Prepared in cooperation with the California Department of Water Resources Construction and Analysis of a Giant Gartersnake (Thamnophis gigas) Population Projection Model Open-File Report 2017–1164 U.S. Department of the Interior U.S. Geological Survey Cover: Photograph showing an adult female giant gartersnake (Thamnophis gigas), Sacramento Valley, California, representing the importance of large adult female snakes for population persistence. Photograph by Alexandria M. Fulton, U.S. Geological Survey, May 10, 2017. Construction and Analysis of a Giant Gartersnake (Thamnophis gigas) Population Projection Model By Jonathan P. Rose, Julia S.M. Ersan, Glenn D. Wylie, Michael L. Casazza, and Brian J. Halstead Prepared in cooperation with the California Department of Water Resources Open-File Report 2017–1164 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior RYAN K. ZINKE, Secretary U.S. Geological Survey William H. Werkheiser, Deputy Director exercising the authority of the Director U.S. Geological Survey, Reston, Virginia: 2018 For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment—visit https://www.usgs.gov/ or call 1–888–ASK–USGS (1–888–275–8747). For an overview of USGS information products, including maps, imagery, and publications, visit https:/store.usgs.gov. Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner. Suggested citation: Rose, J.P., Ersan, J.S.M., Wylie, G.D., Casazza, M.L., and Halstead, B.J., 2018, Construction and analysis of a giant gartersnake (Thamnophis gigas) population projection model: U.S. Geological Survey Open-File Report 2017–1164, 98 p., https://doi.org/10.3133/ofr20171164. ISSN 2331-1258 (online) Acknowledgments We thank the California Department of Water Resources for supporting this project. We also thank the U.S. Fish and Wildlife Service and the Natomas Basin Conservancy for funding projects whose data were used in this report. We thank the Natomas Basin Conservancy, Colusa National Wildlife Refuge, Wildlands, Inc., and several cooperative landowners for access to the study sites. We also thank the giant gartersnake Technical Review Committee for providing feedback and guidance for this study, Julie Yee and Brian Prochazka for reviewing this report, and Tim Tinker and Elizabeth Gallegos for reviews of sections 1 and 2, respectively. We are grateful to Eric Hansen, Rick Scherer and Maxwell Joseph for sharing data and model results on giant gartersnake reproduction. Dr. Ray Wack of the University of California, Davis and the Sacramento Zoo conducted the X-rays and interpreted the number of embryos in each radiograph for the fecundity analysis. We thank Lisa Parker for administrative support. This study would not have been possible without the work of numerous biological technicians who collected field data on giant gartersnakes for the U.S. Geological Survey for many years. This work was performed under Institutional Animal Care and Use Committee protocol WERC-2014-01 and as stipulated in California Department of Fish and Wildlife Scientific Collecting Permit 10779 and U.S. Fish and Wildlife Service Recovery Permits TE- 020548-5 and TE-157216-2. iii This page left intentionally blank iv Contents Acknowledgments ................................................................................................................................................... iii Abstract .................................................................................................................................................................... 1 Background .............................................................................................................................................................. 1 Purpose and Scope ................................................................................................................................................. 3 Giant Gartersnake Biology ....................................................................................................................................... 3 Study Area ............................................................................................................................................................... 4 Goals and Objectives ............................................................................................................................................... 4 Section 1: Fitting a von Bertalanffy Growth Model for Giant Gartersnakes .............................................................. 5 Introduction ........................................................................................................................................................... 5 Methods ................................................................................................................................................................ 6 Data Collection ................................................................................................................................................. 6 Growth Analysis ............................................................................................................................................... 9 Results................................................................................................................................................................ 12 Discussion .......................................................................................................................................................... 17 Section 2: Reproductive Frequency and Size-Dependence of Fecundity in the Giant Gartersnake ....................... 19 Introduction ......................................................................................................................................................... 19 Methods .............................................................................................................................................................. 20 Data Collection ............................................................................................................................................... 20 Statistical Analysis ......................................................................................................................................... 23 Results................................................................................................................................................................ 24 Litter Size, Neonate Size, and Maternal Size ................................................................................................. 25 Annual Proportion of Gravid Females and Reproductive Frequency.............................................................. 29 Discussion .......................................................................................................................................................... 30 Section 3: Integrating Growth and Capture-Mark-Recapture Models to Estimate Size-Dependent Survival in Giant Gartersnakes. ....................................................................................................................... 33 Introduction ......................................................................................................................................................... 33 Methods .............................................................................................................................................................. 35 Data Collection ............................................................................................................................................... 35 Hierarchical Cormack-Jolly-Seber Model ....................................................................................................... 38 Environmental Covariates .............................................................................................................................. 39 Model Comparison and Model Averaging ...................................................................................................... 40 Results................................................................................................................................................................ 40 Capture Data Summary.................................................................................................................................. 40 CJS Survival Model ........................................................................................................................................ 41 Environmental Covariates of Survival............................................................................................................. 48 Discussion .......................................................................................................................................................... 50 Conclusions ........................................................................................................................................................ 54 v Section 4: Development and Elasticity Analysis of an Integral Projection Model for the