Inferring Population Changes of Freshwater Turtles in South Texas




Schultz, Amanda D'Anne

Journal Title

Journal ISSN

Volume Title



After more than 200 million years as a unique evolutionary group, persisting in the great extinction that ended the reign of dinosaurs, many species of turtles are now increasingly threatened with extinction. Leading pressures of additive mortality include exploitation and habitat alteration. Often seen as an issue for Asia, this situation is ongoing for the United States. Before 2007, market hunting of turtles was effectively unregulated within Texas. The 2007 harvest regulation changes permit harvest of turtles only from private water bodies but retain no regulation of harvest numbers, sizes, or sex within the state. Prior to the regulation change, a substantial harvest occurred within counties of the Lower Rio Grande Valley of South Texas, an area of marked human population growth. I repeated a freshwater turtle assessment conducted in 1976 to determine if demographic changes, consequent of harvest and human presence, have occurred in freshwater turtle populations within the Lower Rio Grande Valley. Original trapping locations were re-located and when possible re-trapped with similar trapping effort using baited hoop nets. Original locations rendered unsuitable by anthropogenic or natural changes were replaced with proximal or similar locations. Simultaneously, I repeated a 2008 study to determine if annual fluctuations of the population?s demographics negate significant results in the replication of the 1976 study. The 2008 study attempted to replicate the same 1976 study, but an ongoing drought forced several relocations from original trap sites and consequently, a low overall replication success. Species, sex, carapace length and width, plastron length and width, body depth, and weight were recorded for individual turtles. Data were analyzed for red-eared slider (Trachemys scripta elegans) and Texas spiny softshell (Apalone spinifera emoryi) captures. Capture-rates were compared using unequal variance t-tests or randomization tests. Age class ratios and adult sex-ratios were compared using Chi-square goodness-of-fit tests. Carapace lengths were compared using unequal t-tests or 2 factor crossed ANOVAs and an unbalanced multiple planned comparisons where sample sizes yielded enough temporal replication. The red-eared slider capture-rates showed a decrease over the 3-decade period and a significant increase in female carapace length. Drought conditions of 2008 increased captures, but total captures still do not reach the levels of 1976. I concluded that the red-eared slider population has changed since 1976 and commercial turtle harvest might be one of the causative factors. Further study is required to evaluate whether other additive mortality, such as urbanization is contributing to the decline of turtle populations in the Lower Rio Grande Valley. My results highlight the necessity for further study and likely further restrictive management changes to keep common pond turtles common within Texas.



Freshwater turtle conservation, South Texas, Vertebrate population monitoring


Schultz, A. D. A. (2010). <i>Inferring population changes of freshwater turtles in south Texas</i> (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


Rights Holder

Rights License

Rights URI