Freshwater Pearl Mussel (FWPM) Monitoring

Ellendale environmental ecologists undertook a FWPM survey as part of monitoring at a site in Perthshire where FWPM were translocated to avoid harm and disturbance during the project works.

Standard survey methods have been developed for FWPM through a number of projects and these survey methods are adapted to the individual site. The Freshwater pearl mussel is fully protected under the Wildlife and Countryside Act (1981) and therefore all surveyors must be licensed by SNH.

Survey work can only be undertaken in periods of low water flow and therefore generally cannot be undertaken between October and March due to high water levels. A survey was first made of the river / channel and its substrate types within the survey site, by walking along the river bank and / or by wading in the water. The aim is to identify specific areas that are most likely to harbour mussels using information on their habitat preferences from previous studies and experience. The river was then entered at the nearest point and a search conducted. To ensure compatibility with other surveys undertaken the search is made:

  • using a glass-bottomed viewing bucket.

  • conducted under favourable conditions i.e. bright light, clear water, low flow regime.

  • in water sufficiently shallow for safe wading.

  • in an upstream direction, checking favourable sites e.g. in the shelter of cobbles, boulders or overhanging banks.

The survey found a total of 17 adult FWPM’s along the water course that were found to be open and filtering. This demonstrates that the mussels have not been impacted by the works and are healthy. It was noted that the mussels have moved from the original positions that they were translocated to again showing that they have selected the best areas for filter feeding and to avoid predation.

Stewart Parsons