Successful reproduction of Trachemys scripta in the Altrhein of Kehl (Germany) and simultaneous increase of estimated population size

Abstract

The European Union categorised pond sliders (Trachemys scripta) as an invasive species for which all member countries have to develop an action plan. While T. scripta has been released illegally in thousands of freshwaters in Europe, monitoring programs in Germany are missing. Based on short term surveys, anecdotal data, and assumptions, the action plan in Germany proposes that T. scripta does not represent a problem as the cold weather causes high mortality and does not allow for successful reproduction. Here I present data of five years of monitoring at the Altrhein of Kehl, at the border between Germany and France showing: 1. Apart from T. scripta, five additional species have been released (Pseudemys nelson, Pseudemys concinna, Graptemys pseudogeographica, Mauremys spec., Chrysemys picta.). 2. Population size seemed to increase over the years, with 116 individuals being counted in 2020 compared to 33 individuals in 2016. 3. Successful reproduction of. T. scripta was indicated by the observation of very small individuals and proved by the collection of hatchlings in two years. While the data presented here would benefit from more years of more intensive monitoring, they are sufficient to draw the conclusion that the German action plan is insufficient because individuals survive for long periods and do reproduce successfully. Similar results have been reported from other central European countries. Thus, T. scripta might become invasive not only in south but also in central Europe. In sum, national action plans of how to deal with the invasive T. scripta should to be revised in Germany and many other European countries.