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Assessment of Sea-Turtle Status and Trends: Integrating Demography and Abundance
the remigration interval and the at-sea survival rate. Without information about remigration intervals and adult survival, it is not possible to relate the number of nesting females in a year to the total number of adult females in that year. Third, adult females make up a small part of the overall population. Their number is an index of population abundance only if their proportion in the population remains stable. Taken together, those complications in the use of nest counts as an index of population abundance underscore the importance of demographic and other information in drawing robust conclusions about a sea-turtle population from observations limited to one part of the population at one stage of the lifecycle. Hence, a conceptual model that links population abundance with the key demographic processes in a single coherent framework is needed.
The six species of sea turtles that inhabit U.S. waters share the basic lifecycle characteristics of nesting on land with breaks of a year or more between nesting seasons and varied degrees of site fidelity (see Chapter 2), variable egg survival with an incubation period of about two months and temperature-dependent sex determination, a phase of rapid growth in the open sea, and a protracted juvenile stage of several years. The species then fall into two primary life-history groups that are based on habitat use through their lifecycle. Loggerhead (Caretta caretta), green (Cheloniamydas), hawksbill (Eretmochelys imbricata), and Kemp’s ridley (Lepidochelyskempii) turtles make a developmental shift from pelagic (open ocean) to neritic (coastal, nearshore) habitat as juveniles; the discreteness of the shift may vary (McClellan et al., 2010). Leatherback (Dermochelys coriacea) and olive ridley (Lepidochelys olivacea) turtles, in contrast, remain pelagic throughout their lives. The number of years spent in preadult life stages varies among species, and lifecycle models have had some variability in the number and definition of life stages. All sea turtles undergo extensive migrations during their lives in response to changes in temperature and forage opportunities, and adult males and females migrate for mating and egg laying. With the exception of basking green turtles in Hawaii, only adult females return to land.
A simple but informative conceptual model of loggerhead abundance and demography is shown in Figure 3.1. The representation was developed for causal-loop modeling (Puccia and Levins, 1985), but it provides a generic description of sea-turtle population dynamics (Chaloupka, 2002a, 2003a, 2004) and is not tied to a particular modeling approach. This conceptual model is meant to remind the reader of the big picture and is an effective graphic device to capture in a coherent and integrated framework the key demographic processes and anthropogenic hazards facing