous holes/crevices in which to hide, soil areas in which to construct burrows for the purpose of moulting, a good food supply, and access to fresh and, to a lesser extent, salt water. On this basis coconut crabs generally inhabit near-coastal forest regions, hiding in burrows or crevices amongst eroded limestone outcrops/rubble (Reese, 1965; Helfman, 1970; Amesbury, 1980) or under coral boulders (Amesbury, 1980).

Given the generally poor quality of the soil on Niue, subsistence agriculture as practised by the majority of Niueans for hundreds of years has been by necessity of the shifting slash/burn variety. As a consequence much of the native forest on Niue has been destroyed due to clearing of land for agriculture. The legacy of such practices is the presence of large monospecific stands of Nephrolepsis hirsutula, often referred to as Niuean desert, and extensive regions of poor quality regenerating forest which together cover 77% of the Island. Forest destruction has been exacerbated in recent years by the introduction of bulldozers to clear land for agriculture. During the 1988 census it was found that 80% of agricultural households used bulldozers to clear land with only 18% using the traditional slash/burn methods.

Stands of Nephrolepsis hirsutula have no protective canopy and are characteristically very dry. There is little in the way of holes or crevices and food availability is extremely low. This vegetation type is totally unsuitable for coconut crabs. Regenerating Forest has some canopy cover but in general the vegetation density is too low to provide a high humidity environment. In addition the availability of holes/crevices is very low and the food supply scarce. This vegetation type is also unsuitable as coconut crab habitat.

In preparing newly cleared land for agriculture, Niueans level the area as much as possible, destroying suitable coconut crab hiding places. When the area is allowed to regenerate the nature and rate of regrowth is such that for many years (decades?) The bush regenerating is too sparse to provide the high levels of humidity required by coconut crabs and there is little available shelter. In effect although land cleared for agriculture is ultimately ‘returned’ to the forest, the quality of the regenerating forest as coconut crab habitat is very low. For example coconut crab density in Inland Regenerating Forest is so low that effectively crabs are absent from the region, and no crabs at all are found in Fernland areas. Hence approximately 55% of the available habitat on Niue is totally unsuitable for coconut crabs. Inland High Forest, comprising 5% of the island, has extremely low coconut crab densities and can be considered as relatively inhospitable coconut crab habitat. On this basis 63% of the available habitat on Niue, incorporating much of the central region, is either totally unsuitable or grossly sub-optimal as coconut crab habitat and contains only 4% of the coconut crab population.

At the other end of the spectrum Coastal Forest, which provides excellent coconut crab habitat, comprises only 11.5% of the available habitat yet contains a massive 76% of the coconut crab population. Furthermore coconut crab abundance within the Coastal Forest region is concentrated in the eastern areas of the island such that an estimated 6% of the total land area of Niue 50% of the coconut crab population. Protection of Coastal Forest, particularly in the eastern regions of Niue, is essential as it is the ‘core habitat’ for coconut crabs on Niue and no further clearing of this vegetation category should be permitted.

In addition to the problem of overall reduction of available suitable habitat is the added influence of forest fragmentation. Small regions, or isolates, of what is technically suitable coconut crab habitat surrounded by unsuitable habitat type are effectively unavailable to crabs and are under/not utilised. Hence when investigating the extent of available coconut crab habitat the effect of fragmentation on the perceived area available must be taken into consideration. Fragmentation will also act to exacerbate the effect of increasing distance from the coast on coconut crab distribution/abundance, whereas vegetation corridors (ie. tracks of appropriate habitat connecting 2 or more habitat regions) linking coastal areas with inland regions tend to ameliorate the effect of distance from the coast. On Christmas Island where suitable coconut crab habitat extends inland from the coast for several kilometres and fragmentation is absent, coconut crabs occur in large numbers even 3 or 4 km inland (Schiller 1988a). In contrast Niue has relatively small amounts of good quality coconut crab habitat which is found primarily in near-coastal regions on the eastern side of the island. In addition prevalent forest fragmentation further reduces the effective availability of coconut crab habitat that is present.

