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Density of Holothuria nobilis and distribution patterns of common holothurians on coral reefs of Northwestern Australia

Glenn R. Shiell

University of Western Australia, Crawley, Australia


Broad-scale distribution and density data of sea cucumber inhabiting Ningaloo Reef, Western Australia (WA), are compared with published data from Ashmore Reef, Cartier Reef and Rowley Shoals, northern WA. Results are presented particularly of the black teatfish, Holothuria (Microthele) nobilis. Heavy fishing pressure has affected numbers of H. nobilis on both Ashmore and Cartier Reefs, where population densities of less than 1.0 ind ha-1 have been recorded. Rowley Shoals and Ningaloo Reef, areas that are unlikely to have experienced significant fishing pressure, support black teatfish densities of 9.1 and 19.3-27.2 ind ha-1, respectively. Densities of black teatfish recorded at Coral Bay are approximately equal to or exceed those reported on reefs that are closed to fishing on the Great Barrier Reef.

The distribution patterns of H. nobilis, H. atra and Stichopus chloronotus were examined on Ningaloo Reef using a novel method integrating both Global Positioning System (GPS) and Geographic Information Software (GIS). Differences in patterns of distribution between H. nobilis and S. chloronotus were identified. H. nobilis preferred habitats closer to the reef crest, whilst S. chloronotus were distributed among coral rubble within the inner-reef lagoon. H. atra showed little or no recognisable pattern of distribution. Studies utilising GIS are currently determining the relationship between species distribution and physical habitat characteristics. This process ultimately will help clarify the micro-habitat characteristics of coral reef holothurians. Methods associated with this technique are described.

Management of a potential Western Australian H. nobilis fishery should be approached with caution. Research investigating sources of recruitment and population recovery following fishing should be undertaken as a minimum prerequisite to the adoption of policy concerning beche-de-mer fishery management in WA.

Keywords: Holothurian, GIS, GPS, micro-habitat, fishery, management


Summary of sea cucumber fishing in Western Australia

Sea cucumber fishing in Western Australia (WA) operates on a smaller scale to that conducted in other Australian states. The Department of Fisheries, WA, currently supports six commercial fishing ‘authorisations’, all of which concentrate exclusively on sandfish, Holothuria scabra. Catch data relevant to holothurians are limited to the last eight years and the extent of fishing prior to the introduction of official monitoring is largely unknown. In the last five years, annual reported catch rates of sea cucumber in WA have fluctuated between 30 and 400 tonnes wet-weight (all figures refer to the weight of sandfish measured prior to evisceration) (information provided by David Harvey, The Department of Fisheries, WA).

Fishing of H. nobilis in Western Australia

The limited history of government-sanctioned sea cucumber fishing in WA, combined with the relative isolation of northwestern Australian coral reef-habitats, have precluded H. nobilis from large-scale harvesting by Australian nationals. Most, if not all beche-de-mer fishing in WA has concentrated on sandfish. Black teatfish, however, have been the focus of much international interest and coral reefs of northern WA traditionally have been subjected to fishing pressure by Indonesians (MacKnight, 1976). Foreign visitors to northwestern Australia have, for a number of reasons, exploited H. nobilis commercially: they are readily accessible on remote island reefs; they inhabit clear, relatively shallow water; and as a beche-de-mer product, they fetch some of the highest market prices available.

The significant commercial value of black teatfish on the Great Barrier Reef led to over-exploitation of this species resulting in the total closure of the fishery in 1999 (Uthicke and Benzie, 2000b). Stringent management of black teatfish stocks is clearly necessary, however, knowledge of its biology is limited to a few studies only: reproduction (Conand, 1981), larvae culture (Martinez and Richmond, 1998), feeding selectivity (Uthicke and Karez, 1999), population genetics (Uthicke and Benzie, 2000a) and impacts of fishing (Uthicke and Benzie, 2000b).

