Precision of the Change-in-Ratio Technique for Deer Population Management

Abstract

Utility of the change-in-ratio (CIR) technique for estimating abundance of deer (Odocoileus spp.) populations was investigated by calculating the sample size requirements (n, = prehunt and n2 = posthunt) for 3 levels of accuracy (e = 0.10, 0.25, and 0.50) and a = 0.05 and by applying the technique to the deer population at Remington Farms, Maryland. The calculations indicated that a sample size of n = 1,600 was required to estimate abundance of deer populations with a skewed sex ratio (prehunt proportion of antlered bucks [P,] = 0.10) with f = 0.25. If only antlerless deer were removed from a population with a similar sex ratio and a large change in the ratio (AP = 0.20) was achieved, N, could be estimated for E = 0.25 with n = 325. Application of the technique at Remington Farms indicated that the sample sizes could be achieved for relatively high density herds in open habitat. Biases associated with applying the CIR to deer populations are discussed. J. WILDL. MANAGE. 50(1):125-129 Population estimates are an important component of deer population management, particularly when establishing harvest quotas or evaluating the impacts of previous management practices. Numerous techniques are available to estimate deer abundance, including track counts (McCaffery 1976), pellet group counts (Stormer et al. 1977), aerial counts (Floyd et al. 1979), drives (McCullough 1979), line transects (Evans 1975), remote sensing (Wyatt et al. 1980), markreobservation (Downing et al. 1977), and change-in-ratio (Hess 1985). Because most of these techniques are expensive, biased, or unproven, they have limited utility for management agencies. The CIR technique is an attractive alternative for estimating or indexing populations of white-tailed deer (0. virginianus) because few data are required to execute the method, and the precision of the estimator can be high. Only prehunt and posthunt estimates of the proportions of the population that are antlered and antlerless and the number and sex of deer killed by hunting are required to apply the CIR. Precision of the CIR depends on the prehunt and posthunt ratios and kill removals. An added advantage is the usefulness of sex ratios for establishing harvest regulations, particularly for trophy-buck production, when adjusting the sex ratio is a management goal. Our study objectives were to: (1) evaluate the theoretical precision of the estimator and to calculate the sample size requirements for a range of proportions of antlered and antlerless deer and (2) address the assumptions of the technique relative to its application. We then applied the CIR in a field example to ascertain whether the sample sizes dictated by the theoretical calculations could be achieved. We thank Remington Farms personnel, especially E. C. Soutiere and E. H. Galbreath, for their support, ideas, and encouragement. J. W. Eldrett conducted the fieldwork. Remington Arms Co., Inc., and the Natl. Rifle Assoc. provided funding. C. Powell drafted the figures. DESCRIPTION OF THE TECHNIQUE A brief description of the CIR technique follows; for more detail see Seber (1982) or Paulik and Robson (1969). We consider a closed population with 2 classes of animals, x-type and y-type. For our example, x-type are antlered and y-type are antlerless deer. If the proportion of xand y-type animals in the population changes between 2 periods due to the removal of a known number of animals (in our example by hunting), we can estimate total population size before the removal (NI) as follows: N, = R P (1) N2 P 2 where R, = the number of x-types removed (known), R = Rx + RY = the total number of animals removed (known), P, = X,1N, = the proportion of x-type animals before the removal, and