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ABSTRACT: Currently there is no commercial or subsistence fishing for holothurians on the Cocos (Keeling) Islands and, despite a prominent government presence, no reported cases of poaching or illegal fishing. However, because of recent interest in developing a commercial fishery for holothurians there, a survey was initiated to provide baseline data on the previously unfished local holothurian populations. Fourteen species of holothurians were recorded during the survey, with the most abundant species being Holothuria atra; only 4 other species had relatively high abundances. A total of 20 556 holothurians were counted; however, 97% of these were considered to be of low commercial value. The high-and medium-value species found in this survey were all in extremely low abundances, with restricted distributions. The distribution and abundance of holothurians was closely linked with benthic habitats, with 48% of the variation in holothurian populations explained by 13 habitat variables. Several species displayed distinct habitat preferences: H. atra was associated with sand-dominated habitats, Actinopyga mauritiana was associated with relic reefs and soft corals, while Holothuria fuscopunctata and Stichopus herrmanni were both associated with reef flats. The densities recorded in the present study represent the natural abundance and distribution of holothurian populations at this atoll. Given the low numbers of commercially important species, it is highly unlikely that a commercial fishery would be economically viable at the Cocos (Keeling) Islands and it would be more beneficial to maintain the natural holothurian population. KEY WORDS: Holothurians · Habitat associations · Coral reefs · Exploitation · Unfished populations Resale or republication not permitted without written consent of the publisher Aquat Biol 14: [57][58][59][60][61][62][63][64][65][66] 2011 ans by fishing are unknown. However, given they have important functions in their environment, it is likely that removal of these animals may reduce the overall productivity of coral reefs Holothurians are often amongst the most valuable resources for small tropical island communities The Cocos (Keeling) Islands are an Australian territory approximately 2800 km north-west of Perth, Western Australia The present paper describes a baseline investigation of the abundance and habitat preferences of holothurian species at the Cocos (Keeling) Islands, motivated in part by recent interest in initiating a commercial fishery for holothurians. While the primary aim of fisheries management is ensuring the sustainability of exploited stocks, in recent years increasing emphasis has been placed on broader ecosystem function. A critical step in this process is understanding the relationship between the abundance of organisms and the habitats in which they are found METHODS Study location The Cocos (Keeling) Islands are located in the Indian Ocean (12°12' S, 96°54' E), approximately 2950 km north west of Perth, 900 km west-southwest of Christmas Island and 1000 km southwest of Java Head ( Survey methodology A 4-person team undertook field surveys of holothurian densities between 7 and 24 May 2006. The study was limited to depths of < 29 m and covered all habitats; intertidal reef flats, shallow lagoon habitats and the outer reef slopes. Sampling was conducted systematically across the entire atoll by dividing the atoll into 1 minute blocks This survey used similar rapid assessment techniques to those used in holothurian surveys at Warrior Reef, Torres Strait (Skewes et al. 1998), Timor Sea MOU Box Paired t-tests indicated that there were no significant differences between the observations on the 2 replicate transects at each site, so the abundance of holothurian species and percentage cover of each habitat were averaged across the 2 transects to give the overall density of each holothurian species and the average habitat percentage cover for each site. Habitat categories In the present study habitat is categorised in 2 different ways: fine and broad scale. The fine-scale habitat data were collected during the current survey, using the methods described above; these data were used for the modelling component of the study. The fine-scale habitat categories used in the current study are: sand, rubble (limestone rubble), seagrass, macroalgae, massive coral (corals that are solid and similar in shape in all directions), sub-massive coral (corals less than, or not quite, solid and similar in shape in all directions), plate coral (or laminar, forming a tier), foliose coral (forming a whorl), soft coral (coral without exoskeleton), branching coral (forms with branches), branching coral dead, reef flat (limestone platform) and relict reef (old eroded limestone reef). Fine-scale habitat categories were based on those used by previous studies so comparisons could be made (i.e. Statistical analysis Permutational distance-based approaches were used to examine spatial patterns in holothurian assemblages, as the data were highly skewed