River Dee Freshwater Pearl Mussel Surveys for population recovery work

Tender description

Re-survey of 2023 Site Condition Monitoring Sites containing pearl mussels (17 sites). Produce report on all sites to include condition, restoration potential and threats. Identify potential ARK sites within tributaries. Lot 1: This contract is to continue two of the activities based on recommendations arising from the report (Cooksley et al., 2025). 1. Assessment of selected pearl mussel sites - The 2022 SCM survey established the broad distribution of pearl mussels along the mainstem of the Dee. This involved spot-checks (5 x 1 m transects) made every 100m followed by a 50m transect at 27 sites to gather more detailed information about the population. - In 2024 ten sites were chosen for further analysis being those with the highest numbers of mussels and most suitable habitat. Range of age class, habitat and ongoing recruitment was recorded (Cooksley et al., 2025). - We would like to find out more detailed information on the remaining 17 SCM sites and two further identified sites including population structure at that site, juvenile recruitment/ habitat stability/suitability, host fish habitat presence and pressures. 2. Identification of Ark/refuge sites - The pearl mussels remaining in the main stem are vulnerable to future flood events and also drought as they are located in the very edges of the riverbed, twenty years ago mussels could be found across the width of the main stem in many places and now they are largely confined to the edges as much of the centre of the river now has unstable unsuitable habitat. - We would like to identify ark or refuge sites within the tributaries which would be suitable locations to transfer mussels from captive breeding or translocation, this will help provide a safeguard against losing the whole population if another large flood event were to further impact on the main stem population. - Selected tributaries will be assessed using a desk-based methodology based on available LiDAR survey data (10cm resolution and including photo grammatic images) to identify suitable stable habitat without obvious pressures or pressures which can be remedied which could serve as future potential refuge or ark sites. - Killeen & Moorkens (2016) have identified that suitable receptor sites may have been formerly occupied by mussels and now have now been restored or pressures removed, have a suitable flow regime providing hydrological and hydraulic conditions that encourage stable habitats, and the geomorphological conditions must be suitable or have the ability to be made suitable. - Concurrently a study is being undertaken by NatureScot to identify the best methods to undertake population recovery these are to include analysis of conservation breeding and translocation, the information provided from this survey will feed into the study and provide a baseline for future work.