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Assessment of Sea-Turtle Status and Trends: Integrating Demography and Abundance
understanding sea-turtle population status and trends fully. That knowledge is critical for predicting the changes in sea-turtle populations that will occur with climate change and with oceanic regime shifts that have profound effects on many important sea-turtle habitats.
Demographic parameters are not of equivalent value for diagnosing status and trends in populations. Some vital rates are influenced more than others by environmental factors—probably acting largely through nutrition. For example, nutrition affects age at sexual maturity, clutch frequency, and the number of years between breeding seasons, but it does not affect clutch size (Bjorndal, 1985). In populations with ample high-quality food, somatic growth rates, body condition, and clutch frequency will be high, and interbreeding intervals will be small. Populations that have poor food resources or that are approaching carrying capacity, at which competition for food is high, will exhibit the opposite.
BREEDING RATES AND ADULT-RECRUITMENT PROBABILITIES
In most species of sea turtles, females generally do not reproduce in consecutive years but at variable intervals of two years or more. The probability that a female will reproduce in any given year (breeding rate) is affected by nutrition (Bjorndal, 1985), environmental factors, and migration distance between foraging grounds and nesting beaches (Limpus and Nicholls, 2000; Solow et al., 2002; Troëng and Chaloupka, 2007). Knowledge of breeding rates is critical for understanding the highly variable numbers of clutches deposited in successive years on nesting beaches (Hays, 2000; Broderick et al., 2001; Solow et al., 2002) and for interpreting population trends.
Estimates of breeding rates of females have been derived from mark–recapture studies on nesting beaches using an “open robust design”—a specific mark–recapture method—with hawksbills (Eretmochelys imbricata; Kendall and Bjorkland, 2001) and leatherbacks (Dermochelys coriacea; Dutton et al., 2005). Mean remigration interval (the number of years between successive breeding seasons) has been estimated more commonly in sea-turtle studies and approximates the inverse of breeding rate. Although not as useful as breeding rate for demographic models, the remigration interval does offer important insights into the productivity of the population and population density relative to carrying capacity (Saba et al., 2007; Troëng and Chaloupka, 2007). Remigration interval is usually measured as the number of years that elapse between sightings of individual tagged females at a nesting beach. Thus, values are biased to shorter intervals because of tag loss and human-induced mortality in that the probability of both factors increases with the length of the remigration interval. Values are biased to longer intervals when incomplete sampling