Change-in-Ratio Estimators for Habitat Usage and Relative Population Size

Abstract

SUMMARY New change-in-ratio methods are developed for estimating the proportion of a population in each of several defined regions. The procedures assume population closure so that an observed decline in catch rate in one region must be balanced by increased catch rates elsewhere. Sampling gear catchability can be unknown and variable among regions. The method was used to estimate the proportion of a population of juvenile striped bass occurring in a river region slated for filling. Formulas are also given for constructing an index of abundance and for estimating the relative efficiencies of the sampling gears in the different regions. A given animal population may occur simultaneously in several habitats in proportions that vary over time. If the sampling gears used in the different habitats have unknown relative efficiencies, then it is difficult to assess the relative abundance of animals in the different habitats. Relative abundances may be important information for planners and resource managers seeking rational criteria for siting development projects. Similarly, it is difficult to determine trends in population abundance over time when changes in observed catch rate may be due, at least in part, to changes in geographical distribution. In this paper, we describe new change-in-ratio estimators for the proportion of the population in each defined region and for the relative population size. The method relies on four assumptions: (1) catch rate is directly proportional to abundance within a region; (2) the efficiencies of the sampling gears remain constant over the course of the study; (3) the population is closed; and (4) the proportion of the population within each region is variable over the course of the study. The closure assumption implies a conservation of numbers so that a decline in catch per unit of effort in one region must be balanced by increases in catch per unit effort elsewhere. In other words, changes in observed catch rates are assumed due to a redistribution of the population among regions. Extensive local movements over short periods of time are rather common in the animal kingdom. This suggests that the method may be widely applicable.