Dispersal and mortality of translocated thick-shelled river mussel Unio crassus Philipsson, 1788 adults revealed by radio tracking: Dispersal and mortality of translocated thick-shelled river mussel Unio crassus Philipsson, 1788 adults revealed by radio tracking

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 Biaa (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 (0.20m) 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 (>100m), 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.