Physical microhabitat requirements of freshwater pearl mussels, Margaritifera margaritifera (L.)

The spatial distribution patterns of freshwater pearl mussels, Margaritifera margaritifera (L.), in the River Kerry, north-western Scotland were investigated, together with their relationships to a number of physical parameters (distance from nearest bank, mean water depth, mean current velocity, substratum descriptors, aquatic vegetation cover). The mussels exhibited a highly contagious, non-random spatial distribution pattern. Adult and juvenile M. margaritifera were found to have broadly similar habitat 'preferences', although adults were found over a wider range of physical conditions. Based on computed habitat suitability curves, optimum water depths of 0.3-0.4 m and optimum current velocities of 0.25-0.75 ms(-1) at intermediate water levels were observed. River bed substratum characterisitics appear to be the best physical parameters for describing M. margaritifera habitat. Boulder-stabilised refugia, which contain enough sand for burrowing, are ideal microhabitats for juvenile mussels. Adults are able to tolerate silty or muddy conditions for unknown lengths of time, but juveniles are never found in this type of habitat. Substratum-based discriminant function models were used to predict the presence or absence of mussels, with a success rate of 76-92%.