Post-delisting genetic monitoring reveals population subdivision along river and reservoir localities of the endemic Concho water snake ( Nerodia harteri paucimaculata )

The Concho water snake ( Nerodia harteri paucimaculata ), endemic to the Colorado and Concho Rivers in Texas, was removed from the U.S. Endangered Species Act in 2011. Monitoring of a species removed from threatened/endangered status is required to assess long-term management goals. As such, we conducted extensive surveys along with population genetic structure assessment at the time of delisting. No Concho water snakes were captured along the Concho River, part of the previously known range. Compared to older studies, fewer snakes were captured on the Colorado River. Several individual-based population genetic analyses showed concordant patterns of population substructure along the Colorado River. Specifically, there was significant genetic differentiation, with a signature of isolation by distance, among sampled riffle habitats and the O. H. Ivie Reservoir population. At the level of sampling locations, effective size estimates were low (10 to 20) and bottlenecks were detected. Habitat modification and low water flows during periods of drought may have contributed to existing patterns. Overall, genetic-based results demonstrate that the Concho water snake exists as small subpopulations along the Colorado River. The latter result coupled with the failed detection along the Concho River raises concern for this delisted species. Because much of our sampling was conducted during a drought, we recommend additional monitoring for which our genetic and demographic results will provide a much-needed baseline.