Academic literature on the topic 'Flatback turtle (Natator depressus)'

Cape Domett, located in tropical Western Australia, supports a significant population of flatback turtles, Natator depressus, but the magnitude of this was previously underestimated. We assessed temporal nesting abundance to find that Cape Domett supports one of the largest aggregated nesting flatback turtle populations globally with annual abundance in the order of several thousand individuals (estimated = 3250, 95% CI = 1431–7757). We assessed temporal abundance within a year to find turtles nested throughout the year with peak nesting occurring between August and September. This paper re-evaluates the importance of this major flatback turtle nesting site and shows the first detailed examination of flatback turtle nesting biology in north-western Australia.

The ability of individuals to actively control their movements, especially during the early life stages, can significantly influence the distribution of their population. Most marine turtle species develop oceanic foraging habitats during different life stages. However, flatback turtles ( Natator depressus ) are endemic to Australia and are the only marine turtle species with an exclusive neritic development. To explain the lack of oceanic dispersal of this species, we predicted the dispersal of post-hatchlings in the Great Barrier Reef (GBR), Australia, using oceanographic advection-dispersal models. We included directional swimming in our models and calibrated them against the observed distribution of post-hatchling and adult turtles. We simulated the dispersal of green and loggerhead turtles since they also breed in the same region. Our study suggests that the neritic distribution of flatback post-hatchlings is favoured by the inshore distribution of nesting beaches, the local water circulation and directional swimming during their early dispersal. This combination of factors is important because, under the conditions tested, if flatback post-hatchlings were entirely passively transported, they would be advected into oceanic habitats after 40 days. Our results reinforce the importance of oceanography and directional swimming in the early life stages and their influence on the distribution of a marine turtle species.

Herbivorous turtle, Chelonia mydas, inhabiting the south China Sea and breeding in Peninsular Malaysia, and Natator depressus, a carnivorous turtle inhabiting the Great Barrier Reef and breeding at Curtis Island in Queensland, Australia, differ both in diet and life history. Analysis of plasma metabolites levels and six sex steroid hormones during the peak of their nesting season in both species showed hormonal and metabolite variations. When compared with results from other studies progesterone levels were the highest whereas dihydrotestosterone was the plasma steroid hormone present at the lowest concentration in both C. mydas and N. depressus plasma. Interestingly, oestrone was observed at relatively high concentrations in comparison to oestradiol levels recorded in previous studies suggesting that it plays a significant role in nesting turtles. Also, hormonal correlations between the studied species indicate unique physiological interactions during nesting. Pearson correlation analysis showed that in N. depressus the time of oviposition was associated with elevations in both plasma corticosterone and oestrone levels. Therefore, we conclude that corticosterone and oestrone may influence nesting behaviour and physiology in N. depressus. To summarise, these two nesting turtle species can be distinguished based on the hormonal profile of oestrone, progesterone, and testosterone using discriminant analysis.

The lack of accurate distribution maps and reliable abundance estimates for marine species can limit the ability of managers to design scale-appropriate management measures for a stock or population. Here, we tested the utility of aerial photogrammetry for conducting large-scale surveys of nesting marine turtles at remote locations, with a focus on the flatback turtle (Natator depressus) in the Pilbara region of Western Australia. Aerial surveys were conducted between 29 November and 6 December 2016 to overlap with the peak nesting season for flatback turtles and collected imagery was used to examine marine turtle distribution, abundance, and cumulative exposure to industrial activity relative to overlap with protected areas. Two observers independently reviewed aerial georeferenced photographs of 644 beaches and recorded turtle tracks and other evidence of turtle nesting activity. A total of 375 beaches showed signs of nesting activity by either flatback, green (Chelonia mydas) or hawksbill (Eretmochelys imbricata) turtles. Most of these beaches (85.3%) were located on islands, and the rest (14.7%) on the mainland. Half (n = 174) of the active beaches showed evidence of fresh (0–36 h. old) flatback nesting activity, with track abundance varying from 1.0 to 222.0 tracks·night−1. Six rookeries accounted for 62% of the Pilbara flatback stock. Remarkably, 77% of identified flatback rookeries occurred within protected areas. However, one-third (34%) of those were also located within 5 km of a major industrial site, including eight of the highest abundance beaches (50–250 tracks·night−1). Several key rookeries were also identified as being relatively unexposed to industry-related pressures but currently unprotected, highlighting the need for a cumulative impact assessment to be completed for this flatback stock. Finally, our aerial tallies and multiple ground-survey flatback track tallies were highly correlated and together with low intra- and inter-observer errors suggested that reliable data can be collected via aerial photogrammetry for nesting marine turtles. Such large-scale digitized surveys can therefore be used to assess the cumulative exposure of marine turtles to pressures, and to reveal new conservation opportunities.

