Rec. West. Aust. M"... Suppl. No. 44.1993 Part 1 Historical background, description of the physical environments of Ashmore Reef and eartier Island and notes on exploited species P.F. Berry* Abstract Ashrnore Reef (12°17'S, 123°02'E) and nearby Cartier Island (l2°32'S, 123°33'E) are located on tl.e north-western extremity of the Sahul Shelf. They are approximately 350 km off the Kimberley coast of Australia and 145 km from Roti, Indonesia. The morphology and physical environments of the two reef systems are briefly described as a background to faunal inventories presented in Parts 2-7 ofthis publication. Ashrnore Reef (approximately 26 km long and 14 km wide) is similar in general shape and morphology to other shelf-edge atolls off the north-western coast of Australia, but because of the larger breaks in the reef there is no impounding of water in the lagoon on outgoing tides as at the Rowley Shoals and to a lesser extent, Scott Reef. There are three vegetated islets on Ashmore Reef. Cartier Island, an unvegetated sand cay, is surrounded by an oval-shaped reef platform approximately 4.5 km long by 2.3 km wide. Mean sea surface temperatures range from approximately 24°C in July and August to 30°C between January and March. Spring and neap tidal ranges (semi-diurnal) are 4.7 m and 2.8 m respectively. Observations on species exploited in the traditional Indonesian fishery are recorded. These suggest that composition and abundance of exploited species at Ashmore Reef and Cartier Island reflect a higher level of fishing effort there than at Scott Reef and Rowley Shoals. Introduction Ashmore Reef (12°17'S, 123°02'E) and nearby Cartier Island (12°32'S, 123°33'E) lie on the north-western extremity of the Sahul Shelf 350 km off the Kimberley coast (Figure 1). They are separated from Indonesia (Roti) by the Timor Trough and a distance of approximately 145 km. To the north-east is an extensive and complex system of submerged reefs (the Sahul Banks), some of which are buried in sediment, while others are exposed and reach within 6 m of surface; only Hibernia Reef is emergent. This reef system must once have been comparable to parts of the Great Barrier Reef (Edgerley 1974). To the southwest lies a series ofatolls rising from the continental slope: Seringapatarn Reef, Scott Reef and the Rowley Shoals, for which faunal inventories have been published (Berry 1986). This report describes the results ofa preliminary survey of selected faunal groups at Ashmore Reef and Cartier Island by the Western Australian Museum in September 1986 and also lists the faunas of all these shelf-edge reefs sampled so far. Collections of octocorals and echinoderrns from Ashmore Reef made by the Northern Territory Museum between 1986 and * Western Australian Museum, Francis Street, Perth, Western Australia 6000. Physical enviromnent / N o / A N o c EA N o 100 200 300 I I I I km WE s TER N AUST RA LIA Figure 1 Map ofthe shelf offnorth-western Australia. 1987 have been included. Participants in the survey were: G. Allen (fishes), P. Berry (crustaceans, leader), C. Bryce (molluscs, technical), T. Knight (medical), L. Marsh (echinoderms, cnidarians other than reef-building corals), J. Veron (reef-building corals) and F. Wells (molluscs). Historical Notes Because of their proximity to Roti, Ashmore Reef and to a lesser extent Cartier Island have been for centuries and still are, visited by Indonesian fishermen in traditional sailing prahus. 2 P.E Berry They are also staging posts for longer voyages to other reefs and islands off the Kimberley coast and to the coa<;t itself (MacKnight 1976). The position of Ashmore Reef was reported by Captain S. Ashmore on the Hibernia in 1811 and Great Britain took formal possession of it on behalf of the Western Australian Government in 1878. In the same year it was also claimed on behalf of the United States of America by Captain Webber who referred to it (the three islands) as "the Caller Group" (after his vessel Sadie F. Caller) and a dispute arose as to the rights to mine guano deposits there. However, the United States Government did not support Webber's claim. Carrier Island was annexed by Great Britain in 1909. In 1931 Ashmore Reef and Carrier Island were placed under the authority of the Commonwealth of Australia as an External Territory (the Territory of Ashmore and Carrier Islands) administered until 1938 by Western Australia, then by the Northern Territory until 1978 when control was returned to the Commonwealth. In 1974 a Memorandum of Understanding was signed by the governments of Australia and Indonesia which allowed continuation of certain traditional Indonesian fishing at Ashmore Reef and Carrier Island. In 1983 Ashmore Reef and surrounding waters to approximately the 50 metre depth contour were declared a National Nature