An Approach for Population Assessment in the Absence of Abundance Indices

Population assessment models give managers insight into both the current status of a given population and how that population may respond to management measures. Nearly all of the techniques in both terrestrial and aquatic wildlife management require either an independent estimate of absolute abundance or an index that is consistently proportional to abundance. When neither of these exists, some information can still be mined from the changes in the sex and age ratios within the population over time. When harvesting is highly skewed toward a single sex or age class, the change in these ratios provides information about the exploitation rate and, when combined with absolute numbers removed, also provides information on absolute abundance. Traditional change-in-ratio techniques require independent estimates of other variables in order to be used in a predictive sense for open populations. This paper proposes a new method that combines the synthetic approach of fisheries stock assessment with the information contained in sex and age ratios to allow for the assessment of populations when no estimate of absolute abundance or reliable index of abundance exists. Although estimates of population parameters such as survival rates and fecundity are produced, the primary output of the estimation routine is the current population size and structure and the potential population sizes and structures that may result from future management options. The approach is validated using simulated data and is then applied to data for an elk population in north-central Idaho. Corresponding Editor: B. A. Maurer