Nest site selection is likely to be important for the fitness of sea turtle populations, but data on the environmental drivers of nest site selection and other important parameters like nest site fidelity and inter-nesting and remigration intervals are limited. We address these questions using data on flipper tag resightings and track counts from flatback turtles (Natator depressus) from Delambre Island in Western Australia collected over 2–3 weeks each nesting season across six nesting seasons. The median inter-nesting interval was 13 days (range 9–17 days) and the mean±s.d. remigration interval was 1.99±0.95 years. Turtles had around 10% probability of returning to the same sector of the beach (150-m-long sections). The median distance between subsequent emergences (whether false crawls were included or not) was ~450m. The number of turtles both emerging and successfully nesting was higher when air temperature and humidity were lowest and emergences increased slightly with tide height. Sector of the beach was by far the strongest predictor of nest site, with turtles showing preference for the less exposed side of the island. The results of this study will assist with future monitoring of this population and the management of threats related to coastal development and activities.

Two styles of plastic tags were used on flatback sea turtles (Natator depressus). In parallel with conventional metal flipper tagging, and internal PIT tagging methodologies, a total of 476 plastic tags were applied to 428 individual females in the 1991, 1992, 1993 and 1998 nesting seasons at Wild Duck Island, Queensland. Data are reported up to and including the 2000 nesting season demonstrating that plastic tags are inadequate for the long-term identification of these turtles, with failures of approximately 80% within three years of application.

To address a major knowledge gap for flatback sea turtles (Natator depressus), a species endemic to Australia and considered ‘Data Deficient’ for IUCN Red List assessment, we present the first-ever skeletochronology-derived age and growth rate estimates for this species. Using a rare collection of bone samples gathered from across northern Australia, we applied skeletochronology and characterized the length-at-age relationship, established baseline growth rates from the hatchling to adult life stages, and produced empirical estimates of age-at- and size-at-sexual-maturation (ASM, SSM). We analyzed humeri from 74 flatback sea turtles ranging in body size from 6.0–96.0 cm curved carapace length (CCL), and recovered from Western Australia (n = 48), Eastern Australia (n = 13), central Australia (n = 8; Northern Territory n = 3, the Gulf of Carpentaria n = 5), and unknown locations (n = 5). We identified the onset of sexual maturity for 29 turtles, based on rapprochement growth patterns in the bones. Estimates for ASM ranged from 12.0 to 23.0 years (mean: 16.3 ± 0.53 SE), SSM ranged from 76.1 to 94.0 cm CCL (mean: 84.9 ± 0.90 SE), and maximum observed reproductive longevity was 31 years for a 45-year old male flatback. Growth was modeled as a smoothing spline fit to the size-at-age relationship and at the mean SSM (84.9 cm CCL) corresponded with a spline-predicted maturity age of 18 years (95% CI: 16 to 24), while mean nesting sizes reported in the literature (86.4 to 94 cm CCL) corresponded to estimated ages of 24+ years. A bootstrapped von Bertalanffy growth model was also applied and showed consistencies with the spline curve, yielding an estimated upper size limit, Linf, at 89.2 ± 0.04 cm (95% CI: 85.5 to 95.9 cm) with the intrinsic growth rate parameter, k, at 0.185 ± 0.0004 (0.16 to 0.22); at the same mean SSM (84.9 cm CCL) the estimated ASM was 16.3 ± 0.05 years (95% CI: 12.8 to 27.7 years). Lastly, four of the samples analyzed were collected from deceased adult females that had previous sizes known from on-going mark/recapture studies at nesting sites in Western Australia. The paired CCL data (measured at nesting and back-calculated) did not significantly differ (p = 0.875). This first skeletochronology study for flatback sea turtles generates valuable empirical estimates for ongoing conservation and management efforts.

