Graham J Edgar, Daniela Ceccarelli, Rick D Stuart-Smith, Antonia T Cooper. Report to Parks Australia, Department of the Environment, 2017.

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• Biodiversity survey of Ashmore Reef (Coral Sea) and comparison with surrounding reefs, 2017

Executive summary

Ashmore Reef is located in the northwestern Coral Sea, between the northern extension of the Great Barrier Reef and Papua New Guinea, close to the islands and reefs of the Torres Strait.

Field surveys at Ashmore Reef and adjacent Boot Reef were conducted in 2013 and 2015 by a team of skilled divers from the Reef Life Survey program (www.reeflifesurvey.com) and the University of Tasmania.

Ecological surveys were conducted at varying depths along 155 transects at 78 sites across Ashmore Reef, Boot Reef, and at reefs considered the most comparable.

These reference reefs comprised the pinnacle reefs in the northern Coral Sea (e.g. Bougainville and Osprey Reefs), which are also remote and isolated from Australian continental reefs; and the platform reefs of the far northern Great Barrier Reef (GBR) and Torres Strait, which are in close proximity, but are located on the continental shelf.

Data collected from each site consisted of abundance and size of fishes, abundance of mobile macroinvertebrates and cryptic fishes, and percentage cover of sessile biota.

Ashmore and Boot Reefs form part of the Coral Sea CMR, and due to their northern location and shelf-edge geomorphology they contribute ecological characteristics that are different from other Coral Sea reefs:

• Ashmore and Boot Reefs were more similar to the ‘oceanic’ reefs of the Coral Sea, but with some representation of the ‘inshore’ fish community - which is unusual among the reefs in the Coral Sea CMR; thus, inclusion of this northern area adds to the overall biodiversity coverage of the CMR.
• Ashmore Reef had anomalously high abundance and species richness of invertebrates, similar to the continental shelf reefs and much higher than the oceanic reefs.
• Ashmore and Boot Reefs had high coral cover (~30%) compared to reefs elsewhere in the Coral Sea (average ~18%).
• When compared with other Coral Sea reefs, Ashmore Reef stood out as having high macroinvertebrate diversity, Boot Reef had very high fish abundance, Ashmore and Boot Reefs both had high fish species richness, total biomass and large fish biomass.
• Shark biomass at Ashmore and Boot Reefs was as high as would be expected of highly protected reefs. They are potentially a valuable sanctuary for healthy predator populations.

An “inshore-offshore” gradient in the composition of reef fish assemblages, with some species typical of offshore reefs and others more characteristic of inshore reefs, was clearly evident in this study.

Oceanic reefs at Osprey and Bougainville were characterised by an offshore fish assemblage, dominated by planktivores and small grazers that feed by cropping low-lying turf algae.

The northern GBR and Torres Strait reefs had an ‘inshore’ assemblage, with omnivorous and farming damselfishes, large benthic invertivores and a broader range of grazers that included browsers that feed on fleshy macroalgae.

Ashmore and Boot Reefs were most similar to the ‘oceanic’ reefs, but with some representation of the ‘inshore’ fish community.

Boot Reef had the richest fish assemblage in terms of density and diversity of all fishes, and a high biomass and density of sharks (~29 sharks per hectare) and other piscivores.

Ashmore Reef had relatively low fish biomass, but shark density was high (~20 sharks per hectare).

This was higher than the highest shark density recorded on the GBR (5.5 +/- 0.83SE individuals per hectare) on no-entry reefs, suggesting low fishing pressure and a relatively intact predator community.

Coral assemblages appeared healthy, with coral cover ~30% across sites, with different coral growth forms distinguishing the different sites.

Interestingly, the mobile macroinvertebrate community showed patterns reflecting geographic distances between the reefs.

Like reef fishes, invertebrates rely on larval dispersal between reefs, with negligible opportunity for adult migration.

Ashmore Reef stood out as having high abundance and species richness of invertebrates, similar to the continental shelf reefs and much higher than the oceanic reefs.

Most of the common invertebrates were more typical of offshore areas and exposed reef fronts, with a high proportion of filter-feeding feather stars.

The fauna recorded on Ashmore and Boot Reefs represents a combination of those associated with the more sheltered habitats of the networked continental shelf reefs and the highly exposed and isolated oceanic reefs (even those quite distant to the south).

Large predators are highly abundant, despite the reefs being open to fishing; their remoteness is likely to have protected them from overexploitation thus far.

Prevailing currents are expected to provide good connectivity with the reefs nearby, both on the shelf and in the Coral Sea.

The position of these reefs on the far northern edge of the shelf places them in danger of frequent exposure to high temperature anomalies; at present, however, their seemingly intact community structure suggests the potential for high resilience or an absence of major threats to date.