This report presents distribution and density data of the black teatfish, Holothuria nobilis, from Coral Bay, Ningaloo Reef. Results from Coral Bay are compared with published data from Ashmore and Cartier Reefs (Smith et al., 2001; Smith et al., 2002), and Rowley Shoals (Rees et al., 2003), all of which are off shore coral reefs to the northeast of Coral Bay (Figure 1). Each of these reefs, including Ningaloo, are currently closed to beche-de-mer fishing, however, illegal fishing has, and continues to affect Ashmore and Cartier Reefs, and to a lesser extent, Rowley Shoals. This report, in addition, describes a novel method integrating Global Positioning System (GPS) and Geographic Information Software (GIS) aiming to determine the micro-habitat preferences of dominant holothurian species from Coral Bay. Initial results and the potential applications of this method are presented and described, respectively.

Materials and Methods

Rapid quantitative assessment surveys recorded the density and broad-scale distribution patterns of H. nobilis on major coral reefs of WA, namely, Ashmore and Cartier Reefs (Smith et al., 2001; Smith et al., 2002), Rowley Shoals (specifically Mermaid Reef) (Rees et al., 2003), and Coral Bay, Ningaloo Reef. The mean number of H. nobilis (ind ha-1) distributed among broadly defined habitats on each of these reefs were calculated and plotted as a histogram.

Micro-habitat determination: the application of GPS and GIS technology

Micro-habitat preferences of dominant holothurian species at Coral Bay were determined using a modification of the manta tow as described by English et al. (1997). Manta tows were conducted in the traditional sense, however, instead of counting individual holothurians, the position of each was recorded as a single GPS way-point and plotted on an aerial photograph using GIS. In situations where animals were too numerous to record individually, a single GPS co-ordinate was recorded and the observer estimated the abundance according to the following criteria: <10; <20 or <30 animals.

Standardised signals were developed to ensure the observer relayed accurate information to the GPS operators, for example different coloured gloves, representing individual species. Only two species could be surveyed per manta tow, so it was necessary to repeat tows to incorporate the three species under investigation.

Figure 1. Partial map of Australia showing locations of density and distribution survey sites. 1. Ashmore and Cartier reefs; 2. Rowley shoals; 3. Ningaloo reef.

All manta tow transects were conducted in the afternoons (³ mid-day) when H. nobilis were known to be feeding and more likely to be visible (Shiell, in prep). Each parallel transect was separated by a lateral distance of approximately 50 m. The survey incorporated all sections of inner-reef, including broadly defined habitats such as the lagoon (including open sand), the back-reef, the reef-flat and where possible, the reef-crest. Surveys incorporated the inner-reef between Lotties lagoon, south of Coral Bay, and Point Maud, north of Coral Bay, a total area greater than 700 ha.

Results presented in this report are restricted to examples of distribution patterns obtained through this technique only.

Results and Discussion

The data presented in this paper are two-fold. Firstly, the report investigates broad-scale distribution and density patterns of H. nobilis obtained using traditional random transect techniques on reefs both un-impacted and impacted by fishing. Secondly, it presents initial results from a study using a GIS-based technique to determine micro-habitat preferences or fine scale distribution patterns of holothurians at Coral Bay.

Density and broad-scale distribution patterns of H. nobilis on major reefs of Western Australia

Densities of H. nobilis at Coral Bay differed considerably when compared with published data obtained at Ashmore Reef, Cartier Reef (Smith et al., 2001; Smith et al., 2002) and Mermaid Reef (Rees et al., 2003) (Figure 2). Lower densities of H. nobilis on both Ashmore and Cartier are probably indicative of heavy fishing pressure applied both historically and in recent times by Indonesian fishermen. At these reefs, especially Cartier, populations of H. nobilis have been harvested to the point of near total denudation. Similarly, only small numbers of animals have been observed at the significantly larger Ashmore Reef; 13 in total after 69 transects totalling approximately 8 ha-1 (Smith et al., 2001). Average densities of H. nobilis at both Ashmore and Cartier Reefs were found to be less than 1.0 animal ha-1, a figure approximately 20 times lower than average densities reported on reefs closed to fishing on the Great Barrier Reef (Uthicke and Benzie, 2000b).

