An Evaluation of the Removal Method of Estimating Animal Populations

Moran (1951) presented a method for obtaining maximum likelihood estimates of population size from the results of a series of trappings in which the trapped animals are removed from the population. This type of trapping program may be appropriate when auxiliary studies of the trapped animals are to be made which necessitate their eventual sacrifice, or when economic or health reasons make it inadvisable to return trapped animals to the population.In such situations it is not possible to use the tagging and recapture method which has been discussed by numerous authors in recent years (see e.g. Leslie and Chitty (1951), Bailey (1951)). Moran's method, which will be called the removal method, is a special case of a more general procedure described by De Lury (1947). The present paper will cover the following topics: 1. Development of a rapid graphical procedure for obtaining maximum likelihood estimates of population size from removal method data. 2. Determination of the asymptotic precision of the removal method. 3. Report on the results of experimental sampling which was conducted in order to compare the maximum likelihood estimates with estimates by an alternative regression method (Hayne, 1949) and to study the performance of the removal method on small populations. 4. Determination of the proportion of the total population which must be trapped in order to reduce the coefficient of variation of estimates of population size to specified levels. 5. Tests of the assumptions underlying the removal method and their application to actual trapping data.