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Study site. The northern end of Dirk Hartog Island National Park (25°29’S, 112°59’E) is located in the Shark Bay Marine Park and the World Heritage Area. This protected area is an important nesting habitat for the majority of loggerheads from the Western Australian population. The nesting season extends from November into April (Prince 1994), with the peak of the nesting season occurring in mid- January (Baldwin et al. 2003). Occasional turtle tracks are observed throughout the year (Bob Prince, pers. comm. 2011). Green turtle, Chelonia mydas, tracks have also been occasionally recorded on Dirk Hartog Island (2008/09, 2009/10 and 2011/12).
The rookery is composed of five nesting beaches, interspersed with areas of rocky shore (Fig. 1). The nesting habitat used in this study measured 2.1 km in length. This was a normal length of sand compared with other seasons, but sometimes, whole beaches are stripped of sand during cyclones, building back before the following season. Loggerhead nesting on Dirk Hartog Island is not restricted to these beaches. Other beaches extend south and west of these five survey sites, with southern low-density nesting extending beyond Cape Levillain, as well as intermittently westward around to the lighthouse precinct at Cape Inscription, on other beaches of Dirk Hartog Island and on the mainland to the south of the study site, at Steep Point (Pam Dickenson, pers. comm. 2011).
Figure 1. Loggerhead nesting beaches on the northern end of Dirk Hartog Island.
Track counts are the total number of tracks, including both successful and unsuccessful nesting attempts. From 6-18 January 2011, nesting turtle tracks were counted at sunrise each day by foot or quadbike (all-terrain vehicle-ATV) across the five study site beaches. The new tracks were crossed each day either by ATV tire tracks (Beach 5), or by a drag (a looped rope attached to a length of chain spaced by a stick) pulled behind the person counting on foot (Beaches 1, 2, 3 and 4). The lower-density adjacent beaches to the southeast and west were also counted for tracks. This pilot study, of counting daily nesting attempts, ran alongside the annual mark-recapture program (Prince 2000). As part of the tagging program during the years before and after this study, turtles were tagged regardless of whether they had finished laying their eggs. This meant that turtles often had to emerge more than once because their first nesting attempt was aborted due to human disturbance, thus biasing any track counts. To obtain natural track counts for the one season of this pilot study, the tagging teams had an added stipulation not to disturb any turtles for tagging unless they had laid their eggs, or were already heading back to the water. Tagging teams also took care not to disturb neighboring turtles.
Nesting success varies with beach geomorphology. On Beach 4, turtles often encounter rocks while digging egg chambers. On any of the beaches, turtles also occasionally disturb each other because of the high density of nesting within the study site. For three of the nights during the study period, undisturbed nesting success was quantified over 1.6 km, encompassing 76% of the main nesting beaches. During these three nights, from 19:30 to 05:30, surveyors were assigned a section of beach (ranging from 50 m to 300 m in length) at which to tally the number of turtles coming ashore and designate if a clutch of eggs was laid in each instance. Nesting success was calculated as the proportion of turtles completing successful oviposition divided by the total number coming ashore for each beach section (Limpus et al. 2003). Quantifying nesting success makes track counts meaningful, as the moisture content and depth of sand over beachrock can change seasonally, affecting the proportion of tracks resulting in successful nests. The measurement of undisturbed nesting success may also become a significant parameter for assessing long-term changes in beach quality.
The nesting success rate was multiplied by the daily track count (total number of nesting attempts, regardless of whether they laid eggs or not) to give the number of clutches laid on each section of beach per night over the 13-day survey. Several beaches differing in topography, sand texture and rockiness were sampled to more accurately match any resulting differences in nesting success with individual beach track count data. Each clutch laid represents one female. The internesting interval for loggerheads is typically approximately 14 days (Dodd 1988). Density parameters were calculated at mid-season (January) (clutches km-1 night-1) to enable comparison with data gathered at other loggerhead rookeries, including high-density beaches of Masirah Island, Oman (Ross 1998). It was beyond the scope of this pilot study to extrapolate the mid-season data to estimate nesting numbers for the entire season.
During the 13-day survey, daily track counts averaged 165 per night (min: 102, max: 253) for the total 2.1 km comprising the five rookery beaches. When the track surveys from the adjacent lower- density beaches to the southeast and west were included, average daily track count increased to 193 (min: 121, max: 289 tracks per night, Table 1).
Table 1. Track count data and nesting success from undisturbed loggerhead turtles emerging onto nesting beaches across the northern end of Dirk Hartog Island, and adjacent beaches from Cape Inscription to Sammys Beach. For each beach, the table lists the average number of nesting attempts (tracks) per night, the length of beach having sandy access that was available for nesting, the section length that was sampled for nesting success, and the number of turtles that entered the nesting success sectors during the night to be tallied for clutch deposition.
To derive an estimate of nesting success, 125 (8%) out of the total 1590 tracks on the five beaches were sampled for successful egg laying. Out of the sample size of 125 turtles, 92 turtles laid and 33 did not lay, giving a nesting success rate of 73.6% for the rookery (Table 1). Loggerhead nesting success was highest (87.5%) at the open sandy stretch of Beach 1, and lowest (30%) on Beach 4.