Flatback turtles (Natator depressus) are the only marine turtle endemic to Australia, occurring exclusively in turbid shelf waters and unlike other species of marine turtle, flatback turtles are considerably understudied. Understanding the diving behaviour and ecology of flatback turtles is important because they are the only species of marine turtle without a pelagic oceanic stage, and their limited geographic range means they are more susceptible to anthropogenic threats. In particular, understanding the diving behaviour during the inter-nesting interval is important as known inter-nesting areas often overlap with pressures from industry and coastal development. Flatback turtles (n=5) were equipped with Daily Diary (DD) multi-sensor tags that recorded data from movement sensors including an accelerometer, magnetometer and gyroscope as well as recording pressure (as a measure of depth), global positioning system (GPS) surface locations, speed and temperature at a temporal frequency of 50 Hz. High resolution data collected using multi-sensor tags were used to classify the diving behaviour of flatback turtles using unsupervised k-means clustering into k= 5 clusters, irrespective of dive depth. Here, the results from clustering and dead-reckoning identified that flatback turtles perform a combination of active and inactive dives in the initial days of the internesting interval, putatively performing a range of behaviours from resting to foraging, travelling and exploration. Notably, recreating the underwater movement paths of dives performed by flatback turtles using dead-reckoning revealed a single dive profile can have different functions associated. These results demonstrate that classifying the data collected using multi-sensor tags provide the opportunity to objectively quantify the diving behaviour of inter-nesting flatback turtles, and the activity budgets associated throughout the internesting interval.

During the Australian summers of 1995/1996, 1996/1997 and 1997/1998, the embryonic development of Natator depressus was investigated in the laboratory and in natural nests at the major eastern Australian rookery of Peak Island.

Eggs were incubated under different thermal and hydric conditions on vermiculite substrates. No eggs hatched at 25°C, but eggs incubated between 26 -33°C hatched successfully. Within this range, the thermal environment significantly influenced the water exchange of eggs, incubation duration, nutrient mobilisation of embryos, hatching size and energy reserves. The pivotal temperature for sex determination in this population was close to 29.5°C with a possibly narrow transitional temperature range of 1 Celsius degree. Sexually biased differences were observed at hatching; male hatchlings produced at 26°C and 29°C were larger, but had less energy reserves than females which were produced at 32°C. Thermosensitive developmental stages at masculinising temperatures were different to those at feminising temperatures. For the 26 to 32°C temperature shift, the thermosensitive period was confined to a single developmental stage. Determination of ovaries took place at a later stage than that of testes.

The influence of the hydric environment depended greatly on the range of substrate water potentials used in experiments. Nutrient mobilisation of embryos, size and energy reserves of hatchlings were dependent on total egg water exchange over the range of 2% gain to 29% loss (at ~ -180 to -3500 kPa) of initial egg weight, but independent within the narrower range of 6% gain to 19% loss (at ~ -200 to -650 kPa). Hatching was affected only when eggs lost more than 21% of their initial egg weight (at ~ -1300 kPa). The pivotal temperature for sex determination was not influenced by the hydric environment (~ -180 to -2000 kPa).

A significant effect of clutch on morphological and physiological aspects of developing embryos indicated that genetic/maternal factors influenced these traits. Dietary sources of the female possibly contribute to maternal factors, through processes such as preferential accumulation of specific fatty acids such as oleic acid into the egg yolk. A high proportion of egg yolk lipids (35%) suggests considerable maternal investment. Only 26% of these lipids were used for embryogenesis whereas 74% remained in the form of hatchling fat bodies or residual material in the yolk sac.

Eggs in natural nests incubated over a temperature range of 25.5 to 36.5°C and experienced an average increase in temperature of 7 Celsius degrees during incubation. Water content of sand surrounding nests at the beginning of incubation varied from 2.6 to 7.8%. Hatching and emergence success were not influenced by the position of the nest on the beach, but were positively related to clutch size. Neither clutch size nor hatching and emergence success varied significantly between subsequent clutches of a female. Seasonal changes in the sex ratio of hatchlings are likely to take place at Peak Island, with an overall female biased sex ratio.