In contrast, populations of H. nobilis at both Mermaid Reef and Coral Bay were substantially greater than on those reefs described above, at 9.3 and 19.3 to 27.2 ind ha-1, respectively (Figure 2). In the case of Coral Bay, these figures represent abundances that are at least equal to, or greater than, those described on reefs closed to fishing on the Great Barrier Reef (Uthicke and Benzie, 2000b). Surveys at Mermaid Reef recorded densities slightly lower than this suggesting that fishing has been conducted at some time in the past (Anon, 1973), although probably not to the same degree as that conducted at Ashmore and Cartier Reefs.

Figure 2. Mean density of H. nobilis within each of the identified surveyed habitats: note the differences between heavily impacted reefs, Ashmore and Cartier (representing heavily fished reefs); and relatively un-impacted reefs, Mermaid and Coral Bay. Density and distribution results relevant to Ashmore, Cartier and Mermaid are compiled from Smith et al. (2001), Smith et al. (2002) and Rees et al. (2003).

Broad-scale distribution patterns

Populations of H. nobilis on Ashmore Reef, Cartier Reef, Mermaid Reef and Ningaloo Reef showed distinct preferences for outer-reef zones, specifically, the reef-flat and reef-crest. Although average densities were greatest on reef-flats, substantial densities were also observed upon the shallower and more exposed reef-crests. For example, certain sections of the reef-crests contained densities of up to 108 ind ha-1, however, the average values presented in Figure 2 were deflated because of the patchiness of suitable habitats (i.e. appropriate sand patches) within the crest itself. Large error bars present on the histograms representing reef-crests at both Mermaid and Coral Bay reflect this trend.

Micro-habitat determinations: The application of GPS and GIS technology

GPS-derived data obtained in Coral Bay provide some of the clearest illustrations of holothurian distribution patterns to date (Figures 3 and 4). Clear preferences for distinct zones within the reef are obvious in the case of H. nobilis and S. chloronotus, but, conversely, are random in the case of H. atra. Although apparently random distributions of H. atra have been documented, for example in Baker (1929) and Massin and Doumen (1986), patterns of distribution between size classes of H. atra have received little attention. Anecdotal observations made at Coral Bay found concentrations of smaller specimens of H. atra on areas of sand and coral rubble adjacent to the shoreline within the reef-lagoon. This observation may be indicative of asexual reproduction, a process reported to result in congregations of smaller sea cucumber in areas closer to the shoreline. This phenomenon has been reported previously by Conand (1996) and Uthicke (1997) on La Réunion and the Great Barrier Reef, respectively, however, further work is required to determine whether this process is important to the patterns observed at Coral Bay.

Individuals of H. nobilis appeared to be concentrated on sand patches either on, or near, the reef-flat and reef-crest zones. Other habitats within the area surveyed, for example, patches of sand in the central lagoon region of the bay, also contained H. nobilis, but in lower concentrations. These habitats were observed at considerable distances away from the preferred habitat zones described in the literature (Conand, 1981; Uthicke and Benzie, 2000b; Benzie and Uthicke, 2003), e.g. the reef-flat and back-reef slope. Further work is being conducted to isolate and identify the environmental factors consistent to these areas where individuals of H. nobilis were observed in an effort to clarify the micro-habitat characteristics of this species.

