Detection, occupancy, and abundance of the alligator snapping turtle in Texas

Abstract

The alligator snapping turtle (Macrochelys temminckii) is a long‐lived species that is widely distributed across the southeastern United States. Both these characteristics make monitoring its occupancy and abundance particularly challenging. In 2020 and 2021, we conducted hoop trap surveys at 51 permanent water bodies across the Texas, USA, range of the species to estimate occupancy and average abundance with site occupancy models and multinomial N‐mixture models, respectively. We confirmed the species to be present at 61% of surveyed locations. Mean detection probability was 0.77, and was influenced methodologically by the spacing of traps, and environmentally by lunar phase and water flow velocity. Mean occupancy probability was 0.82, and mean estimated abundance was 7.57 turtles/site. Our data provided evidence that the species has a higher occurrence probability in waters of highly forested catchments, and is most probable to occur in highest abundances at median latitudes within its Texas range. Despite known microhabitat preferences of the species, instream habitat characteristics (e.g., canopy coverage, depth, sinuosity) did not strongly influence its occupancy or abundance. Spatial dynamics of the occupancy and abundance of the species in Texas are influenced by landscape features more than local habitat. These results further corroborate the importance of watershed land cover to aquatic organisms, and our methods provide a standardized template for monitoring and quantifying important demographic parameters of the species across multiple locations while accounting for imperfect detection.

Comparing cost, effort, and performance of environmental DNA sampling and trapping for detecting an elusive freshwater turtle

“…Further, our eDNA sampling stream locations were chosen based on ease of access (i.e., road crossings), but targeted sampling could be conducted in microhabitats where M. temminckii are most likely to occur (e.g., in deep bends with submerged logs: Howey & Dinkelacker, 2009;Riedle et al, 2006) and summer months when they are more active (Harrel et al, 1996). While eDNA is certainly subject to false negatives (Burian et al, 2021;Darling et al, 2021;Schultz & Lance, 2015), we note that conventional sampling is as well, particularly for M. temminckii (Rosenbaum et al, 2023). Conversely, it should also be noted that eDNA detections at each site within a stream are not necessarily independent given the transport of DNA.…”

“…We sampled year‐round to address seasonality of M. temminckii detection. Trapping consisted of 1‐m diameter aquatic hoop traps (Memphis Net and Twine) baited with sardines, similar to previous studies for the species (e.g., Rosenbaum et al., 2023). At each site, 3–6 traps were set, and then checked each day for up to four consecutive days.…”

Using Local Ecological Knowledge to Document Distribution and Temporal Patterns of Macrochelys temminckii in Texas

“…A previous state-wide assessment of Alligator Snapping Turtles had a similar occupancy rate (69.6%) when selecting sites based solely on habitat and accessibility (Rudolph et al 2002). More recently, a state-wide assessment of Alligator Snapping Turtles re-evaluated areas of historic occupancies and areas of uncertain occupancy and found a 61% occupancy rate over 51 locations (Rosenbaum 2022, Rosenbaum et al 2023a). This study also documented occupancy in counties within the Brazos and San Jacinto–Brazos watersheds, though exact location and proximity to watershed boundaries is unclear.…”

Intraspecific Variation in Mercury Contamination of Alligator Snapping Turtles (Macrochelys temminckii)

“…Our efforts to acquire M. temminckii THg samples occurred from March through September 2021 within the major watersheds of eastern Texas, which included the San Jacinto, Trinity, Neches, Sabine, and Mississippi watersheds (Rosenbaum et al, 2023a, 2023b). The Mississippi watershed contains the Red, Sulphur, and Cypress watersheds, which confluence in Louisiana, while the other watersheds are contained in Texas and drain directly into the Gulf of Mexico (Figure 1).…”

“…This dimorphism is not apparent in sexually immature individuals, so we categorized small individuals whose sex was unclear (all of which were less than 40 cm in carapace length) as juveniles. The species also exhibits male-biased sexual size dimorphism, with females and males in our study area averaging 42.8 and 51.6 cm in carapace length, respectively (Rosenbaum et al, 2023a). We did not consider sex as a covariate of THg concentration in analyses because we could not confirm the sex of small individuals, and the three resultant demographic categories (juveniles of undetermined sex, adult females, and adult males) were effectively three partially overlapping size classes.…”