A tolerance to high incubation temperatures and severe moisture stress by N. depressus eggs may be reflected in the short incubation duration of this species relative to other species of sea turtles.

An increased global demand for natural resources has driven a recent expansion in Western Australia's industry resource sector, notably within the North West Shelf (NWS) region. This demand has increased industry resource activities both offshore e.g. exploration, drilling, production, and nearshore of the NWS's coastal boundary e.g. dredging, construction, underwater blasting. Elsewhere, these activities are known to present a threat to marine turtles and there is a potential for the expanding NWS industry resource sector to present a threat and risk of impact to flatback turtles that are known to occur within the same region. Threats posed to flatback turtles by developments and activities associated with the industry resource sector are managed through the Environmental Impact Assessment (EIA) process. The process includes two important phases: a screening/referral exercise that considers the potential presence of protected species within the development's footprint and determines the subsequent scale of the EIA; and an Environmental Scoping Document (ESD) which includes a risk-assessment process that helps inform the need and design of control measures required to remove or reduce the risk of impact to a species from a particular activity. To be effective, both phases require baseline species information and prior knowledge gained from follow-up case studies involving the species and a similar activity. For flatback turtles and proposed industry resource sector developments/activities on the NWS, there are knowledge gaps that may prevent effective screening/referral and ESD phases, potentially resulting in an insufficient level of protection during construction or operation. This thesis has therefore been applied in nature to address these gaps and contribute information and knowledge that can be applied during the different EIA phases outlined above and ultimately to contribute to the conservation of flatback turtles within the region. My first objective was to identify the baseline spatial movement and distribution of flatback turtles on the NWS and determine the extent of the industry resource sector threat by investigating their potential for interaction during different life phases. To achieve this objective, I used data from satellite tracking units that were attached to nesting flatback turtles at multiple rookeries on the NWS to investigate their movements and behaviour during their inter-nesting (Chapter 2) and subsequent post-nesting foraging (Chapter 3) life phases. I undertook a broad scale assessment of the potential likelihood for interaction and threat from the industry resource sector by identifying their overlap with areas that have the potential to host activities associated with the industry resource sector in the region. I found differences between rookeries with regards to the extent of the threat from the industry resource sector. Flatback turtles tracked from offshore islands (Thevenard and Barrow) demonstrated the largest overlap of their inter-nesting home range and time with areas that have the potential to host industry resource sector activities. Extended inter-nesting movements from these offshore islands to the coastline close to the mainland also increased their exposure to current and planned major resource developments. I found no overlap of inter-nesting home range areas and time with areas that have the potential to host industry resource sector activities for turtles tracked from mainland rookeries (Mundabullangana and Port Hedland). Following the completion of their inter-nesting phase, I further investigated their movements, behaviour and likelihood of interaction with the industry resource sector during their foraging life phase (Chapter 3). I found that foraging areas were broadly dispersed across the region, with the furthest foraging area situated 2511 km from the original nesting site (Port Hedland) within the Gulf of Carpentaria in Queensland state waters. I delineated five main areas of concentrated foraging use. I recorded an overlap of habitat use by flatback turtles from multiple rookeries within the same RMU for the first time, with some individual foraging areas utilised by flatback turtles tracked from rookeries of different origin. I considered that during the flatback turtle foraging life phase, the extent of the threat from the industry resource sector was lower compared to their inter-nesting life phase. Nearly half of their foraging areas were situated within an existing protected area and there was a smaller overlap of their home range areas with petroleum title areas when foraging. Their behaviour appeared more flexible when foraging compared to inter-nesting, showing low site fidelity and moving between multiple areas distributed across a broad area. My second objective involved investigating the environmental variables that influenced flatback turtle distribution during their inter-nesting life phase and generating a habitat suitability model to identify areas of the NWS where flatback turtles may be present and specific areas where they have the highest likelihood of impact from industry resource sector activities. I used an ensemble ecological niche-modelling approach to identify the environmental variables that influenced inter-nesting flatback turtle distribution across the NWS study area (Chapter 4). Inputs into the model included selected environmental variables and flatback turtle presence data based on inter-nesting tracking positions from multiple rookeries in the region. Outputs of the model included the importance of each variable and a regional flatback turtle inter-nesting habitat suitability map. I compared the inter-nesting habitat suitability map with a cumulative resource sector impact layer to produce a regional risk map and identify specific inter-nesting areas with the highest likelihood of interaction across the region. I found the primary environmental variables that influenced flatback turtle inter-nesting distribution were bathymetry, distance from coastline and sea surface temperature. The habitat suitability map demonstrated areas of inter-nesting habitat in close proximity to many known flatback turtle rookeries across the region. I found areas of suitable inter-nesting habitat overlapped spatially with resource sector impact areas in close proximity to nearly all known flatback rookeries within the NWS study area, with notable overlaps of highly suitable habitat with areas of high cumulative impact in areas offshore from the Gorgon Liquefied Natural Gas (LNG) development at Barrow Island and the existing port at Port Hedland. My third objective was to contribute an EIA follow-up case study by evaluating the predicted vs. actual consequence of the Gorgon LNG dredging operation at Barrow Island to inter-nesting flatback turtles. I also considered the suitability of implemented control measures by comparing flatback turtle movement and behaviour at different phases of the dredging operation and combining this with actual survivorship data as represented by injury/mortality observations recorded by onboard Marine Fauna Observers (MFOs). To achieve this objective, I attached satellite tracking units and time-depth recorders to nesting flatback turtles at different phases of the Gorgon LNG dredging operation: before (baseline), during (dredging) and after (post-dredging). I compared specific inter-nesting movement and behavioural characteristics recorded during each of these dredging phases and reviewed the observation records of onboard MFOs. I found that during the active dredging operation, flatback turtles had a substantially higher use of the dredging areas compared to the baseline and post-dredging phases (Chapter 5). During the dredging operation, they used the areas being dredged to undertake longer and deeper dives compared to baseline and post-dredging phases, utilising the now deeper and highly turbid waters of the dredging areas. Despite their increase in time spent within the active dredging areas and subsequent increase in potential exposure to entrainment or vessel strike, no events of injury or mortality were detected by the onboard MFOs. I considered that the implemented control measures may have been effective in preventing their injury or mortality, however, based on the results showing that turtles remained within the active dredging areas, the spatial scale of the control measures' effectiveness in deterring turtles from the area may be smaller and less effective than first anticipated. I further reviewed the potential drivers behind their increased use of the dredging areas during the active dredging operation. The most likely driver was considered to be a combination of the increase in turbidity and acoustic noise within the dredging area; potentially resulting in an area that was predator-free and reduced the likelihood of predator detection. This thesis demonstrates that the expanding industry resource sector provides a risk of impact to NWS flatback turtles when inter-nesting and foraging offshore, though any realised impact from this threat is likely dependent on the scale that it is assessed at. At a project-by-project scale, the potential for an individual development or activity to provide a population wide impact to flatback turtles situated offshore is limited due to the existing regulated EIA process and variations in flatback turtle spatio-temporal movement and behaviour characteristics demonstrated at multiple rookeries in this study. However, at a regional scale, the movement and behaviour characteristics, spatial extent of the industry resource sector and limitations within the EIA process for assessing cumulative impact, provides potential for population wide impacts to NWS flatback turtle rookeries from the industry resource sector.

