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THE IMPACT OF A CATESTROPHIC FLOOD ON A SCOTTISH FRESHWATER PEARL MUSSEL POPULATION OF EUROPEAN IMPORTANCE
  • Peter Cosgrove
Peter Cosgrove
Alba Ecology

Corresponding Author:[email protected]

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Abstract

Following a huge river channel-changing flood in December 2015 during the severe weather event named ‘Storm Frank’, a freshwater pearl mussel (Margaritifera margaritifera) survey of the River Dee Special Area of Conservation in the northeast of Scotland was undertaken in 2022-2023 to understand the current population and distribution of pearl mussels. This survey was compared with previous survey data collected in 2002 that used the same survey and assessment methods. Main findings • During 2022-2023 the River Dee between Aberdeen and Braemar was surveyed under licence for pearl mussels using a standardised survey methodology. • A total of 985 5 m x 1 m spot-checks were conducted. These found a total of 172 visible pearl mussels. 95% of spot-checks recorded no pearl mussels. Twenty-seven 50 m x 1 m transects were conducted, which held 771 visible pearl mussels. • All pearl mussels were recorded close to a bank edge, with twenty-eight spot-checks having pearl mussels recorded within 0-2 m of the bank edge and twenty spot-checks having pearl mussels recorded within 3-5 m of the bank edge. • The size of the River Dee pearl mussel population in 2022-2023 is estimated to be ca. 16,000 individuals, compared to ca. 1.3 million in 2002. This represents a ca. 99% decline in pearl mussel abundance since 2002. • The only evidence recorded of direct threats to pearl mussels were the substantial changes in channel morphology and substrate attributed to Storm Frank. • The catastrophic decline in the number of pearl mussels estimated between the 2002 and 2022-2023 is unprecedented. Most of the River Dee now appears to have highly unstable instream substrate making this habitat wholly unsuitable for pearl mussels. • The threat posed to pearl mussels by climate change induced increases in severe weather events is discussed and conservation actions identified