The Hawksbill turtle, Eretmochelys imbricate, (Linnaeus 1766): ecological insights of a resident population in the Northern Great Barrier Reef, Queensland, Australia

Bell, Ian Philip (2012) The Hawksbill turtle, Eretmochelys imbricate, (Linnaeus 1766): ecological insights of a resident population in the Northern Great Barrier Reef, Queensland, Australia. PhD thesis, James Cook University.

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The life history of Eretmochelys imbricata is complex and until now, there has been a paucity of data describing even fundamental population parameters for the species in the western Pacific Ocean. A baseline understanding of a population‟s structure, its dynamics and general foraging ecology are essential for determining its conservation status, and for developing management strategies if required. Ongoing monitoring is also vital to detect trends in population size, or if a population is responding to conservation management strategies.

In this thesis I provide a detailed description of the biometry, demographics, population dynamics, genetic structure, diet and growth rates of a population of E. imbricata, resident within a collection of reefs in the northern Great Barrier Reef. Eretmochelys imbricata were captured from a group of 13 reefs, which are collectively known as the "Howick Group". The area is remote, relatively undisturbed and falls within the Far Northern Section of the Great Barrier Reef Marine Park. Reef flats were searched using small outboard motor powered dinghies, and when spotted, E. imbricata were hand-captured by jumping from the boat.

I conducted eight annual surveys, of ~ 18 days duration (n = 143, R = 1 - 33; SD = 9.9) during the Austral winter period (June - July) between 1997 and 2008 at the Howick Group. Over this time, 665 E. imbricata were "first-time" captures with 148 turtles being recaptured on at least one other successive survey. This resulted in a total of 813 turtle encounters recorded.

An almost even number of juvenile, pubescent and adult E. imbricata were captured, with a high female to male gender ratio bias displayed across all three age-classes; adult E. imbricata comprised 9.0:1 (n = 243 f; 27 m), pubescent turtles: 4.7:1 (n = 221 f; 49 m) and juveniles displayed the highest proportion of females with a 10.0:1 ratio (n = 99 f; 9 m).

The mean curved carapace length (CCL) for adult female E. imbricata was 84.9 cm (n = 243; R = 74.5 - 97.7 cm; SD = 3.72 cm) and 82.5 cm (n = 27; R = 74.6 - 87.5 cm; SD = 3.2 cm) for males. The somatic growth rate of sub-adult turtles was non-monotonic, however the rate was monotonic and declined to zero growth upon reaching maturity. A Generalised Additive Modelling approach revealed a significant difference (P = 0.02) in growth rates between pubescent males and females. Pubescent males grew an average of 1.17 cm/yr (n = 7) compared to pubescent females, which grew 0.98 cm/year (n = 28). The mean growth rate for juvenile female E. imbricata was 0.32 cm/year (SD = 0.48 cm; R = -0.13 - 1.57 cm; n = 12), whereas the mean annual growth increment for the single juvenile male turtle caught was 0.8 cm/yr.

I used Bayesian Mixed Stock Analysis methodology to identify the genetic structure of turtles feeding in the Howick Group. From these data I determined the natal rookeries. The majority of E. imbricata (87%, 95% CI = 78 - 95%) came from eastern Bismarck-Solomon Sea eco-region natal beaches. Only 11% (95% CI = 2 - 21%) of E. imbricata originated from rookeries within the nGBR (e.g., Milman Island) and possibly the Northern Territory, and two percent from an unknown source.

Annual survival probabilities and population densities were determined using a Cormack-Jolly-Seber capture-mark-recapture model. Marked differences in survival probability between adult male (0.71) and female (0.92) E. imbricata were found. The mean annual population density estimates were consistently greater for adult female E. imbricata (n = 333.7; SD = 135.6; R = 221 - 581) than for adult males (n = 32.4; SD = 33.4; R = 8 - 98), with both adult males and females displaying high survivorship rates (71.1%; 92.2%, respectively). This was also apparent in immature age-classes, with male and female turtles showing similarly high survivorship likelihoods (78.0%; 93.0%, respectively).

I conducted gastric lavage sampling and examined the buccal cavities for prey items selected by 120 individual E. imbricata. A total of 467 gastric lavage and 71 buccal cavity ingesta items were identified. I found that during periods of high-tide, when access to the reef top was possible, turtles were primarily feeding on marine algae from the genera Laurencia and Gelidiella; refuting the commonly held notion that the species are principally spongivorous. During periods of low-tide, their diet principally comprised the same two algae, however a range of vertebrate and invertebrate prey, found typically occurring in a deeper (~ 6 – 8 m) water habitat were also recorded in diet samples. Eretmochelys imbricata were found to show strong fidelity to the reef upon which they were caught, with no movement of animals between reefs being recorded.

These data present an in-depth description of many life history aspects of all age classes of E. imbricata found in the northern Great Barrier Reef, which, until now, were undescribed for the western Pacific. The data, derived from this mark-recapture, genetics, diet and growth rate study, may now be used to develop management strategies to protect northern Great Barrier Reef E. imbricata populations in a rapidly changing marine environment.

Item ID: 28951
Item Type: Thesis (PhD)
Keywords: Great Barrier Reef, GBR; North Queensland; Howick Group; Hawksbill turtle; Eretmochelys imbricate; population structure; population dynamics; life history
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Publications arising from this thesis are available from the Related URLs field. The publications are:

Chapter 4: Bell, Ian, Schwarzkopf, Lin, and Manicom, Carryn (2012) High survivorship of an annually decreasing aggregation of hawksbill turtles, Eretmochelys imbricata, found foraging in the northern Great Barrier Reef. Aquatic Conservation: marine and freshwater ecosystems , 22 (5). pp. 673-682.

Chapter 6: Bell, Ian (2013) Algivory in hawksbill turtles: Eretmochelys imbricata food selection within a foraging area on the Northern Great Barrier Reef. Marine Ecology, 34 (1). pp. 43-55.

Chapter 7: Bell, Ian, and Pike, David A. (2012) Somatic growth rates of hawksbill turtles Eretmochelys imbricata in a northern Great Barrier Reef foraging area. Marine Ecology Progress Series, 446 . pp. 275-283.

Date Deposited: 05 Sep 2013 22:38
FoR Codes: 06 BIOLOGICAL SCIENCES > 0602 Ecology > 060205 Marine and Estuarine Ecology (incl Marine Ichthyology) @ 50%
06 BIOLOGICAL SCIENCES > 0602 Ecology > 060207 Population Ecology @ 50%
SEO Codes: 96 ENVIRONMENT > 9608 Flora, Fauna and Biodiversity > 960808 Marine Flora, Fauna and Biodiversity @ 100%
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