A New Approach to Estimating Populations by the Removal Method

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Abstract
A new approach to the use of removal data in estimating the size of a population of fish or other animals is presented. The theory admits a variety of assumptions on how catchability varies among fishings including the assumption of constant catchability, which underlies most previous work. The methods hinge on maximum likelihood estimation, and they can be used both to decide objectively if the data justify rejecting constant catchability and to determine confidence intervals for the parameters. A new method of assigning confidence to the population estimate is included and problems are pointed out in methods currently available in the literature, even in the case of constant catchability. The theory is applied both to data in historical literature and to more recent data from steams in New Brunswick, Canada. The assumption of constant catchability can frequently lead to serious errors in data interpretation. In some cases, the conclusion that the population size is well known may be false, and reasonable estimates may be impossible without further data.