Ashmore Reef

• Category: Marine and Oceanic Biomes.
• Geographic Location: Indian Ocean.
• Summary: Ashmore Reef, a large platform reef in the eastern Indian Ocean, is uniquely positioned to provide vital habitat for a vast range of marine and avian fauna.

Ashmore Reef is a large platform reef consisting of an atoll-like structure with three low-lying, vegetated islands; several shifting sand cays; and two lagoons. The Ashmore Reef National Nature Reserve and the Cartier Island Marine Reserve aid in conserving regionally and internationally important sea turtles, dugongs (Dugong dugon), and birds (both migratory and seabird species), as well as an unusually high diversity and abundance of sea snakes. The Ashmore Reserve, located off the coast of northwestern Australia, encompasses approximately 225 square miles (583 square kilometers). About 25 nautical miles (46 kilometers) southeast of Ashmore lies Cartier Island Marine Reserve, which covers approximately 64 square miles (167 square kilometers).

The surface waters around the reserves, which are warm and generally nutrient-poor, are dominated by the Indonesian Throughflow Current. During the summer months, the current weakens, resulting in a northward movement of surface water due to the Eastern Gyre Current. At the same time, cooler, nutrient-rich water, thought to be associated with the northward-flowing West Australian Current, is flowing just beneath the surface. Mixing of both bodies of water is thought to occur along shelf-break areas adjacent to the reef systems, which results in localized upwelling and increased nutrient availability.

The Indonesian Throughflow Current is responsible for transporting genetic material southward from the biologically diverse reefs of the Philippines and Indonesia. Consequently, the Australian North West Shelf reefs and shoals play primary roles in maintaining the biodiversity in reefs to the south. In addition, the Leeuwin Current, which is the major southern-flowing current originating in the reserves' area, and the other currents with which it interacts play important roles in maintaining coral reef and algal communities to the south. As a result, the Cartier and Ashmore reefs facilitate the transport of biological material to the reef systems along the western Australian coast.

Vital Species Base

Ashmore Reef National Nature Reserve encompasses two lagoons, several sand flats, shifting sand cays, an extensive reef flat, and the three vegetated islands simply called East, Middle, and West Islands. Together these are the base for a variety of marine habitats. The reef front and crest are composed of hard and soft corals, gorgonians, sponges, and encrusting organisms. In addition, this area provides abundant niches for fish, crustaceans, and echinoderms. Based on recent research, the total number of fish species at Ashmore and Cartier reefs may be as high as 747; such high density and diversity is attributed to the vast diversity of habitat types.

The reef flats are composed of seagrasses, which provide critical habitat for numerous species, including dugongs and sea turtles. The sand flats support species including foraging sea turtles, sting rays, echinoderms, mollusks, and crustaceans. In addition, the sand flats are important grounds for migratory wading and shorebirds. Both lagoons support a variety of fish, as well as apex predators such as sharks and sea snakes, and provide additional feeding areas for both dugongs and sea turtles.

Due to seasonally shifting marine conditions and diverse benthic (seafloor) habitats, Ashmore Reef supports distinct benthic and pelagic communities. Also unique to Ashmore Reef is the fact that it resides in a transitional area between algal-dominated communities to the north and coral communities in the south. The reef is recognized for its high biological diversity, supporting the greatest number of reef-building and non-reef-building corals on any reef off the western Australian coast. As a critical habitat for sea snakes, the reserves are internationally significant. A total of 17 species of sea snakes in the subfamily Aipysurini have been recorded in the reserves—the greatest number of the species recorded at any location worldwide. Three of these species were endemic (uniquely evolved to fit a specific biome niche) to Australia's North West Shelf, however two of the three are believed to be locally extinct. The third, the short nosed sea snake (Aipysurus apraefrontalis) was also presumed locally extinct, and then rediscovered in 2021. While historically the reef boasted large numbers of sea snakes, heavy declines occurred in the 2000s.

