Survival of Western Cottonmouths (agkistrodon Piscivorus Leucostoma) in a Pulsing Environment

We present the first robust estimates of apparent survival of western cottonmouths (Agkistrodon piscivorus leucostoma) in central Texas. Estimates presented were obtained using the Cormack–Jolly-Seber Model that accounts for detectability. Apparent annual probability of survival of western cottonmouths located at Honey Creek, Comal County, Texas, a spring-fed stream flowing 3.2 km to its confluence with the Guadalupe River, was 0.81 and was consistent with estimates for similar species of snakes. Despite low probability of detection (0.12) and relatively small sample (n 5 51), the estimate of survival was reasonably precise (coefficient of variation was 4%). One benefit of our study is that we used a long-term dataset (11 years) that encompassed multiple floods and droughts, and therefore, represents a relatively wide range of conditions to which western cottonmouths are exposed at this locality. RESUMEN—Presentamos las primeras estimaciones robustas de supervivencia aparente de la mocası́n de boca de algodón (Agkistrodon piscivorus leucostoma) en Texas central. Las estimaciones presentadas fueron obtenidas utilizando el modelo de Cormack-Jolly-Seber que se ajusta por la detectabilidad. La probabilidad anual aparente de supervivencia de la mocası́n de boca de algodón localizada en Honey Creek, condado de Comal, Texas, una corriente alimentada por un manantial que fluye 3.2 km a su confluencia con el rı́o Guadalupe, fue 0.81 y fue consistente con estimaciones para especies semejantes de serpientes. A pesar de la probabilidad baja del descubrimiento (0.12) y la muestra relativamente pequeña (n 5 51), la estimación de supervivencia fue razonablemente precisa (coeficiente de variación de 4%). Un beneficio de nuestro estudio es que utilizamos un conjunto de datos a largo plazo (11 años) que abarcó inundaciones y sequı́as múltiples, y por lo tanto, representa una relativamente gran variedad de condiciones a las que las mocasines de boca de algodón están expuestas en esta localidad. The western cottonmouth, Agkistrodon piscivorus leucostoma, is an iteroparous pit viper that inhabits a wide variety of habitats (Gloyd and Conant, 1990; Werler and Dixon, 2000). In Texas, the species is ubiquitous in the eastern one-half of the state and is found from coastal marshes to inland streams (Werler and Dixon, 2000). In north-central Texas, it is associated with mesic habitats, where it migrates seasonally between woodlands and river bottoms (Tennant, 1984; Ford, 2002). Because of its wide distribution and abundance, the western cottonmouth might be an important predator of fish, amphibians, birds, and small mammals (Savitzky, 1992). Furthermore, many habitats occupied by western cottonmouths, particularly the ones characterized by pulsing environments (e.g., streams and rivers) are largely influenced by activities of humans. Little is known, however, about demography of this important species, and estimates of survival based on robust statistical methods are lacking. The objective of our study was to provide estimates of survival for western cottonmouths in central Texas, based on robust approaches to statistical estimation. In the research reported herein we used results of an 11-year mark-recapture study to investigate apparent survivorship (1—mortality and permanent emigration) and probabilities of detection within a population of western cottonmouths resident in Honey Creek, a typical small, highly variable, and low-productivity stream in The Southwestern Naturalist swna-55-01-02.3d 1/1/10 05:48:11 11 Cust # WL-28 THE SOUTHWESTERN NATURALIST 55(1):11–15 MARCH 2010