Introduction

The phrase ‘capture–recapture methods’ refers to studies in which a sample of animals is marked and then some, but usually not all, of them are recovered on one or more subsequent occasions. Goals of capture–recapture studies include estimating population size, survival, and certain other variables discussed in this Chapter and studying associations between these and other variables such as how survival rates vary with age or across years. In this Chapter we review the basic concepts that underlie capture–recapture studies, identify major branches of the field, and describe recent developments. Although the methods were originally used primarily to estimate population size and survival rates, contemporary methods provide a rigorous approach for studying a wide variety of other issues of interest in behavior, ecology, and evolution (Nichols 1992). The methods, however, are complex and new refinements appear constantly. We therefore suggest consulting a specialist in capture–recapture methods before undertaking work in this field, and we do not attempt to provide a comprehensive guide to the methods.

Rationale

The basic rationale in capture–recapture methods is to estimate what fraction of the animals marked and present in the study area was counted during each sampling period. This fraction is then used to estimate quantities of interest. Two general approaches might be distinguished for estimating the fraction. First, suppose we can assume that all animals (marked and unmarked) present at the start of the study are still present on each recapture occasion. We will also assume that the marked and unmarked animals, in a given cohort, have the same recapture probabilities.