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

Abstract

1. 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, Rabaçal 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 Rabaçal 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.

Effect of glochidia infection on growth of fish: freshwater pearl mussel Margaritifera margaritifera and brown trout Salmo trutta

“…Results from this study add to the list of negative effects of M. margaritifera on its salmonid host (Treasurer & Turnbull, 2000;Taeubert & Geist, 2013;Ö sterling et al, 2014;Thomas et al, 2014;Filipsson et al, 2016Filipsson et al, , 2017. However, as the growth-effect on brown trout can be expected to be low in natural infection intensities of less than 1000 glochidia fish -1 , this result should encourage testing the introduction of artificially infected fish, (Wellmann, 1943;Buddensiek, 1995;Geist et al, 2006;Thomas et al, 2010;Sousa et al, 2013Sousa et al, , 2015Sousa et al, , 2018 as a conservation measure of this endangered species. Finally, together with the observed lack of mortality effects, this may turn public opinion favorable for M. margaritifera conservation even if the salmonid host population is important for commercial or recreational fishing.…”

“…From a conservation point of view, the observed growth-effect of M. margaritifera on the salmonid host can be regarded as a disadvantage. If glochidia-infected stockings are used to strengthen or restore M. margaritifera populations (see Wellmann, 1943;Buddensiek, 1995;Geist et al, 2006;Thomas et al, 2010;Sousa et al, 2013Sousa et al, , 2015Sousa et al, , 2018, it would be attractive to encourage landowners, fishery managers, fishing right owners' associations and salmonid conservation programmes to use glochidia-infected juvenile Salmo spp. in their stockings.…”

“…Margaritifera margaritifera is an endangered species (e.g., Geist, 2010;Lopes-Lima et al, 2017;Oulasvirta et al, 2017) whose glochidia are produced in captive breeding programmes using brown trout as fish host (e.g., Buddensiek, 1995;Thomas et al, 2010;Gum et al, 2011;Eybe et al, 2015;Moorkens, 2018). One-way to strengthen declining M. margaritifera populations-or to restore the extinct populationswould be to stock juvenile brown trout infected with M. margaritifera glochidia to the target rivers (Wellmann, 1943;Buddensiek, 1995;Geist et al 2006;Thomas et al, 2010;Sousa et al, 2013Sousa et al, , 2015Sousa et al, , 2018. This is a realistic, though little utilized option in M. margaritifera areas where juvenile salmonids are stocked, either for recreational fishing purposes or to strengthen the salmonid populations, and where conditions for completion of the life cycle of M. margaritifera are otherwise favorable, like in northern Fennoscandia.…”

Water mill canals as habitat for Margaritifera margaritifera: Stable refuge or an ecological trap?

“…The climate is typically temperate and precipitation have high seasonal and inter-annual variability (Oliveira et al, 2012). Floods may occur during winter/early spring, with a gradual decline in river flow throughout the year, reaching minimal values in the late summer/early autumn (Sousa et al, 2012(Sousa et al, , 2018. The Tuela basin is under low human pressure and is mostly forested (Ferreira et al, 2007).…”

“…The upstream areas are inside the Montesinho Natural Park, which support a high terrestrial and aquatic biodiversity. In these upstream areas, a high abundance of freshwater pearl mussels is found, turning this river the second most important (after Rabaçal River) for the conservation of this species in Portugal (Sousa et al, 2015(Sousa et al, , 2018. Although with a very low human pressure, the upstream area of the Tuela River present some old weirs connected to water mill canals.…”