MODELS FOR POPULATION ASSESSMENT

Mathematical models are powerful tools for species assessment and evaluation. The reliability and utility of models depend on the quality and availability of data and on the assumptions conferred by model structure. Population models for sea turtles have been reviewed by Chaloupka and Musick (1997), Heppell et al. (2002), and others. Published models have ranged from regression fits to nesting-numbers data, deterministic lifecycle analyses, and complex simulation models—all with varied data requirements and assumptions. There are tradeoffs in model construction among precision, realism, and generality. Levins (1966) argued that a particular model can achieve at most two of those three qualities. Appropriate model complexity depends heavily on the question asked. The results of a simple model might be robust in uncertainty in lifecycle parameters but qualitative or incapable of supporting the precise estimates of population size or the effects of removal of individuals from a population. In contrast, detailed simulation models may require a large amount of biological information to produce precise or reliable estimates of population size or to predict response to perturbations. Regardless, models that are to be used for assessment, prediction, and management decisions require solid demographic data, preferably as time series of information that can be analyzed for changes in response to stressors, population density, or environmental variability (Hilborn and Mangel, 1997).

TOOLS FOR ASSESSMENT

Sea-turtle management issues vary by region, but quantitative assessment generally focuses on the following four primary issues:

  • Evaluation of trends in nesting and foraging population abundance as an indicator of population status

  • Diagnosis of the potential causes of those trends

  • Evaluation of the effects of natural and anthropogenic hazards on population viability

  • Definition of recovery criteria

Here the committee reviews a variety of available modeling approaches to questions about sea-turtle status and trends and notes the data requirements for each (Table 6.1). Unlike fishery assessment, the focus for sea-turtle management in the United States is not on sustainable harvest. Nevertheless, many of the quantitative tools used in fishery assessments are applicable to sea turtles and other threatened species. The results of the approaches identified here vary from qualitative to highly quantitative (Table 6.1).



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