Raine Island - Part 1: Raine Island - the biggest green turtle rookery left in the world.
Marine turtles have been around for more than 120 million years reaching back to the late Triassic period and the time of the dinosaurs. The modern day species Chelonia mydas, the green turtle, has evolved into a highly successful seagrass grazer with enormous 'herds' of 'tens of thousands' existing right up into recorded history. It was said that mariners could be sure they were heading in the right direction if the bumps on the hull of their vessels from migrating turtles remained consistent (Jackson 1997).
However these populations have declined alarmingly over the past several hundred years (Jackson, 1997) making sites like Raine Island all the more important with every passing year and its role as a potential producer of 100's of thousands maybe over a million hatchlings in a year can hardly be overstated. This population, the northern Great Barrier Reef genetic stock, has somehow 'decided' that it is best that they focus all their effort at this one small cay (along with nearby Moulter Cay) rather than spread more widely across the region. Just why it has become a focus of such heavy nesting activity remains a mystery. Its position at a northern entrance to the reef (with water dropping off precipitously to over 350m just off the island edge may play a part.
It is estimated that Raine Island has been a significant turtle rookery for at least 1100 years (Limpus 2008). Despite recent concerns that the island may be losing sand, and that this may be part of the problem with incubating eggs, it appears that the island has actually accumulated approximately 68 000m³ of sand over the past 40 years (Dawson and Smithers 2010). The island is subject to the prevailing south-east trade-winds (April – October) and occasional cyclonic conditions during the monsoon season (November – March) resulting in seasonal shifts in beach sand profiles.
Every year from late October to February egg laden females having migrated from many parts of northern Australia, eastern Indonesia, Papua New Guinea and as far away as Vanuatu and New Caledonia (a migration of over 2700km) crawl up the beach at night and attempt to lay their approximately 100 eggs. They are generally expected do this 4-6 times in a nesting season and then not return again for the next two to six years depending in large part on how well they have been able to build up fat reserves back at seagrass feeding grounds. Cyclones, El Nino/La Nina weather patterns and coastal water quality all impact on seagrass quality and therefore how often green turtles reproduce (Limpus 2000, Limpus 2008).
Once successfully laid, clutches of eggs generally take around 60 days to incubate before the hatchlings, as a group, emerge from the sand and scurry down the beach to the sea where they have to dodge crabs, birds and fish before they can make it over the reef flat and into the relative safety of deeper water.
The island and its turtles, although perhaps accessed fairly regularly by maritime Aboriginal and Torres Strait Islander groups (Raine Island Management Statement, QLD Government 2006) were probably relatively untouched until the mid-nineteenth century. Impacts to the nesting turtle and seabird populations have been far greater from European use of the island, first as the site of a navigation beacon constructed in 1844 from phosphate rock cut from the island and then as a guano mine (1890 to 1892). Accounts from this period describe sailors arriving at the island and crushing all the seabird eggs they can see so they would know that anything they collected in the coming days would be fresh (Stoddart et. al. 1981). Turtles were also an important food source at this time.
In late 1958 a sea turtle harvesting business was set up by the Whittakers from Cairns (Limpus et. al. 2003). This was following a change to the Queensland Fisheries Act that meant turtles could be harvested without restriction north of 15º in Queensland. During the 1958-59 nesting season at Raine Island approximately 1200 turtles were collected, butchered and offered for sale through Cairns. The harvest did not continue beyond this one season as it was not profitable with only small volumes of the meat sold (Limpus et. al. 2003).
By 1973 the Queensland government's interest in the island had shifted significantly from a source of income to one of conservation and the first systematic study of the rookery occurred in the 1974/75 nesting season. Within a few years data was showing that this was a site of major international importance for a number of species and in particular exceeded in size other green turtle rookeries such as Tortuguero made famous by Dr Archie Carr in his book 'So Excellent a Fishe'. By 1981 when the Great Barrier Reef was declared a World Heritage Area Raine Island was specifically mentioned under Criterion (X) for its 'outstanding universal value' from the point of view of science and conservation - see https://whc.unesco.org/en/list/154
In total the island has approximately 50 000m² of nesting beach with a central depression that during the wet season is well vegetated by shrubs and grasses (there are no trees on the island) and populated by thousands of nesting seabirds with 16 breeding populations including masked and brown boobies, lesser frigatebirds, red-tailed tropicbirds and the endangered herald petrel. This seabird nesting area is largely protected from wandering sea turtles by a low (approximately 1 - 1.5M) phosphate rock cliff formed by phosphates leaching from avian guano cementing sediments together (Dawson and Smithers 2010). The sand of the island is made up primarily of Foraminifera tests (43%), mollusc fragments (25%) and coralline algae and fine coral rubble (21%) with this supply being generated from the 210 hectare reef flat to the south-east of the cay (Dawson and Smithers 2010).
Foram based sand binds poorly and dries quickly. Not a good combination if you are a turtle trying to dig a 50cm deep egg-chamber with your hind flippers in the dark, out of your natural watery home, and while constantly being jostled by fellow females. Rates of nesting failure are high, particularly when the sand is dry and females expend enormous amount of energy just trying to get their eggs safely into a nest and covered. These eggs then have to survive approximately 8-9 weeks in the sand and not get dug up by another female and not succumb to other identified risks (discussed below).
The island is spectacular, of that there is no doubt, but it is also at significant risk. Sea level rise and climate change is predicted to have an increasing impact on these low-lying cays (Fuentes et. al. 2010) and there is already concern that seabird nesting numbers are suffering from irregular fish supplies at their off-shore feeding areas (Congdon et. al. 2007). In particular there is clearly something wrong with green turtle hatchling production (QPWS Internal Report 2013).
There is real concern that Raine Island's future as a centrepiece of the Great Barrier Reef's World Heritage listing is in doubt. Major management actions are now underway in an attempt to tackle this and they are discussed in Part Two.
A summation of the biology of the turtles of the island and its human use can be found at https://parks.des.qld.gov.au/parks/raine-island