Double boost for Scotland’s fresh water pearl mussels

A watercourse survey carried out ahead of work to replace culverts has uncovered a previously unknown colony of freshwater pearl mussels in north Highland.

The habitat improvement work – supported with a £170,000 Biodiversity Challenge Fund grant from Scottish Natural Heritage (SNH) and managed by Forestry and Land Scotland (FLS) – will see 8,000 native broadleaf trees planted in groups along 6.5km of water course and the removal of six barriers to fish passage from tributaries.

The discovery was made as a survey was being carried out by Alba Ecology’s Pete Cosgrove.

Freshwater mussel populations depend upon these fish (Atlantic salmon, brown/sea trout) to support the larval stages of their lifecycle, when they latch on to gill filaments and grow until they detach themselves the following spring. They need to land in clean sand or gravel in order to grow.

Suzanne Dolby, FLS environment forester, said: ‘Our reputation rests mainly on our forestry expertise – but this entails a whole lot more than just knowing about trees. Species conservation is a huge part of what we do and that includes looking after over a quarter of all of Scotland’s pearl mussel watercourses.

‘This is a significant responsibility for a species that has declined by 95 per cent in central Europe and is classified by the IUCN as being critically endangered in Europe.

‘We take practical action to meet these responsibilities and discoveries such as this highlight the importance of sustainable forest management and the type of work that we continue to build on to improve in-stream habitat for fish and pearl mussels.’ The new culverts restore the natural river bed, allow for a more natural flow of water and allows fish to pass freely along the tributaries to access additional spawning areas on the outskirts of the forest.

As the pearl mussels hitch a ride on the gills of young fish, this also allows for larval-stage mussels to colonise new stretches of a watercourse and replenish existing populations.

Iain Sime, SNH freshwater and wetlands advice manager, said: ‘Freshwater pearl mussels can live to well over 100 years old, but the species is critically endangered in Europe.

‘These rare molluscs are found in fast-flowing, unpolluted rivers and streams – often in catchments that are partially or wholly forested – and prefer a stony, well-oxygenated sand or gravel bed that is free from siltation.

‘The sustainable management of forests – including the careful harvesting and restocking of trees – can therefore play a crucial role in their survival, alongside managing any barriers on the river bed.

‘This work funded by the Biodiversity Challenge Fund will restore this vital habitat to help ensure that this rare species will have more than a fighting chance.’

The work is an example of how Biodiversity Challenge Fund projects are helping to improve our biodiversity. The next round of successful projects is due to be announced this spring following a £2m increase to the fund announced by the Scottish Government in its 2019 Programme for Government.

The fund encourages applicants with innovative projects that improve biodiversity and address the impact of climate change, increasing the resilience of our most at-risk habitats and species and creating large areas of brand new or restored habitat.

‘Species conservation is a huge part of what we do and includes looking after over a quarter of all Scotland’s pearl mussel watercourses.’