A crackdown on illegal pearl fishing is being launched following a spate of recent incidents threatening a vulnerable species.

Investigators in the Riverwatch initiative have found three cases of illegal fishing for freshwater pearl mussels recently in different rivers in West Sutherland.

In the biggest incident, thousands of empty shells were found discarded in a river, resulting in most of the population in the river being killed.

Experts say it is likely to take decades for the population to recover as pearl mussels are very slow-growing.

Scotland's rivers, including those in Assynt, contain many of the world's most important remaining pearl mussel populations.

Conservationists are now launching a Riverwatch scheme in Assynt to raise awareness of the threats to freshwater pearl mussels.

Natalie Young, a riverwatcher for the Pearls in Peril Project, said: "Pearl fishing damages already-threatened populations of a species that is significantly important to the Highland region as it is one of the few remaining strongholds for the mussel species.

"We are committed to taking action to prevent further illegal acts of fishing and we are now working with Police Scotland and other partners to ensure that this is stamped out in this area of Assynt."

The Riverwatch scheme works in collaboration with landowners, local communities, river users, fishery boards and trusts, and Police Scotland.

It urges people to look out for piles of shells in the river or the bank, people wading in the river with a glass bottom bucket, or any works on rivers that may disturb a pearl mussel habitat, and report any suspicious activity to the police or to the riverwatcher.

Pearl fishing involves breaking open the mussel shell to look for pearls, which only very rarely form inside the mussel.

Chief Inspector Colin Gough, wildlife crime co-ordinator for the Highlands, said: "Police Scotland fully supports the Riverwatch scheme operating in the west of Sutherland, which assists with disseminating information to reduce crime against freshwater pearl mussels, in addition to encouraging the reporting of illegal pearl fishing.

"Protected by law, freshwater pearl mussels are a very important part of a river's habitat, with illegal pearl fishing having the potential to devaste their fragile populations.

"Police Scotland will continue to work in close partnership with the Pearls in Peril LIFE + project by supporting their river patrols, investigating any reports of this type of crime and undertaking enforcement action wherever possible."

The Riverwatch initiative is particularly focused on a number of local Special Areas of Conservation (SAC) that have been designated as sites of European importance for the conservation of pearl mussels under the EC Habitats Directive.

These include the Abhainn Clais an Eas and Allt a'Mhuilinn SAC and the Inverpolly SAC near Lochinver, and the Loch a'Mhuilinn Woodlands SAC near Kylesku.

Freshwater pearl mussels are similar in shape to common marine mussels but grow much larger, and can grow as large as the size of a hand.

They also live far longer and can live for more than 100 years, making them one of the longest-lived invertebrates.

As well as being threatened by illegal pearl fishing, they are also extremely vulnerable to water pollution and engineering work in rivers.