This thesis reports on a study of fungi found in nests of green (Chelonia mydas), loggerhead (Caretta caretta), flatback (Natator depressus) and hawksbill (Eretmochelys imbricata) sea turtle nests at Heron Is., Wreck Is., Mon Repos, Peak Is. and Milman Is., eastern Australia, examined during the 1995/96-1998/99 nesting seasons. Egg mortality and fungal colonisation of eggs were significantly greater in loggerhead turtle nests at Heron Is. and Wreck Is. than in green turtle nests at the same two rookeries, and also greater than in loggerhead, green, flatback and hawksbill turtle nests at other rookeries. Three species of fungi, Fusarium oxysporum, Fusarium solani and Pseudallescheria boydii were frequently isolated from failed eggs in nests of all turtle species at all rookeries. The fungi are all ubiquitous soil species, and probably originated from the nest substrate. However, there was some evidence for acute, intra-seasonal oviductal contamination of eggs with fungi, accumulated in the cloaca and oviduct during nesting behaviour. The Pisonia grandis forest and high seabird density, present only at Heron Is. and Wreck Is. of all the rookeries investigated, did not appear to harbour fungi colonising sea turtle eggs at these locations. Within the sea turtle nest, fungi first appeared on an egg that had failed from other (natural) causes. Spores and hyphal fragments in the sand are likely to be disturbed during the nesting process and may settle on the exterior of sea turtle eggs. Experiments showed that the infection of viable eggs of all sea turtle species with fungi is possibly inhibited by the anti-fungal properties of mucus secreted during oviposition, egg albumen and the dense ultra-structure of the eggshell. However, if an egg dies the lytic enzymes and acids produced by F. oxysporum, F. solani and P. boydii could allow penetration of the eggshell and utilisation of egg contents. Using this nutrient source, hyphae could then expand to adjacent, viable eggs. The linear growth rate of all fungi varied with the thermal and hydric conditions during incubation. Embryo mortality as hyphae spread across a viable egg is probably due to inhibition of the respiratory surface area or possibly to calcium deprivation. Analyses of hatchlings from nests with different levels of fungal colonisation suggested that hatchlings emerging from nests that have a high percentage of eggs colonised by fungi should have a similar fitness to those from nests without fungi. Any nest characteristic that enhances egg failure, particularly early in incubation, will markedly accelerate fungal invasion of the sea turtle nest. Each additional failed egg greatly increases the likelihood of a focus of growth being available for fungi. The increased vulnerability of loggerhead nests at Heron Is. and Wreck Is. to egg failure and subsequent colonisation by fungi is likely to be to due characteristics of the nest. Significantly high substrate conductivity at Heron Is. and Wreck Is. is likely to impose osmotic stress on loggerhead eggs, which are smaller than green turtle eggs in adjacent nests, and result in higher egg mortality. Once fungal invasion of the nest is established, equivalent linear growth of fungus will cover a greater surface of the egg and allow faster access to adjacent eggs in loggerhead turtle nests than green turtle nests. This results in a significantly lower hatch success of loggerhead turtle nests.