5.3.3 Over-Exploitation

The Niuean practice of hunting coconut crabs for extended periods using the same bait line has resulted in numerous regions of localised depletion or, in some areas, extinction of the coconut crab. The nett effect has been to produce a small population fragmented over space.

Under conditions of stable food supply coconut crabs tend to exhibit a relatively high site fidelity, not roaming more than 100–200 m from their burrows/crevices. As a consequence crabs removed from an area by hunting are not readily replaced by crabs ‘emigrating’ from other regions and hence recolonising of exploited regions is slow. In the face of continuous hunting the number of crabs in an area will decline. This effect may become more pronounced as the size of the coconut crab population declines and the available ‘pool’ of new immigrants to hunted regions becomes smaller and more dispersed. The most noticeable areas of localised extinction resulting from over-exploitation occur in the regions surrounding Namukulu/Hikutavake, Toi, Mutalau, Hakupu, and along the coast from Alofi to Avatale. Unfortunately there is little prospect of a build-up of coconut crab numbers in such over-exploited areas.

5.4 Discussion

The Niue coconut crab population estimate of 181440 is low considering the extent of suitable crab habitat on the island. If just Coastal Forest, the vegetation category with the highest recorded density of crabs, is considered there are an estimated 140000 crabs contained within a total land area of 29.99 km2. Similar habitats, with respect to coconut crab habitat suitability, of the same land area in Vanuatu (in a low exploitation region) would be expected to contain approximately 402000 coconut crabs (Fletcher 1988) and at Christmas Island would contain approximately 420000 coconut crabs (Schiller 1988a). If other vegetation categories are similarly compared the contrast between Niue and Vanuatu or Christmas Island becomes even more pronounced. On this basis it is suggested that the density of coconut crabs in the vegetation categories of a. Coastal Forest, b. Coastal High forest, and c. Coastal Light and Scattered Forest is far below the maximum carrying capacity of each category type.

Available evidence suggests that the observed low coconut crab densities result from a combination of prolonged poor recruitment and over-exploitation superimposed on the problem of diminishing and increasingly fragmented areas of suitable habitat. In order to suggest effective remedial measures to increase coconut crab densities, it is necessary that the relative influence of poor recruitment and over-exploitation be understood.

Unfortunately this is not just a simple matter of examining size and size-frequency data because mean body size of coconut crabs is influenced by factors other than just the exploitation history of the crab population, namely recruitment patterns and growth rates.

Forest areas highly exploited by hunters are characterised by low CPUE levels, low mean and maximum sizes of crabs and often a low M:F ratio. This has the effect of fragmenting the coconut crab population which in turn acts to increase the possibility of recruitment failure via an overall decrease in reproductive output.

Of the 7 sub-categories comprising the Coastal Forest category, the greatest sub-regional CPUE occurred in the area with the most rugged terrain-characterised by large and extensive limestone pinnacles/outcrops. The effect of extremely rugged terrain is twofold. Firstly the more rugged the terrain, the greater the number of available crevices and holes for coconut crabs to inhabit. Secondly, rugged terrain is an effective deterrent to hunters and so exploitation is reduced. It is suggested that, within limits, the ruggedness of a vegetation category is proportional to the number of crabs potentially present and inversely proportional to the level of exploitation occurring. It would be expected then that Coastal High Forest, with moderate to low ruggedness, would contain few crabs as a result of high exploitation of resident coconut crab stock. Whereas Coastal Forest, with high ruggedness, should contain many crabs. These scenarios have been substantiated by survey results.

The largest individuals recorded, male or female, were well below the asymptotic size limits of either sex and were much smaller than the maximum size crabs recorded during coconut crab population surveys in Vanuatu and Christmas Island. This situation can be directly attributed to sustained high exploitation preferentially removing larger crabs. Large coconut crabs are easier to see and catch, feed more people, and fetch higher prices at market, than small crabs, and so tend to be the first removed from the population (Amesbury, 1980). Comparison of female size-frequency for coconut crabs on Niue, Vanuatu and Christmas Island (Figure 7) graphically illustrates the predominance of small size-classes and absence of large size-classes in the Niue population.