Individuals of S. chloronotus were concentrated on patches of sand among coral rubble, with the majority of animals positioned in either the central lagoon region of the bay or closer to the shore. Particularly dense congregations of this species were observed in sheltered areas of coral rubble northwest of Monks Head (Figure 3). Similar to H. atra, the average size of the observed specimens and the elevated densities of S. chloronotus at this location suggest that asexual reproduction may be causing this pattern. As with H. nobilis, congregations of S. chloronotus were not restricted to the areas described above. Transects run along some of the shallowest, high-energy sections of the reef-crest, consistently found numerous S. chloronotus clinging to the limestone platform.

Figure 3 shows clearly the contrasting patterns of distribution between S. chloronotus and H. nobilis. The pattern in this figure is supported by previous observations on the distribution of these species (Baker, 1929; Conand, 1981; Uthicke and Benzie, 2000b).

Very few animals of any of the surveyed species were present on open sand away from reef habitat or upon habitats containing ripple marks on the sand surface. This finding is consistent with at least two observations made of holothurian distribution (Moriarty, 1982; Massin and Doumen, 1986).

Preliminary application of this new technique has succeeded in highlighting clear differences in the distribution of the 3 surveyed species, H. nobilis, H. atra and S. chloronotus. Further work will investigate relationships between physical habitat characteristics and the GPS derived distribution patterns of these holothurians. Physical habitat parameters investigated include: sediment grain size, sediment type (geology), sediment nutrient content, water temperature, hydrodynamics and bathymetry. These data have been collected at points along five strategically placed transects. Ultimately, through the application of GIS software, the study aims to determine the environmental characteristics of each habitat containing individual species to help clarify the micro-habitat preferences of coral reef holothurians. Further use of GIS may lead to calculation of distances between individuals to investigate fertilisation kinetics of these broadcast spawners. The application of this technique to determining more accurately species specific density estimates may also be investigated.

Figure 3. Distribution of H. nobilis (red points) and S. chloronotus (white points), Coral Bay, Western Australia. Locations of selected transects A, B and C, through distinct holothurian distribution groups. Water temperature, bathymetry and sediment characteristics were recorded at measured intervals along each transect.

Figure 4. Distribution of H. atra, Coral Bay, Western Australia.

Recommendations for management of H. nobilis populations in Western Australia

Management of a potential WA H. nobilis beche-de-mer fishery would currently be difficult given major limitations in information concerning the ecology and biology of endemic holothurian populations. In contrast to WA, a study of H. nobilis stocks on coral reefs in Queensland provided excellent baseline knowledge of this species (Benzie and Uthicke, 2003). Major components of this study examined stock size, gene flow, recruitment patterns, and stock recovery following fishing. Although this study provided excellent management information pertinent to the Great Barrier Reef fishery, many of the processes described by it may not be relevant to WA. For example, there are currently no data about the degree of gene flow or possible sources of recruitment between WA H. nobilis populations; biological processes that may be distinctly different relative to those of Queensland.

Although there are biological similarities between eastern and western Australian populations of H. nobilis, for example, in the timing of their spawning events (Shiell, unpublished data), sources of population recruitment are fewer in WA when compared to recruitment sources on the Great Barrier Reef, Queensland. Appropriate coral reef habitats are relatively sparse in northern WA and combined with the denuded nature of at least two of these, i.e. Ashmore and Cartier Reefs, recruitment of H. nobilis larvae to Mermaid Reef and, potentially, Ningaloo Reef may already be impeded.

Hence, management of a potential WA H. nobilis fishery should be approached with absolute caution. As a minimum prerequisite a study of the genetic similarity and the degree of gene flow between H. nobilis populations should be conducted before changes to current management policy are considered.


I express my gratitude to: Dr Brenton Knott for guidance and support; Dr Sven Uthicke for discussion and suggestions that benefited greatly the final manuscript. I thank Max Rees and Luke Smith of the Australian Institute of Marine Science for help with the preparation of this report; David Harvey, The Department of Fisheries, WA, and Roberto Ycaza for valuable personal communication. Many thanks also to my field helpers, particularly Nathalie Malo, Hamish Maitland and Svea-Mara Wolkenhauer.


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