Reserve under the National Parks and Wildlife Conservation Act of 1975. The position of Carrier Island was rust reeorded in 1799 by Captain Nash on the vessel Cartier. Little has been published on the marine fauna of Ashmore Reef and Carrier Island. In 1952 Serventy provided a list of birds encountered on his visit in 1949, although he did not land at Carrier Island. The 1972-73 visit by the US Alpha Helix resulted in a small colleetion offishes, deposited at the Scripps Institute of Oceanography and a publication on the seasnake fauna (Dunson 1975). In 1978 the USSR vessel Professor Bogorov visited Ashmore Reef, resulting in an unpublished report on the echinoderm fauna and reeording the presence of four genera of Alcyonacea (Anon. 1979). Attempts to obtain the results of fauna sampling the same year by the USSR vessel Kallisto were unsuccessful. Reports produced by staff of the National Parks and Wildlife Service and Western Australian Department ofFisheries cover mainly birds, turtles, dugongs and seagrasses (e.g. Forbes 1982; Hicks 1982, 1983; Sarti 1983). Physical Environment The climate is monsoonal with a prevailing westerly and northwesterly rainbearing monsoon from November to March and dry southeasterly trade winds from May to September. No rainfall figures are available. Ashmore reef lies to the north of the main belt of tropical cyclones which form in the Timor sea, mostly between January and March and generally move to the south west Between 1975 and 1992 only five passed close to it compared with ten which passed within 10 of Rowley Shoals. Reef morphology indicates that the prevailing swell is from the south rather than from the west or south west as at Rowley Shoals and Scou Reef. The mean spring and neap tidal ranges are 4.7 m and 2.8 m respectively. (Tidal predictions for Ashmore Reef were supplied by the Tidal Laboratory of the Flinders Institute for Atmospheric and Marine Sciences, copyright reserved). Because of the large breaks in the reef there is no impounding of water within the lagoons at Ashmore Reef as occurs at the Rowley Shoals (Berry and Marsh 1986). No details of currents prevailing around Ashmore Reef are available but evidence of large scale transport from the Pacific Ocean through the Indonesian Archipelago into the Indian 3 Physical environment ASHMORE REEF N ... ~ (J/ 12\>1 I .. ... .. ,-.-. ' .. ..... .. .... .. ....• . ... .... 4..... .-.. .. .. ~REEF III INTERTIDAL SAND FLATS o, 5, 10km, [J] SHALLOW LAGOON Flgure 2 Ashmore Reefdrawn from a LANDSAT image, showing localities ofsampling stations. Ocean is provided by Wyrtki (1961), Godfrey and Golding (1981), Godfrey and Ridgeway (1984,1985) and Fine (1985). On the basis of examination of temperature charts from 1984-1986 derived from satellite imagery, the mean weekly sea surface temperature at Ashmore Reef was found to range from approximately 30°C from January to March to approximately 24°C in July and August Reef Morphology Ashmore Reef is a shelf-edge atoll similar in shape and general morphology to the other atolls of the northwest, particularly the Rowley Shoals but also Scott and Seringapatarn Reefs (Berry and Marsh 1986). It differs from these in being orientated east-west instead of north­ south. It also occurs on a platform with an extensive shoal area less than 50 m deep and 12 km wide to its east instead dropping off into deep water on all sides; the 200 m contour lies close to its southern and western sides but the shallow platform on the north and east sides is part of the Sahul Shelf which is generally less than 100 m deep. In addition it has the most extensive areas of sand habitat ofall these atolls. According to Woodroffe (1984) the irregular outline of the reef crest of Ashmore Reef probably reflects the configuration of the underlying Tertiary basement encountered at a depth of 272 m in a drillhole on the reef (Anon. 1968). 4 P.F. Berry The reef (see Figure 2) is approximately 26 km long and 14 km wide (150 sq km). The windward (southern) reef margin reaches a maximum width of approx. 6 km, is unbroken and is backed by extensive sandflats. The leaward (northern) margin is narrow (approx. 1.4 km maximum), and broken by several broad passages which lead into two extensive shallow lagoon basins. There are three low (2.5 m) vegetated supratidal cays: West Island (approximately 1400 x 500 m, 33 ha), Middle Island (approximately 1200 x 400 m, 13 ha) and East Island (approx. 800 x 600 m; 16 ha). Woodroffe (1984) gives an account of the distribution of outcrops of beachrock and adjacent cemented sands and phosphatized boulders occurring on the islands, as well as describing their sedimentary characteristics. He describes the sediments as skeletal carbonate sands, principally composed of fragments of foraminifera tests and mollusc shells.