Movements, habitats, and nesting ecology of spiny softshells in the Missouri River : the influence of natural and anthropogenic factors

Abstract

Sparse information exists about the ecology of spiny softshell turtles in large rivers and in Montana where they are at the northern extent of their range, disjunct from downstream populations, and a Montana species of concern. We described spatial and temporal patterns in movements, habitats, and nesting ecology of spiny softshell turtles in relation to natural and anthropogenic factors in the Missouri River from August 2009 through July 2012. Movement rates and home ranges were generally highest in summer, and lowest in winter. Turtles aggregated and showed interannual fidelity to active-period and overwintering-period habitats that were distinct and separated by 0.2-23.3 km. Microhabitat characteristics at turtle locations varied between active and overwintering periods; shallow and slow velocity areas were inhabited from May-September whereas deeper areas with moderate water velocities were inhabited from October-April. We located 25 nests in 2011 and 97 in 2012. Nesting followed peak river stage, and occurred mostly in the afternoon when no humans were present. Nesting and emergence occurred later in 2011 than in 2012, but incubation periods for successfully-emerged nests were similar between years, and fewer nests were successful in 2011. Nearly all nests were in mixed-gravel substrates with sparse vegetative cover. Nest sites during the nesting period were lower and closer to water than nest sites during the emergence period. More islands were nested on in 2012 than 2011 and depredation rate was three times lower on islands than mainland shores in both years. Substrate temperatures in simulated nests during the incubation period were warmer in gravel than sand substrates. Flooding in 2011 probably decreased nesting success by inundating potential nesting habitats and reducing habitat availability thereby delaying the onset of nesting. However, nesting habitats are created and maintained by floods. During winter, freezing episodes occurred at all depths in all simulated nests. The northern range of the species is probably limited by incubation period because hatchlings are unable to overwinter in the nest. Preservation of natural streamflow regimes and protection of habitats from anthropogenic disturbance may facilitate continued existence of spiny softshell turtles in the Missouri River in Montana.