Density, Apparent Survival, and Local Population Size of Louisiana Pigtoe (Pleurobema riddellii) in the Neches River, Texas

Abstract

Most North American unionids are imperiled to some degree, including the Louisiana Pigtoe, Pleurobema riddellii, which is currently under review for listing under the U.S. Endangered Species Act. Understanding a species' population dynamics, including spatial and temporal variation in survival, density, recruitment, and population size, is vital for conservation, but this information is lacking for P. riddellii. We conducted a mark–recapture study to estimate apparent survival, density, recruitment, and population size of P. riddellii within a 25-m2 area at three sites (75 m2 total) in the Neches River, Texas from 2014 to 2019. We used the program MARK to evaluate POPAN models for estimating population parameters. We collected a total of 392 unique individuals of P. riddellii over the 5-yr period and the observed recapture rate averaged 55.6%. The most parsimonious POPAN model indicated that apparent survival varied temporally, the recapture rate varied temporally and spatially, and both the entry probability (recruitment) and population size varied spatially. Apparent survival averaged 83.3% ± 3.4% (SE)/yr, overall population size across the three sites was 429 individuals (overall density = 5.7/ m2), and recruitment averaged 6.3%/yr. High survival, relatively high density, the presence of recruitment, and the lack of temporal changes in population size suggest that these populations are stable. The presence of P. riddellii throughout a long section of the river, including localized patches of higher abundance, suggests that the total population size in the Neches River is relatively large and the river is a global stronghold for the species.