The ecological characteristics of dipteran larvae collected from green (Chelonia mydas), loggerhead (Caretta caretta) and flatback (Natator depressus) sea turtle nests at Heron Is., Mon Repos, and Peak Is., Central Queensland, were examined during the 2002/03, 2003/04, and 2004/05 nesting seasons.Investigation of the relationship between nest productivity measures and larval infestation rates was undertaken at both the rookery and individual nest level. The approximate timeframes for nest infestation were investigated for the three dominant nest invaders. Experiments examining the predisposition of nests to infestation suggested that the most important factor was the number of dead embryos and hatchlings in a nest, in support of the major finding that dipteran larvae were scavengers.

Sea turtles are reptiles that have inhabited the earth for 100 million years. These are divided into 2 families (Cheloniidae and Dermochelyidae) and 7 species of sea turtles in the world: the leatherback turtle (Dermochelys coriacea); hawksbill turtle (Eretmochelys imbricata); Kemp’s ridley (Lepidochelys kempii); olive ridley (L. olivacea); Loggerhead turtle (Caretta caretta); flatback sea turtle (Natator depressus) and green turtle (Chelonia mydas). In particular, Kemp’s ridley is included in the red list of IUCN categorized as “critically endangered”. The most important site around the Word is in Rancho Nuevo, Tamaulipas, Mexico. Where 80–95% of the world’s nesting is concentrated. Other nesting areas are Tepeguajes and Barra del Tordo, in Tamaulipas, and with less intensity in Veracruz (Lechuguillas and El Raudal beaches) and South Padre Island, Texas, USA. They deposit an average of about 90 eggs and hatching takes 40 to 60 days. Therefore, they are vulnerable to different anthropogenic activities and sources of pollution, such as heavy metals, which can cause toxic effects that are harmful to the turtles, damage their physiology and health. To understand the real situation about health and genetic parameters it is necessary to analyze biochemical and molecular factors in this species.