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Assessment of Sea-Turtle Status and Trends: Integrating Demography and Abundance
tope signatures of carbon and nitrogen in scute tissue (the keratin covering of the upper shell that is inert after deposition) provide a history of diet and habitat that can be used to identify recent recruits (Reich et al., 2007). Reliable, rapid, and non-invasive methods of identifying recruits are needed.
SOMATIC GROWTH AND AGE AT SEXUAL MATURITY
Somatic growth has been measured in a number of sea-turtle populations. Adult females essentially stop growing after attaining sexual maturity, at which point resources are allocated away from somatic growth to reproduction. In immature turtles of a given species, growth varies spatially and temporally (Diez and van Dam, 2002; Balazs and Chaloupka, 2004b; Chaloupka et al., 2004b; Kubis et al., 2009). Known sources of variation are body size (Chaloupka and Musick, 1997), population density (Bjorndal et al., 2000a), habitat quality (Diez and Van Dam, 2002), nutrient quality of diet (Wood and Wood, 1981), disease status (Chaloupka and Balazs, 2005), and compensatory growth (Bjorndal et al., 2003a; Roark et al., 2009a). A combination of somatic growth rates with indexes of body condition is the best current measure of habitat quality and population status on foraging grounds (Bjorndal et al., 2000a; Diez and van Dam, 2002; Kubis et al., 2009).
The most common method of measuring growth rates in turtles has been mark–recapture study. Because population and environmental conditions can be monitored throughout a mark–recapture study, this technique offers the best approach for evaluating the mechanisms that regulate growth. Mark–recapture studies are of necessity long term and labor intensive and are successful only when recapture probabilities are relatively high. Because that condition is not always met, other techniques have been used.
Skeletochronology, the use of markers in skeletal material (primarily humeri and eye ossicles), has been used in many studies to estimate somatic growth rates (Zug et al., 1986; Bjorndal et al., 2003a; Snover and Hohn, 2004; Snover et al., in press). Caution in the interpretation of marks is critical, the technique is not practical for live animals, and remodeling of internal bone layers can be problematic. Those and other challenges in the application of skeletochronology have been well reviewed (Snover et al., 2007; Avens et al., 2009). Advantages of the technique are that turtles do not have to be captured, skeletal elements can be gathered from the large number of carcasses that strand on the U.S. coast each year, and longitudinal sampling of individuals can be exploited. Longitudinal sampling is possible only with multiple recaptures in mark–recapture studies. With skeletochronology, growth-increment analysis of the humeri can be