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Freshwater mussels of the order Unionida provide important ecosystem functions and services. Unfortunately, some previously widespread species are now seriously endangered. To restore the historical range of the population of Unio crassus in the Biała River, southern Poland, the species was reintroduced into a series of 'stepping stones' joining two remnant populations. During the first phase of the study, the relationships between the abundance of U. crassus, physical habitat, and water quality were studied to assess reintroduction potential. In general, chemical water quality improved upstream from the existing population, favouring the decision for reintroduction, whereas morphological variables worsened. Mussel abundance was correlated negatively with the elevation and slope of channel, organic matter contents, and pH (exceeding 8.0), but positively with silt presence, water conductivity, and concentrations of HCO 3 - , Ca 2+ , and NO 3 - . During the second phase, adult individuals were introduced into one type of functional habitat-marginal channel sectors with still water and fine sediment. Despite the initial very high rate of reproduction in some parts of the upper reach of the river, the juveniles were ultimately recruited only in the lower part of the restored range, resulting in a very rapid change in recruitment at a channel slope of 1.8‰. Recruitment was positively related to silt content, conductivity, and Ca 2+ and HCO 3 - ions, negatively to channel elevation and slope, and water pH. The host fish species showed no correlation with abiotic habitat features within the studied reach. These results imply that most of the habitat traits related to U. crassus occurrence depended on the river's longitudinal profile, not on the chemical water quality, and that final success of introduction should be evaluated after several years., (Copyright © 2017 Elsevier B.V. All rights reserved.)

This article reports a large‐scale die‐off of approximately one‐third of the unionid mussels inhabiting a shallow, naturally eutrophic, floodplain lake (Zalew Pińczowski), which took place within the space of just a few days.Mortality was the highest in Pseudanodonta complanata (81%), followed by Unio pictorum (58%), Anodonta cygnea (26%), Unio tumidus (15%), and Anodonta anatina (5%). It was significantly related to mussel size (age) only in A. cygnea, with the mortality rate higher in smaller (young) mussels.Pseudanodonta complanata was already the rarest species in the lake, and the number of survivors decreased to such an extent as to trigger a possible Allee effect.The most likely cause of the die‐off was a motor‐boating incident that had directly preceded the die‐off. Analysis of the water samples collected just after the die‐off revealed an extremely high concentration of phosphate (up to 0.5 mg dm−3) and nitrite (up to 0.06 mg dm−3). We suggest that the boat movements must have disturbed the lake sediments, thereby disrupting the mussels' physiology and causing mass mortality.The high sensitivity of P. complanata to environmental stress suggests that it may be an effective umbrella species for the protection of co‐occurring mussel species in eutrophic lakes. [ABSTRACT FROM AUTHOR]

Freshwater mussels are one of the most endangered groups of animals. Translocation is a common conservation measure for protecting them, but the success of this procedure varies widely, and the reasons for failure are frequently unknown.The dispersal and mortality of freshwater mussels was studied using a novel method: the radio tracking of Unio crassus translocated to new sites during a reintroduction project for this threatened species in the River Biała (Poland).Individual mussels were equipped with radio transmitters (10 mussels at each of four sites during low water in autumn, and 10 mussels at each of 11 sites during the spring flood). After being released into the water, the mussels generally moved away from the bank, and then performed erratic movements, dispersing over a larger area on flat parts of the riverbed, finally congregating on steep parts of the river bed in shallow water (0.20 m) near the bank.Mussel mortality was surprisingly low: all survived the autumn trial, and 80% of the radio‐tracked adults survived the spring flood. Some of them were displaced quite far within the channel (>100 m), apparently suffering no harm. The confirmed cases of mortality were a few individuals that had become stranded on gravel bars formed during the flood that were now drying out.Long‐distance displacement was more frequent in lotic or regulated parts of the channel and in the parts where the channel morphology was unstable during the flood.Radiotelemetry should be used to test the suitability of receptor sites before translocation, in order to find places where mussels will choose to stay, and to standardize mussel detectability when monitoring relocation success. [ABSTRACT FROM AUTHOR]

Abstract: Freshwater mussels (Unionoida) are globally threatened. The key to the development of effective conservation strategies for this group is to unravel their complicated life cycle, including the obligatory larva, which parasitizes the external tissues of fish. Artificial infestation was conducted for three fish species – Barbus barbus, Chondrostoma nasus, and Squalius cephalus – in order to study the prevalence, site, and intensity of infestation, in relation to the fish species and their individual body size. Fish gut contents were also examined, in order to analyse the possibility of glochidia consumption. Glochidia of Unio crassus are considered parasites of fish gills; however, this study revealed that U. crassus glochidia were attached to the fins of all the host species tested. The number of glochidia attached to C. nasus fins was four times higher than the number of glochidia attached to gills. The prevalence of fin infestation was higher for C. nasus and S. cephalus than for B. barbus. Infestation intensity was highest for C. nasus and lowest for B. barbus, and was also dependent on fin type. The number of glochidia attached to fins demonstrated a nonlinear (log10) relationship with fish body length. Inspection of dissected fish guts showed that all of the fish species tested foraged on glochidia: S. cephalus and B. barbus more frequently than C. nasus. Using predatory fish host species with non‐predatory species in mixed assemblages for artificial rearing of mussels led to increased mortality of fish and decreased infestation intensity. The best conservation practices for freshwater mussels should therefore be based on single fish species that are easy to infest and that do not prey on glochidia. [ABSTRACT FROM AUTHOR]