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

Abstract

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 (Rabaçal 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 Rabaçal 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.

Small hydropower plants as a threat to the endangered pearl mussel Margaritifera margaritifera

“…effects resulting from the extreme climatic events were similar in the three rivers and resulted in higher mortalities in the upstream areas (Sousa et al 2012: Nogueira et al, 2020. Nevertheless, it is important to mention that mortalities reported for the 2017 drought occurred after the sampling performed in this study (i.e.…”

“…Indeed, human disturbance is quite low and we are not aware of possible pollution sources or sedimentation problems in the study area that could explain such an abrupt decline in abundance and absence of juveniles. While in the last years some negative effects on pearl mussel abundance have been reported from the predation of the signal crayfish Pacifastacus leniusculus (Sousa et al, 2019), the occurrence of an extreme flood in 2010 (Sousa et al, 2012) and an extreme drought in 2017 (Sousa et al, 2018;Nogueira et al, 2020), these factors are not enough to explain the patterns found. In fact, the impacts of the signal crayfish predation are more pronounced in upstream areas (Sousa et al 2019), and the Fig.…”

“…we considered a low abundance site when after 30 min of sampling b5 individuals were collected) sampled with a higher effort. Even in sites within reservoirs, it was possible to snorkel due to the very low water level during the 2017 summer, in response to a severe drought (see for details Sousa et al, 2018 andNogueira et al, 2020). In all sites, the number of pearl mussels was counted and each individual was measured using a digital caliper.…”

“…This is partly accomplished by the Montesinho Natural Park, but most of the conservation attention is devoted to the terrestrial biodiversity, overlooking freshwater species. Therefore, more attention should be given to upstream areas sheltering healthy pearl mussel populations and, for example, mitigate some of the already described impacts such as those related with droughts (Sousa et al, 2018;Nogueira et al, 2020) and the presence of non-native predators such as crayfish (Sousa et al, 2019). Also, and because a great part of upstream areas suitable for pearl mussels are outside the Montesinho Natural Park, a downstream extension of its coverage could considerably enhance the protection of freshwater biodiversity.…”

Assessment of a terrestrial protected area for the conservation of freshwater biodiversity

“…To record the possible presence of freshwater mussels (Order Unionida), different habitats (riffles, pools, near the banks and in the centre of the channel) were also sampled. Individuals were surveyed by snorkelling and hand searching (detailed methodology described in Sousa et al, 2013, and Nogueira et al, in press) and four to six replicates were collected at each site. All the live specimens were identified, counted and measured (total length) with a Vernier calliper (to 0.1 mm), and returned to their habitat in their original position.…”