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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• Reef Life Survey Assessment of Coral Reef Biodiversity in the North-West Commonwealth Network, 2017

Executive summary

The North-west marine bioregion extends from the Western Australian - Northern Territory border to Kalbarri, south of Shark Bay, in Western Australia (WA).

The major coral reefs in the North-west marine bioregion include: Ashmore, Cartier, Hibernia, Scott, Seringapatam, and Mermaid Reef in the Rowley Shoals (which consist of Mermaid, Clerke and Imperieuse Reefs), all of which host high coral and fish diversity.

Three of these reefs are protected as Commonwealth Marine Reserves (CMRs):

• Ashmore Reef CMR
• Cartier Reef CMR
• Mermaid Reef CMR.

Within Ashmore Reef CMR, 550 km2 is strictly protected within a IUCN Ia Sanctuary Zone, and 33 km2 is a IUCN IV Recreational Use Zone, where some fishing is permitted; Mermaid Reef is protected as a IUCN II Marine National Park.

The other Rowley Shoals are zoned under WA State legislation and are partially protected from fishing.

This report presents the findings of the most recent surveys across the North-west CMR Network’s reefs, with a focus on comparing coral reef communities on open and protected reefs.

Reef Life Survey (RLS) dive teams surveyed 172 transects at 94 sites on reefs within the North-west marine bioregion, including 18 transects at 12 sites within the Ashmore Reef CMR and 37 transects at 18 sites in the Mermaid CMR (Appendix 2).

RLS involves recreational divers trained to a scientific level of data-gathering to allow ecological surveys to be conducted across broad geographic areas in a cost-effective manner.

The surveys found clear distinctions in the fish and invertebrate communities of the northern offshore reefs (Ashmore, Scott and Hibernia) and the Rowley Shoals (Mermaid, Clerke and Imperieuse).

Also, fish and invertebrate communities were more distinct between Ashmore, Scott and Hibernia Reefs than they were between the Rowley Shoals, reflecting the greater geographic distance between the individual northern reefs.

Contrary to expectations, diversity was not higher at Ashmore Reef than at reefs less protected (e.g. Hibernia and Scott) or further south (Rowley Shoals).

The Rowley Shoals hosted an overall richer and more diverse fauna, with higher biomass and abundance of sharks, fishes and invertebrates; this like reflects greater exploitation pressure experienced by the northern reefs, including illegal harvesting at Ashmore Reef, rather than the impacts associated with oil spills or climate change.

Further research would be needed to tease apart potential relative contributions of exploitation versus climate change and indirect effects of coral bleaching, for example.

Benthic communities were dominated by live hard coral at most surveyed reefs; only Ashmore Reef had comparatively low cover of all morphologies of live hard coral.

The Rowley Shoals in particular appear to have maintained historically high cover of branching corals.

Fish and invertebrate communities seemed closely aligned to live coral cover, with richer assemblages at reefs with higher coral cover.

Ashmore, Scott and Hibernia Reefs appear to host a general Indo-Pacific fauna, due to their proximity to the Indonesian archipelago and the Indonesian Throughflow, while the Rowley Shoals are characterised by a few species typical of the Indian Ocean not found on the Pacific side of the Indo-Australian archipelago.

The success of no-take (IUCN II) zoning (and possibly enhanced compliance through proximity to state MPAs and likely reduced access by international fishers) is clear in the case of the Mermaid Reef CMR; higher biomass, abundance and species richness of fishes and invertebrates, higher numbers of protected species and higher coral cover all indicate a well functioning ecosystem with an intact trophic structure.

Assessing the differences between the CMR and reference sites is assisted with an ideal situation of two almost identical reefs in close proximity.

In the case of Ashmore Reef, the effects of management are more difficult to detect.

There is a history of disturbance and illegal fishing in this area, and comparison with Scott Reef could imply that impacts are greatest at reefs closest to Indonesia, regardless of protection levels.

This could possibly be the result of the footprint of historical exploitation, or contemporary illegal fishing.

However, with the Scott and Seringapatam Reef reference sites further away and with different habitat structure to those at Ashmore, time-series data are likely going to be important for detecting the response of previously exploited populations to CMR protection.