Die-offs of the endangered pearl mussel Margaritifera margaritifera during an extreme drought

Abstract

Droughts may be responsible for important ecological impacts in freshwater ecosystems, including the death of rare species. 2. This study assessed the mortality of the endangered pearl mussel Margaritifera margaritifera (Linnaeus, 1758) in the Mente, Rabacal and Tuela rivers (north west of the Iberian Peninsula) during the extreme summer drought of 2017. 3. Mortality occurred as a result of the low river flow, which led to mussel stranding near the banks and consumption (by predation or scavenging) by wild boar Sus scrofa (Linnaeus, 1758). 4. Mortality differed across sites. The shell lengths of live mussels (measured before the drought) and dead mussels significantly differed in the Rabacal River, but not in the Mente and Tuela rivers. 5. Extreme droughts are predicted to increase in number and intensity in the future, and possible impacts on rare species such as M. margaritifera should be carefully monitored. Several M. margaritifera populations in Iberia (and elsewhere) may now be at increased risk, and measures should be implemented in order to mitigate the impacts of future extreme droughts.