and contained many zeros Standing biomass was estimated by calculating the average density of holothurians per 400 m 2 , for each broad-scale habitat type (from Williams 1994), converting individuals per hectar and then multiplying by the hectares of each habitat type. This was done for all holothurians and then for each economic category (high, medium, low, unknown; RESULTS Holothurians of the Cocos (Keeling) Islands Fourteen species of holothurians belonging to 7 genera were recorded on the transects Habitats of the Cocos (Keeling) Islands The fine-scale habitat data were collected at the scale of transects and used for modelling habitat associations. Within the lagoon the primary habitat is sand, with seagrass beds mainly occurring along the eastern and southern sides of the lagoon. In fact, sand was found at 76% of the sites surveyed and accounted for approximately 31% of the habitat. Massive and branching corals were also common, surveyed at 62 and 56% of the sites, respectively, but accounting for only 8 and 6.5% of the cover. Soft corals were less common (42% of sites) and accounted for 11.5% of the cover. Relic reef (13.5%) and reef flats (11.1%) were the other major habitats with respect to cover, although their distribution was less common (48.1 and 11.4%, respectively). Habitat preferences of holothurian species The distance-based linear regression model indicated strong relationships be tween holothurian assemblages and habitat variables. The most parsimonious model contained 8 of the 13 habitat variables and explained a total of 48% of the variation in holothurians, with no significant colinearity among the habitat variables. The first dbRDA axis accounted for 29.3% of the total variation in the holothurian assemblage and distinguished sites dominated by soft corals and relic reefs from soft bottom communities. The second dbRDA axis accounted for 7.3% of the variation in the holothurian assemblage and largely discriminated living corals from reef flats Only 4 holothurian species were strongly correlated with the dbRDA axes. Holo thuria atra was positively correlated with Axis 1, indicating it was positively associated with sand and negatively associated with relic reef and soft corals. Axis 2 indicated H. atra was also positively associated with reef flats, but to a lesser extent. Actinopyga mauri tiana (surf redfish) was negatively correlated with Axis 1, indicating a positive association with relic reefs and soft corals, while the positive correlations of H. fuscopunctata and Stichopus chloronotus with Axis 2 indicated that they are associated with reef flats. Sites were then qualitatively assigned to 4 groups based on the distribution of the sites DISCUSSION The current study identified a total of 14 species of holothurians at the sites surveyed, with a further 4 species identified outside the survey sites. A previous survey of holothurians at Cocos (Keeling) Islands identified 34 species of holothurians (Marsh 1994). However, it is difficult to compare the results of the 2 studies as Marsh (1994) focused on identifying species diversity by searching discrete areas, while the current study focused on quantifying the abundance ) of the 5 most abundant holothurians by group and distribution of holothurians across the entire atoll using a spatially stratified survey. While Marsh (1994) conducted a qualitative survey, some observations on the abundance of 9 species were made. Marsh (1994) states that Holothuria atra was the most common species; H. nobilis and Thelenota ananas were the most valuable species, but they were not abundant; Actino pyga echinites and A. mauritiana were common to abundant on reef flats; and Bohadschia marmorata, H. scabra and B. argus were also common. With the exception of A. mauritiana, which was not found in the current survey, these statements are reflected in the results of our survey. In contrast, Hender et al. In the current study, an average total density of 3252 individuals ha The distribution and abundance of holothurians at the Cocos (Keeling) Islands is closely linked with the distribution and nature of benthic habitats. Previous authors have reported that holothurians exhibit distinct habitat preferences Acknowledgements. This work was funded by the Attorney General's Office, Australian Government, which provided salaries for L.M.B. and S.N.E., as well as funding for project logistics. P.L. was supported by the Natural Sciences and Engineering Research Council of Canada and as a Visiting Professor at the University of Western Australia. We are grateful to Parks Australia Cocos (Keeling) Islands, in particular Ismail Macrae, Mohammad-Said Chongkin and Chris Boland for their advice and assistance in the field. We thank Dave Abdo for his contribution to earlier versions of this manuscript, Fiona Webster and Dovid Clarke for their invaluable assistance in the field, and Dieter Gerhard from Cocos Dive. We also thank the residents of the Cocos (Keeling) Islands who provided advice and assistance and several colleagues whose comments greatly improved this manuscript