INTEGRATING CAPTURE-MARK-RECAPTURE AND INDIVIDUAL GROWTH MODELS REVEALS SIZE-DEPENDENT SURVIVAL IN AN ELUSIVE SNAKE

Jonathan P Rose; Western Ecological Research Center, U.S. Geological Survey; jprose@usgs.gov; Glenn D. Wylie, Michael L. Casazza, Brian J. Halstead

Estimating survival for many species is difficult because recapture rates are low, and the fate of marked individuals is often unknown. Low recapture rates also make it difficult to evaluate the importance of individual and environmental covariates for survival. Individual covariates such as size are challenging to include in capture-mark-recapture models because the state of the individual is unknown when it is not captured. We integrated a von Bertalanffy growth model with a robust-design Cormack-Jolly-Seber model to test for a relationship between body size and survival in the threatened giant gartersnake, Thamnophis gigas. We estimated the size of an individual when it was not captured using a Bayesian model, which fully propagates uncertainty in this unobserved covariate. We found clear support for a positive relationship between snake size and annual survival. Survival increased with size up to a peak for adult snakes and declined slightly for the largest individuals. Survival of giant gartersnakes was also positively related to the abundance of fish and the cover of emergent and floating vegetation at a site. Our results have implications for the management of this threatened species and illustrate the utility of hierarchical Bayesian models to the study of survival in elusive species.

Wildlife Techniques and Technologies