Bomb radiocarbon dating - Sea turtles
Hawksbill turtle - Validation of 40 year longevity
Saturday, April 30, 2016
UPDATE: Published with Proceedings of the Royal Society B
Time in tortoiseshell: a bomb radiocarbon-validated chronology in sea turtle scutes
Abstract: Some of the most basic questions of sea turtle life history are also the most elusive. Many uncertainties surround lifespan, growth rates, maturity and spatial structure, yet these are critical factors in assessing population status. Here we examine the keratinized hard tissues of the hawksbill (Eretmochelys imbricata) carapace and use bomb radiocarbon dating to estimate growth and maturity. Scutes have an established dietary record, yet the large keratin deposits of hawksbills evoke a reliable chronology. We sectioned, polished and imaged posterior marginal scutes from 36 individual hawksbills representing all life stages, several Pacific populations and spanning eight decades. We counted the apparent growth lines, microsampled along growth contours and calibrated Δ14C values to reference coral series. We fit von Bertalanffy growth function (VBGF) models to the results, producing a range of age estimates for each turtle. We find Hawaii hawksbills deposit eight growth lines annually (range 5–14), with model ensembles producing a somatic growth parameter (k) of 0.13 (range 0.1–0.2) and AGE OF breeding for
MEDIAN LENGTH OF MATURE ADULTS [sic] was 29 years (range 23–36). Recent bomb radiocarbon values also suggest declining trophic status. Together, our results may reflect long-term changes in the benthic community structure of Hawaii reefs, and possibly shed light on the critical population status for Hawaii hawksbills.
IMPORTANT NOTE:The validated age of this species was based on 3 individuals and the use of variations in K (growth coefficient) relative to the limits of regional bomb radiocarbon records (similar to estimates made for white abalone). Some assumptions were made that may not properly reflect growth, such as asymptotic size (L-infinity) and its use in describing growth and the age of some individuals (actual turtle size was artificially reduced). However, based on bomb radiocarbon dating, the maximum age of this species is likely near 40 years and may approach 50 years. In addition, the “age at first maturity” was calculated to be near 17-20 years.
PROJECT BACKGROUND
In this study we investigated an application of bomb radiocarbon dating to the scutes of Hawksbill sea turtle (Eretmochelys imbricata). Microstructural observations by Kyle Van Houtan (Marine Turtle Assessment Program, Protected Species Division) and Andrew Niccum (Hollings Scholar summer intern at PIFSC) were used to establish the experimental design that provided validated estimates of age and growth. The focus was to establish a temporal basis for observed growth zones in cross sections of the shell scutes and to describe the age, growth and longevity of this species, but scute lines were not well defined with an inconsistent periodicity. At present the lifespan of hawksbill turtle is now known to exceed 35 years, but bomb radiocarbon dating was limited by the scutes available and time of capture.
Work was performed using of our micromilling machine in the Life History Program at PIFSC. Pictured is the micromill with a sectioned scute sample and dentist drill bit (at right) that is controlled by the computer to select specific extraction tracks (in background), of which all of this is controlled by the geek on the left (head magnifier helps with viewing the extraction progress).
Above is a close up of the micromill in action. The drill bit (called a bur) has just traced out a series of close surface scans to remove material is a slow and guided manner. This material (keratin) is like your fingernails. The powder liberated from the track line is very light and is easily lost. This small pile was only about 0.001 g (1 mg) of material, close to the good-precision limits of Accelerator Mass Spectrometry for radiocarbon. All extractions were performed with New Wave Research micromilling machine (Elemental Scientific Lasers, Bozeman, MT, USA; http://www.nwrlasers.com/milling/micromill/).
These foil packets hold the 1 mg samples from the turtle shell. They were sent to Woods Hole Oceanographic Institute (NOSAMS) for radiocarbon analysis.
Hawksbill turtles are circumtropical, usually occurring from 30° N to 30° S latitude in the Atlantic, Pacific, and Indian Oceans and associated bodies of water. Hawksbills are widely distributed throughout the Caribbean Sea and western Atlantic Ocean, regularly occurring in southern Florida and the Gulf of Mexico (especially Texas), in the Greater and Lesser Antilles, and along the Central American mainland south to Brazil. Hawksbills do not occur in the Mediterranean Sea. The largest populations of hawksbills are found in the Caribbean, the Republic of Seychelles, Indonesia, and Australia (OPR).
The hawksbill turtle was listed under the Endangered Species Act (ESA) as endangered in 1970.
See Office of Protected Resources, NOAA Fisheries website for a comprehensive synopsis on this vulnerable species.
The Hawksbill turtle is locally named Taku in South Pacific island nation of Fiji.
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For information about the concerns for hawksbill turtle, see the synopsis at IUCN.
