Work will start on Monday to remove rock armour riverbank protection from sections of an Angus river to safeguard the future of freshwater pearl mussels.
The Pearls in Peril project (PIP) will see 873 metres of boulders removed from sections of the River South Esk and its White Water tributary in Glen Clova and Glen Doll.
The work is part of a £3.5 million UK project and the South Esk is a high priority site due to an increase in illegal exploitation reported in the area.
The bank protection was installed in the 1990s to limit riverbank erosion and channel movement.
This reduced river habitat quality for freshwater pearl mussels, salmon and trout.
The boulders’ removal will make the riverbank more accessible for water voles around one of the restoration sites.
This new population was only found last summer by PIP survey staff the first record of these rare mammals for this part of Glen Clova.
Dr Lorna Wilkie, project officer from the Pearls in Peril Project, said: “Freshwater pearl mussels in the River South Esk have been affected by pollution, illegal fishing, and river engineering.
“Habitat loss has made it impossible for populations to recover, but by increasing suitable habitat for juvenile mussels and spawning salmon PIP aims to increase their number and distribution.
“Although changes to the river environment will be immediately noticeable, we do not expect to see an increase in the mussel population for many years.
“Freshwater pearl mussels are very slow-growing they don’t start breeding until they are 15-20 years old and can live for up to 120 years.”
Freshwater pearl mussels live in the gravel beds of clean rivers and feed by filtering water and removing fine particles.
This helps keep rivers clean and benefits other species like salmon and trout.
The mussels are critically endangered and Scotland is one of their few sole strongholds.
Freshwater pearl mussels have historically been fished for the pearls they can produce, similar to an oyster.
However, they very rarely contain pearls and they are fully protected under law it is a crime to kill, injure, take or disturb them.
Chief Inspector Colin Gough of Police Scotland said: “We would encourage members of the public to remain vigilant when out and about over the summer and to report any suspicious behaviour to police.”
During the works there may be minor disruption to footpaths and tracks.
Signs and alternative routes will be in place with information displayed.