A chi-square test identified significant differences in nesting success between beaches (Beach 1: B4 χ2 = 22.5, df = 1, p = <0.001; B2: B4 χ2 = 15.7, df = 1, p = <0.001; B1: B5 χ2 = 4.0, df = 1, p = 0.04; B4: B5 χ2 = 6.4, df = 1, p = 0.01). Beach-specific nesting success rates were applied when possible.
Of the 125 turtle emergences followed for nesting success, five (4%) turtles had partially dug up an existing nest.
When nesting success was applied to the track count data, there was an average of 122 clutches laid per night for the five study beaches and 143 clutches laid per night for the whole rookery. Limpus (2008) reported mean internesting intervals for east coast Australian loggerheads of 13.9 days (Mon Repos, n = 2,959) and 14.5 days (Heron Island, n = 37). Assuming the Dirk Hartog Island loggerhead internesting period is no less than 13 days, a minimal estimate of 1,853 individual turtles came ashore on Dirk Hartog Island during the 13-night period (Table 1). Approximately 86% (n = 1,590) of these nested on the five study beaches. Each female came ashore an average of 1.4 times before successfully laying a clutch.
Over the survey period, the 2.1 km of study beaches had an overall average density of 58 clutches laid km-1 night-1. Beach 2 had the highest density with 99 clutches km-1 night-1, followed by Beach 1 with 91 clutches km-1 night-1. The 890 m of Beaches 1 and 2 together (Turtle Bay) had a density of 94 clutches km-1 night-1. Together, the Turtle Bay beaches had a track density of 109 tracks km-1 night-1.
This study, although limited to the peak of one loggerhead season due to a coinciding tagging program, has allowed researchers to document and better understand the high-density loggerhead nesting occurring in Western Australia. Anecdotal data (WAMTP unpublished data) from other seasons indicate the 2010/11 nesting season on Dirk Hartog Island was a low year compared to other recent years. No estimates of nesting success were made in those years of greater density. The data from this study support Dirk Hartog Island as the largest loggerhead turtle nesting aggregation in Australia and also within the southern hemisphere. Such high densities are not currently reported for any other loggerhead nesting beach in the world.
Dirk Hartog Island’s 890 m Turtle Bay beaches nightly peak- season loggerhead nesting density of 94 clutches km-1 night-1 is greater than the 63 clutches km-1 night -1 recorded for the densest 350 m beach of the Cape Verde Islands (peak two weeks of 2012 season, Estación Biológica de Doñana, CSIC, Sevilla, Spain, Adolfo Marco, pers. comm. 2013). Densities for other significant loggerhead nesting beaches include 48 clutches km-1 night -1 for Japan’s densest 1,000 m beach (peak two weeks of 2013 season, NPO Yakushima Umigame-kan, Shotaro Koide, pers. comm. 2013) and 29 clutches km-1 night-1 for the densest 810 m section of the Florida coast (peak two weeks of 2012 season, FWC/FWRI Index Nesting Beach Survey Program Database as of 16 Oct 2012, Blair Witherington and Beth Brost, pers. comm. 2013). Dirk Hartog’s peak nesting density is several times the maximum of eight turtles km-1 night-1 recently recorded on Masirah Island, Oman (Mendonça et al. 2010), but is within the range of historical densities in Oman (Ross 1997).
Counts of tracks and nesting success presented by this study have confirmed Dirk Hartog Island as a globally significant high-density rookery for loggerhead sea turtles. Annual monitoring of tracks and nesting success needs to continue, both to enable comparison across years at the rookery, and to give more robust data to contribute to the annual survey of the species. These data should give scientists a better view of the global loggerhead population. The Dirk Hartog rookery is at the cooler end of the nesting range for the species in Western Australia, and is on a relatively remote island reserve, free from the pressures of coastal development or foxes or feral pigs, making it a potential stronghold for the future of the species. Whereas once dense loggerhead rookeries have declined elsewhere (Mendonça et al. 2010), current high densities on Dirk Hartog should be seen for decades to come.
It would also be important to monitor hatchling emergence success as a long-term biological parameter of the rookery. Trocini (2013) reported a hatchling emergence success rate for Dirk Hartog Island in the 2006/07 and 2007/08 seasons of only 49.8% ± 30.3SD (n = 121). This includes 11.6% of nests that were eroded by a cyclone/ storm surge during 2007/08. Of the 121 marked nests monitored throughout their development, 10.7% were at least partially disturbed by other nesting turtles (Trocini 2013). High nesting density may also attribute in part to low hatchling emergence success, through high clutch densities and fungal contamination. Periodic loss and replenishment of beach sand due to natural cycles of cyclone/storm surges and sand deposition, although destroying clutches, may be beneficial to clear the nesting habitat of old nests and associated microbial pathogens.
Acknowledgements. We thank the 18 volunteers and staff that worked all night on the nesting success sectors and did track counts every morning. Colin Limpus gave advice on methodologies in the planning stage of the project. Bob Prince shared his knowledge of earlier work on Western Australian loggerheads and commented on a draft. Holly Smith produced the map. Wayne Moroney and Dave Holley provided logistical support. Karen Hattingh, Kim Friedman, Mark Hamann, Scott Whiting, Bryan Wallace and three anonymous referees commented on drafts. This study was conducted under Regulation 17 permits SC1161 and SC1237 issued by the Western Australian Department of Environment and Conservation.
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