THE DISTRIBUTION OF MARGARITIFERA-MARGARITIFERA (L) IN SOUTHERN IRISHRIVERS AND STREAMS

The distribution of Margaritifera margaritifera in southern Irish rive rs and streams is described using records collected since 1987. Some 1 800 sites on almost 400 rivers and streams were examined of which 8.1 and 14.6% respectively yielded shells. The mapping method employed, ba sed on 10-km squares, distinguishes between sites with living specimen s and empty valves. Although it was recorded in a wide range of sites, some analyses of the site characteristics indicate the following fluv ial habitat preferences: sites within a distance of 5-40 km from sourc e; sites with a channel width of 11-30 m; sites with non-calcareous so ils and strata (though not a strictly calcifuge species); sites on low land terrain; unpolluted sites with a shaded aspect. It is concluded t hat although M. margaritifera is widespread in the region and that its true distribution was hitherto poorly recorded, a decline in some of its former sites is nonetheless apparent.