Successful reintroduction of the European pond turtle Emys orbicularis with a small number of founders: results from a 20-years small-scale experiment

Reintroduction programs can be an effective management tool for conservation of threatened chelonians, yet the success of such programs relies highly on numerous factors, such as the number of founders and the size of introduced individuals. We conducted a reintroduction experiment of the European pond turtle Emys orbicularis in a pond located within its historical range, using a small number of founders (N = 20), most of which were hatchlings maintained over winter and then released in June 2004 (mean weight about 33 g, head-started cohort, N = 7), and others were released within one month after hatching in October 2006 (mean weight about 8 g, control cohort, N = 8). One female contributed two hatchlings (out of seven) to the 2004 cohort and the eight hatchlings released in 2006, so most hatchlings were half siblings. Two adult males and three females from nearby recovery centres were also released in order to increase the genetic variability of the population. At the end of 2023, at least five founder individuals were alive and their weight was about 500 g. The growth rate was similar for both cohorts and is among the highest recorded for the species, suggesting that the habitat in the main pond is optimal. Based on Jolly-Seber and Manly-Parr estimates, reproduction within this population was confirmed by the increase to 42-44 individuals by 2023, respectively. Genetic analysis confirmed the expected low level of genetic diversity, the lowest for the subspecies E. o. occidentalis distributed across the Iberian Peninsula and northern Morocco.. For this population to thrive, a management plan with long-term monitoring is needed to maintain a favourable habitat and genetic diversity, and thus minimize the risk of extinction. Our findings suggest that a small number of E. orbicularis hatchlings can be used as founders to successfully reintroduce the species in suitable habitats, even without headstarting.