First Contribution on Abundance, Population Structure, and Threats of the Baja California and Río Fuerte Sliders (Trachemys nebulosa Van Denburgh 1895)

Freshwater turtles in Mexico remain poorly studied, limiting effective conservation assessments for many species. The Baja California and Rio Fuerte sliders (Trachemys nebulosa nebulosa and T. n. hiltoni) are no exception, and despite their limited range no field studies have been conducted on their populations. In this study we evaluated abundance, population structure, morphology, and potential threats across 19 sites spanning Baja California Sur, Sonora, and Sinaloa. Using baited hoop nets, we conducted 234 trap nights and captured 75 individuals, including 43 unique T. n. hiltoni (23 recaptures) and 9 unique T. n. nebulosa. Capture rates were low across most sites (0–2.00 turtles/trap night), suggesting that this species exists in low densities across their range. Populations were dominated by large adults above 200 mm carapace length (CL, n = 45), and subadults below 200 mm CL were infrequent (n = 8) except for 1 ephemeral water body in Sonora where we trapped 8. Females were significantly larger than males in both subspecies, T. n. hiltoni female carapace length (CL): x̄ = 277.9 ± 48.2 (n = 18), males: x̄ = 236.4 ± 35.7 (n = 18), and T. n. nebulosa female CL: x̄ = 299.3 ± 11.6 (n = 6), and males: x̄ = 265.3 ± 19.1 (n = 3), consistent with female-biased sexual size dimorphism reported for this species. Interviews with local residents revealed that turtle consumption and gillnet use were the most widespread and severe threats, particularly in Sinaloa, alongside sewage runoff, invasive turtles, and habitat degradation from agricultural water diversion. Our findings provide the first range-wide ecological assessment of T. nebulosa and highlight declining detectability and multiple anthropogenic pressures. These results underscore the need for more surveys and an updated conservation status for both subspecies.