Population Structure of the Spiny Softshell Turtle (Apalone spinifera) in Five Montana Rivers

Abstract. To conserve or restore riverine turtles, managers need baseline information on subpopulation structure and abundance in multiple rivers across large geographic areas. Assessing the demographics and morphological characteristics of different subpopulations can increase our understanding of how anthropogenic factors influence mortality and reproduction. We examined spiny softshell turtles (Apalone spinifera) in 5 rivers at the western edge of the species' range in southcentral Montana, where no commercial harvest is allowed. Over 4 yrs, we captured 637 spiny softshell turtles with fish-baited hoop traps. Our objective was to compare the subpopulation demographics in the Yellowstone River—considered one of the most intact rivers in the conterminous United States—to 3 Yellowstone River tributaries (Bighorn and Clarks Fork rivers and Pryor Creek) and the adjacent Musselshell River. Subpopulations differed significantly based on the demographic metrics we examined (e.g., mean sizes and sex ratios), and we documented limited numbers of males (4%–15%). Reproductive potential and mortality of adults among rivers appeared distinct based on juvenile and size class distribution of length-frequency histograms. This information from unharvested populations illustrates the variability in subpopulation demographics of riverine turtles.