Reliability of Aerial Cache Surveys to Monitor Beaver Population Trends on Prairie Rivers in Montana

Abstract

An aerial cache survey was unreliable in indicating population size or trend of beaver (Castor canadensis) on 2 prairie rivers in Montana. Accuracy in locating caches was high (about 90%) and constant among years and areas, but colony size varied among areas and years. Data on age, sex, and reproduction are needed to determine colony size and properly evaluate aerial cache-survey results. J. WILDL. MANAGE. 47(3):697-703 Aerial censuses of beaver using sign observed have been described by Swank and Glover (1948), Fuller (1953), Townsend and Newby (1955), Hay (1958), and Payne (1981). Aerial surveys of caches (submerged piles of branches stored in fall for winter food) agreed well with ground surveys of active colonies in Colorado (Hay 1958) and in Newfoundland (Bergerud and Miller 1977, Payne 1981), where beaver built dams and lodges. The aerial cache-survey technique was inaccurate on streams in British Columbia (Slough and Sadleir 1977), but has not been evaluated on prairie rivers where beaver build dams rarely, live in bank dens, and often secure their caches near the bank. Even if aerial surveys accurately determine the number of colonies present, changes in colony counts may not necessarily reflect changes in population size, because colony size can change from year to year (Townsend 1953, Bergerud and Miller 1977). To properly evaluate the reliability of aerial cache surveys for estimating beaver populations, the accuracy of observing caches and the relationship between colony counts and population size must be examined. This paper evaluates the aerial beaver-cache survey on 2 prairie rivers in southeastern Montana and compares cache counts to population numbers. We thank the following trappers who provided jaws and reproductive tracts from beaver: D. Delcamp, R. Brauer, W. McConnachie, M. and M. Howry, and D. Schaff. We also thank D. Palmisciano for estimating the age of beaver jaws and H. E. Hodgdon, J. S. Larson, H. S. Hash, and R. P. Brooks for constructive reviews of the manuscript. This is a contribution from Montana Federal Aid to Wildlife Restoration Project W-130-R.