Examination of female size data for 1988 and 1990 reveals that both mean and modal thoracic lengths increased by 5.5% over the 2 year period. Given a scenario of relatively constant annual levels of recruitment, mortality and exploitation (sustainable type), no significant annual differences in mean size or size-structure of the coconut crab population on Niue would be expected. A change in mean size, either smaller or larger, over time may be indicative of either a decline or an increase in the size and viability of the population - an increasing mean size does not necessarily mean that an animal population is ‘healthy’ and increasing, while a decreasing mean size does not necessarily mean that a population is in decline. For instance a decrease in mean size over time could indicate that extensive recruitment, together with continued exploitation of larger crabs, has occurred resulting in a proliferation of small individuals. The net result being that the number of smaller individuals in the population has increased while the number of larger individuals has remained static - this is a scenario for a healthy and robust population. Conversely an increase in mean size over time may be indicative of poor recruitment and a static or changing level of exploitation, resulting in a decrease in abundance of small individuals as well as a decline in the size of the population in general. This would cause a higher mean size but the situation is obviously non-viable in that the population is being harvested with little or no replacement of individuals occurring.

The low mean thoracic length, low mean weight, and small size difference between male and female crabs are almost certainly consequences of severe hunting pressure. Comparative size frequency data for female (Figure 7) and male (Figure 8) coconut crabs clearly highlights the effect of increasing levels of exploitation on the mean size and size frequency distribution of coconut crab populations. Christmas Island (Indian Ocean) typifies a coconut crab population which experiences no exploitation. Male and female mean sizes are high and the size frequency distribution approximates a normal curve. Vanuatu, or more specifically the Hog Harbour area of Espiritu Santo, experiences medium to high levels of coconut crab exploitation. As a consequence male and female mean sizes are much lower than for Christmas Island and the size frequency distribution incorporates significant skewness as a result of hunting pressure removing disproportionate numbers of larger crabs. The Niue mean male and female sizes are very lower than those for Christmas island and the size frequency distributions exhibit pronounced kurtosis and skewness. Given the relationship between hunting pressure and the resultant nature/shape of the size frequency distribution suggested by the Vanuatu and Christmas Island data, it is inferred that the skewness and kurtosis present in the Niue size frequency distributions are the result of very high, and prolonged, levels of coconut crab exploitation.

The observed changes in female mean/modal thoracic length and size structure from 1988 to 1990, the present male and female size structures and the negative relationship between patterns of exploitation and crab density and distribution all serve to indicate that previous/current rates of exploitation are in excess of recruitment-based replenishment, resulting in a population in decline. Because estimates of annual coconut crab harvest rates are unavailable (refer section 3.3.1), the actual rate of decline is impossible to quantify but is inextricably linked to the nature of recruitment as it occurs on Niue and is discussed in the next section.

The lack of a male or female 17–19 mm TL size class, the general paucity of small crabs (TL < 25mm) located, and the change from a left-skewed to a right-skewed female size-frequency distribution during the 1988–90 period suggests that recruitment levels have been very low. This is not an unusual situation as it is suggested by Fletcher et. al. (1991) that substantial recruitment may only occur every five to ten years (refer section 4.1).

Prolonged over-exploitation in conjunction with low and sporadic recruitment has resulted in a small population of small individuals concentrated in the small regions of Coastal Forest around the island, particularly areas of rugged terrain. The tendency of hunters to hunt continuously in established hunting areas has resulted in numerous regions of localised depletion, both in terms of coconut crab abundance and general body size, around Niue. The small size and high degree of fragmentation of the coconut crab population on Niue suggests that recovery, in terms of an increase in abundance, of coconut crab sub-populations exposed to long term exploitation pressure would be extremely slow or nonexistent. This is reflected in the relatively large areas of suitable habitat, especially on the western side of Niue, that through previous over-hunting now contain very few crabs. Further destruction and fragmentation of Niue's coastal vegetation categories, particularly Coastal Forest which comprises the ‘core habitat’ of the coconut crab, must be avoided if a reproductively viable coconut crab population is to continue on the island.

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