The status of the freshwater pearl mussel Margaritifera margaritifera Linn. in Scotland

1. One hundred and sixty rivers in Scotland with historical records of fres hwater pearl mussel Margaritifera margaritifera were surveyed between 1996 and 1999 for the presence of the species. 2. M. margaritifera populations were classed as either 'extinct' (no mussel s remaining),'not currently viable' (ranging from only dead shells present to no juveniles present, regardless of the number of adults found), or 'fun ctional' (at least one juvenile was found, regardless of the overall number s of adults present). 3. M. margaritifera populations could be classed as functional: in only 52 rivers (34% of total surveyed) and in only 17 of these were juveniles below 20 mm (5 yrs old) present. Furthermore, in only 10 of these functional riv ers were mussels also still considered either to be common or abundant and these included both large easterly-flowing rivers and small western rivers and streams. 4. In approximately two-thirds (101) of the 155 rivers occupied 100 years a go, M. margaritifera is now extinct or is about to become extinct. Furtherm ore, analysis indicates that the rate of mussel population extinction has a ccelerated since 1970, with a recent average of two mussel river extinction s per year. 5. The predominant threat to the mussel populations has undoubtedly been pe arl fishing. This has been occurring at every river, even in the most remot e northwest areas, where most of the 'functional' populations remain. Recen t legislation has provided full protection to M. margaritifera, so that all pearl harvesting is now illegal. 6. Locally, river engineering and host salmonid stock decline poses a serio us threat and eutrophication has already eliminated populations in southern and eastern Scotland. 7. In every part of its global range, M. margaritifera has declined substan tially and is now either threatened with extinction or is highly vulnerable . Based on recent estimates from across the species range, it appears that Scotland now probably holds at least half of the world's remaining function al M. margaritifera populations. Copyright (C) 2000 John Wiley & Soils, Ltd .