Rio Grande Beaver (Castor canadensis mexicanus) Survey in Big Bend National Park
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The Rio Grande River forms the 176 km boundary of Big Bend National Park with Mexico and is home to the southwestern subspecies of North American beaver, Castor canadensis mexicanus. The last survey for the Rio Grande beaver, sometimes known as the Mexican beaver, in Big Bend National Park was conducted in 1981 by P. Strong and J. Bissonette. Our objectives were to document centers of beaver activity and estimate the population of beaver in Big Bend National Park. We surveyed the Rio Grande River on float trips with kayaks and canoes from the mouth of Terlingua Creek to the mouth of Boquillas Canyon. We recorded water depth, type of vegetation, sign of active beaver colonies (presence of dens, beaver tracks, scat, and cuttings), and ranked the amount of beaver activity within each colony as high, medium or low. From these data, we created a map using ArcGIS showing bathymetry of the river, vegetation profiles, and active beaver sign. We delineated a total of 98 active beaver colonies in the study area occupying deeper pools along the Rio Grande. We conducted camera surveys on 11 of the colonies to estimate the number of beaver in each colony. With camera survey data and activity ranking of each colony, we estimated a population of 185 beavers occupying the 98 colonies. This represents a 38% increase in the beaver population along the Rio Grande since 1981.