Age-specific growth and maturity estimates for the flatback sea turtle (Natator depressus) by skeletochronology

Abstract

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.

Fig 1. Study area.

Map showing location of samples collected from 4 regions: Western Australia, Northern Territory, Gulf of Carpentaria, and Eastern Australia. (Map made using open source R software and R package ‘ggplot2’ (https://github.com/tidyverse/ggplot2) and ‘ozmaps’ (https://github.com/mdsumner/ozmaps).

Fig 2. Size and age at stranding.

Distribution of non-hatchling flatback sea turtles’ size and estimated age at stranding (n = 38).

Fig 3. Flatback size-at-age.

Range of estimated size (curved carapace length, CCL, cm) at each estimated age (years) for all 72 aged flatback sea turtles.

Fig 4. Growth curves.

a) Back-calculated estimated curved carapace length (CCL) and corresponding estimated age (years) for 72 flatback sea turtles with the generalized additive mixed model (GAMM) smoothing spline (solid line) and 95% CI (dashed lines). b) von Bertalanffy growth curve using growth increments and estimated CCL. See Methods for details. c) size and age of turtles separated by geographic regions, Eastern Australia, Northern Territory, and Western Australia, to show samples collected at each location.

Fig 5. Flatback maturation.

Estimated a) age-at-sexual-maturation (ASM) and b) size-at-sexual-maturation (SSM) for n = 29 flatback sea turtles.

Fig 6. Flatbacks with nesting histories.

The skeletochronology-assigned year and curved carapace length (CCL, cm) are shown for four analyzed humeri that were recovered from adult female flatback turtles with previous nesting histories observed during on-going nesting beach monitoring in Western Australia. Filled black circles show the skeletochronology-assigned maturation year for each turtle, with corresponding year, age (years) and CCL (cm) shown. Black asterisks show the year for each observed nesting event for each turtle.