Lower Rio Grande Valley Freshwater Turtle Populations: Three Decades of Change




Brown, Donald J.

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Substantial commercial harvesting of wild freshwater turtles has occurred in the lower Rio Grande Valley of Texas since the 1990s. State regulations were created in 2007 to eHminate turtle harvesting in public waters, while common turtle species have no harvest protection in private waters. In addition to harvest, road mortality may be increasing due to extensive human population growth since the 1970s. I repeated a study conducted in 1976 to determine if demographic changes have occurred in freshwater turtle populations over the last three decades. 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. 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 and carapace lengths were compared using unequal variance !-tests or randomization tests, adult sex-ratios were compared using Chi-square goodness-of-fit tests, and correlations between red-eared slider carapace lengths and roads were tested using Spearman Rank Correlation Coefficient tests. The 1976 data were analyzed by season to determine if the 2008 results were potentially biased. The mean red-eared slider capture-rate was significantly lower in 2008, but only when all counties were included in the analysis. The mean carapace length for male red-eared sliders was significantly shorter in 2008 for Cameron County. Mean carapace lengths for male and female red-eared sliders were significantly longer in 2008 for Hidalgo County and all counties combined. Sex-ratios for red-eared slider adults were typically more male-biased in 2008. The mean carapace length for female Texas spiny softshells was significantly longer in 2008, and the adult sex-ratio was significantly more male-biased. A significant positive weak correlation was detected for carapace lengths and road density within 1 km and 5 km of trapping locations for female red-eared sliders. Capture-rates and carapace lengths were significantly different between May to July and August to November 1976. Sex-ratios were significantly different between May to July and August to November 1976. The changes detected cannot be attributed solely to harvest. They are likely the result of several factors including harvest, differential mortality, changes in habitat availability, and natural fluctuations.



freshwater turtles, animal populations, Lower Rio Grande Valley, populations


Brown, D. J. (2008). Lower Rio Grande Valley freshwater turtle populations: Three decades of change (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


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