Estimating demographic parameters for a critically endangered marine species with frequent reproductive omission: hawksbill turtles nesting at Varanus Island,Western Australia |
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Authors: | Robert I T Prince Milani Chaloupka |
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Institution: | (1) Marine Science Programme, Science Division, Department of Environment and Conservation, Locked Bag 104, Bentley Delivery Centre, Bentley, WA, 6983, Australia;(2) Ecological Modelling Services P/L, University of Queensland, P.O. Box 6150, St Lucia, QLD, 4067, Australia |
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Abstract: | The hawksbill marine turtle (Eretmochelys imbricata) is listed on the IUCN Red List as critically endangered but little is known about its demography to support robust diagnosis
of population trends. Moreover, adult female hawksbills do not nest each year due to environmentally mediated physiological
constraints and this skipped breeding behaviour presents a major challenge in data collection and for estimating demographic
parameters from such data sets. We estimated demographic parameters such as survival and breeding probabilities for a major
Indo-Pacific nesting hawksbill population using a capture-mark-recapture (CMR) study and a multistate open robust design statistical
modelling approach, which accounts for breeding omission and the staggered arrival and departure of nesters during each season.
Our study used CMR histories for 413 nesting hawksbills tagged on Varanus Island (Western Australia) over a 4-month sampling
period each year for 20 austral summer nesting seasons between 1987 and 2007. The estimated annual survival probability for
these nesting hawksbills was constant over the 20 years at ca. 0.947 (95% CI: 0.91–0.97), which is encouragingly high for
a population associated with industry. The estimated annual conditional nesting (breeding) probability for female hawksbills
that had skipped the previous nesting season was time-specific ranging from 0.07 to 0.29 (mean = 0.18, CV = 41.3%), which
presumably reflects the interaction between turtle physiology and in-water habitat quality. The mean conditional probability
of breeding again having skipped 2 prior consecutive nesting seasons was ca. 0.83 (95% CI: 0.73–0.89), indicating a high frequency
of breeding season omission. The annual nesting probability for females that had nested the previous season was 0, reflecting
known obligate skipped breeding (reproductive omission) that is characteristic of hawksbill populations in response to high
energy demands of vitellogenesis and breeding migration. These are the first estimates of annual survival and state-dependent
breeding probabilities for any Indo-Pacific hawksbill stock that provide a basis for developing a better understanding of
regional population dynamics for this critically endangered species. |
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