NatureScot Research Report 1315 - Life in our Lochs - are there freshwater pearl mussel populations in our lochs as well as our rivers?

Year of publication: 2023

Authors: O’Leary, D. (Trex Ecology Ltd), Robinson, G. (Ocean Ecology), Cooksley, S. (James Hutton Institute) and McDermott, T. (Trex Ecology Ltd).

Cite as: O’Leary, D., Robinson, G., Cooksley, S. and McDermott, T. 2023. Life in our Lochs - are there freshwater pearl mussel populations in our lochs as well as our rivers? NatureScot Research Report 1315.

Keywords

freshwater pearl mussels; salmonids; loch; remotely operated vehicle (ROV); Scottish Government Central Research Fund

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Background

The recent discovery of a significant population of breeding freshwater pearl mussels (FPM) in an Irish lough has led to the consideration of whether unknown populations are residing in some of Scotland’s freshwater lochs. The primary aim of this project is to develop a survey method and to survey three lochs to determine if FPM are present. This will further our understanding of FPM habitat range, requirements, and tolerances which will inform future catchment management and restoration projects. In addition, a survey of juvenile salmonid fish was undertaken in two lochs in order to investigate the presence of potential hosts for FPM glochidia.

The survey method was developed to incorporate the use of both snorkelling and boat-based surveys with an underwater remotely operated vehicle fitted with camera equipment.

Main findings

  • Three individual mussels were found in one loch. All three mussels were close to the outflow into another loch from which a river with a known population of mussels flows. 
  • One mussel was also found near the outflow of a second loch, approximately 6 km from where mussels had previously been found in the loch. Dead shells were also found in this loch, two at the inflow and one at the outflow.
  • All mussels were found at depths between 0.3-6 metres using the ROV method, demonstrating the value of using this approach.
  • Juvenile salmonid surveys were undertaken in two lochs in May when glochidia would be visible on the fish gills if present. However, no juvenile fish were caught. A small number of larger fish ranging from 150-400 mm in length were caught but no glochidial encystment was observed.
  • Lessons were learned on ways to improve the survey method from this trial, including changes in equipment and technique which would allow for more thorough investigations in future.
  • If further surveys find mussels in lochs, it could extend our understanding of FPM ecology and opens many new avenues and considerations for the conservation of the species.