NatureScot Research Report 1353 - Site Condition Monitoring of freshwater pearl mussel in the River Dee 2022-2023

https://www.nature.scot/doc/naturescot-research-report-1353-site-condition-monitoring-freshwater-pearl-mussel-river-dee-2022

Published: 2024

Authors: Cosgrove, P. (Alba Ecology Ltd), Massey, K. (Alba Ecology Ltd), Shields, D. (Alba Ecology Ltd), Picton, J. (Alba Ecology Ltd), Cosgrove, C. (Alba Ecology Ltd), Towers, M. (NatureScot), and Tree, A. (NatureScot)

Cite as: Cosgrove, P., Massey, K., Shields, D., Picton, J., Cosgrove, C., Towers, M., and Tree, A. 2024. Site Condition Monitoring of freshwater pearl mussel in the River Dee 2022–2023. NatureScot/CNPA Research Report 1353.

Keywords

freshwater pearl mussel, Margaritifera margaritifera, population collapse, flood, climate change, Storm Frank, Scotland, River Dee

Confidential reports

NatureScot and the Cairngorms National Park Authority are committed to making the findings of all of its research publicly available whenever possible. However, some animals and plants are threatened by or vulnerable to human activities and therefore require protection. Making some of our research confidential is a way of protecting these species and may help their survival. To be transparent about our work on protected species we have published a short redacted version of the full confidential report. If you would like a copy of the full report please email [email protected].

Background

The River Dee, Aberdeenshire is designated a Special Area of Conservation (SAC) under the European Union Habitats Directive. The critically endangered freshwater pearl mussel (Margaritifera margaritifera) is one of the species covered by this designation. In accordance with the Directive, routine Site Condition Monitoring (SCM) is undertaken to assess the state of the species. This report covers the monitoring survey commissioned by the Cairngorms National Park Authority, funded by it and NatureScot, and included in the Dee Catchment Partnership’s delivery plan. The survey was conducted between August 2022 and April 2023 and was the first to be done since the December 2015 channel-changing flood caused by Storm Frank. To understand the current state of the freshwater pearl mussel population and to illustrate any changes in it, the methods used in the 2002 baseline survey were repeated.

Main findings

  • The River Dee between the Bridge of Dee, Aberdeen and the Quoich Water, Braemar was surveyed under licence for freshwater pearl mussels. The survey methods described in the Joint Nature Conservation Committee’s Common Standard’s Monitoring Guidance for Freshwater fauna were applied.
  • 938 5 m x 1 m spot-checks were conducted. A total of 172 live freshwater pearl mussels was found. No freshwater pearl mussels were recorded in the vast majority (c.95%) of these spot-checks.
  • 27 50 m x 1 m transects were surveyed, which held 771 visible freshwater pearl mussels. Where live pearl mussels were found in a transect, the relative abundance code was generally D (Rare).
  • All freshwater pearl mussels were recorded close to, i.e. less than 5 m from, the bank edge. In 28 spot-checks they were recorded within 0–3 m of the bank edge, and in 20 spot-checks they were recorded within 3–5 m of the bank edge. In all circumstances, freshwater pearl mussels were highly localised and within a narrow c.2–3 m wide corridor close and parallel to one bank edge. In 2002 the width of suitable habitat was estimated to be c.20 m.
  • There has been a c.90% decline in overall freshwater pearl mussel abundance in the River Dee SAC between the 2002 and 2022–2023 SCM surveys.
  • The catastrophic decrease in the number of freshwater pearl mussels is unprecedented. The approximately 90% decrease is attributed primarily to the effects of Storm Frank; no evidence was recorded of any other immediate significant threat or pressure that could have caused a rapid population loss of such magnitude. However, it is recognised that other climate change-related factors, not just an increase in extreme spates such as that caused by Storm Frank, are also likely to be contributing to the freshwater pearl mussel population collapse. These include land management in the middle to upper catchment, droughts, warmer water temperatures, and declining numbers of host fish.
  • Storm Frank caused large changes to freshwater pearl mussel habitat and is thought to be the most likely cause of the unprecedented freshwater pearl mussel population decline. The substrate of most of the River Dee now appears to be highly unstable and so unsuitable for freshwater pearl mussels.
  • No evidence of other direct threats was recorded, such as illegal pearl fishing which was previously considered to be a significant and preventable cause of mortality.
  • Catchment-wide restoration is needed, to mitigate the effects of climate change, and to halt the decline and ensure the recovery of the River Dee SAC’s freshwater pearl mussel population.