Effects of an extreme drought on the endangered pearl mussel Margaritifera margaritifera: a before/after assessment

Droughts can change the distribution and alter the behaviour, phenology and physiology of many species, leading in extreme cases to high mortalities or even extirpation. In this study, a before/after comparison was performed in two Portuguese river basins (Rabac¸al and Tuela) to assess the possible effects of an extreme drought on the abundance and size structure of two pearl mussel Margaritifera margaritifera populations, a Critically Endangered species in Europe. The hydrological year of 2017 was extremely dry in Portugal and this situation was responsible for a major reduction of the river flow and rapid decrease in the water levels during summer. One year after the drought, a significant reduction in abundance was observed (i.e. 27.6% for the Rabac¸al and 38.7% for the Tuela basin populations). However, no differences were detected in the size structure of the two pearl mussel populations. Droughts are predicted to increase in number and intensity and it is crucial to monitor M. margaritifera populations, given the low resistance (and possibly low resilience) of this species to these extreme conditions. Management measures that could help the conservation of pearl mussels are discussed.This work was supported by: (i) POSEUR-03-2215-FC-000096 and ICNF funds under Project CP01-MARG-QUERCUS/2018 and (ii) European Investment Funds by FEDER/COMPETE/POCI – Operational Competitiveness and Internationalization Program, under Project POCI-01-0145-FEDER-006958 and National Funds by FCT - Portuguese Foundation for Science and Technology, under the project UID/AGR/04033/2013. FCT also support MLL with a doctoral Grant (SFRH/BD/115728/2016). We are grateful to Jose´ Pedro Ramia˜o for the help with the precipitation data.