ESTIMATING CLOSED POPULATION-SIZE AND NUMBER OF MARKED ANIMALS FROM SIGHTING DATA

We describe a new estimator of population size that can be formed when independent sightings are made of marked and unmarked animals in a closed population where a subset of the population is individually marked. Each marked animal must bear a unique mark but the number of marked animals alive in the population is unknown. The estimate can be used when no recaptures or removals of animals are possible during the experiment. An example is estimating the number of immature bald eagles (Haliaeetus leucocephalus) on a lake some years after banding of nestlings. We derive the maximum likelihood estimates for population size and number of marks, and we show how to develop confidence intervals and perform goodness-of-fit tests. Criteria are developed for determining the number of sightings required to yield satisfactory estimates.