Providing a base for both foraging and breeding, Ashmore Reef is crucial for sea turtles, and supports a large and significant population of endangered loggerhead (Caretta caretta), green (Celonia mydas), and hawksbill (Eretmochelys imbricate) sea turtles. These individuals, present year-round, may spend decades foraging in the reserves before migrating up to 1,243 miles (2,000 kilometers) to other reefs for nesting. Adult sea turtles that feed elsewhere as juveniles return to the reserves for breeding and nesting. Undisturbed reef flats and sandy beaches are critical in supporting these populations and ensuring reproductive success in perpetuity. The small population of dugongs on Ashmore Reef is thought to have a range that extends to Cartier Island and to some of the submerged shoals in the region that support seagrass beds.

The Ashmore islands provide important nesting habitat for seabirds and migratory shorebirds. Despite the smallness of the islands, Ashmore supports some of the most important seabird rookeries on the Australian North West Shelf; it is a staging point for migratory wetland birds, especially waders. A total of 78 species have been recorded at Ashmore, of which 17 have been recorded breeding. Among these are 35 species that are noted in international agreements between the Australian government and the governments of China, Japan, and the Republic of Korea concerning the conservation of migratory birds and their habitats.

Fisheries and Stresses

The reserves are within an area subject to a Memorandum of Understanding (MOU) between Indonesia and Australia, which allows for continued Indonesian traditional fishing activities in an area referred to as the MOU Box. Fishing on this scale has been of mixed impact to the environment of Ashmore and Cartier, but it is seen as a component in the sustainable future of the species that depend on the habitat here.

The reserves were created by the Commonwealth of Australia to protect biologically diverse marine ecosystems within Australia's North West Shelf and to facilitate scientific research. Established in 1983 and 2000 respectively, the Ashmore and the Cartier Reserves protect an important genetic resource for the region, while the MOU underpins vital economic needs of many Indonesian subsistence and small-scale commercial fishers.

Global climate change is considered a factor in future planning for the protection of these habitats, and is being analyzed along with El Niño events for effects on the reef systems here. Two bleaching events, in 1998 and 2003, affected the coral reefs at Ashmore by the action of higher-than-normal water temperatures. However, the reefs rapidly recovered, with a tripling of hard corals and a doubling of soft coral covering during the 2005–09 survey period, according to an Australian government team. The survey cited the relatively light impact of human activities in the reserves, and the particular success of the fast-growing Acropora and Pocillopora coral genera. While the survey found a shift in dominance among the coral species, the habitat base seemed to have been quite well preserved over the course of these events. Rising temperatures have concerned scientists, and researchers continue to closely monitor the reef. High sea temperatures from 2020 to 2024 left the reefs intact and unscathed, however scientist warned that continued increased temperatures could be catastrophic.

Bibliography

Berry, P. F., editor. Marine Faunal Surveys of Ashmore Reef and Cartier Island. Western Australia Museum, 1993.

Bustos, Mark. "Ashmore Reef Marine Park in Australia Navigated, Mapped in Landmark Study." The Science Times. 11 May 2021, www.sciencetimes.com/articles/31106/20210511/ashmore-reef-marine-park-australia-navigated-mapped-landmark-study.htm. Accessed 23 Oct. 2024.

Ceccarelli, Daniela, et al. “Rapid Increase in Coral Cover on an Isolated Coral Reef, the Ashmore Reef, North-Western Australia.” Marine and Freshwater Research, vol. 62, no. 10, 2011.

Commonwealth of Australia. Ashmore Reef National Nature Reserve and Cartier Island Marine Reserve (Commonwealth Waters) Management Plans. Environment Australia, 2002.

Lukoschek, Vimoksalehi, et al. “Enigmatic Declines of Australia’s Sea Snakes from a Biodiversity Hotspot.” Biological Conservation, vol. 166, Oct. 2013, pp. 191–202. EBSCOhost, doi.org/10.1016/j.biocon.2013.07.004. Accessed 23 Oct. 2024.

Mills, Vanessa. “Storms Bring Heat Reprieve to Ashmore and Scott Reefs But Scientists Fear for Long-Term Future.” ABC News, 28 May 2024, www.abc.net.au/news/2024-05-29/storms-bring-heat-reprieve-to-ashmore-and-scott-reefs/103901632. Accessed 23 Oct. 2024.

Smith, L., et al. Survey 2000: Beche-de-mer and Trochus Populations at Ashmore Reef. Australian Institute of Marine Science, 2000.