Scottish police forces and environmental agencies have issued a warning to pearl hunters.

They have told the public not to interfere with freshwater pearl mussels which are on the verge of extinction.

The mussels are a vital link in the life of salmon rivers but can also produce valuable gems.

The Kellie pearl, found in an Aberdeenshire river in 1621, is now in the Scottish crown jewels.

The Scottish pearl mussel is nearly extinct

The police said the mussels are a protected species and anyone taking, disturbing or harming them is liable to prosecution.

The authorities are renewing efforts to catch the illegal pearl fishers, launched two years ago under the title Operation Necklace.

The conservation body, Scottish Natural Heritage (SNH), said because of continuing raids on the mussel stocks, it is now one of the most endangered species in Scotland.

In 70% of the areas in which they were once found, the pearl mussel is now extinct or about to die out.

A spokesman for SNH said: "Even today criminals still raid mussel beds, ripping out mussels up to 80 years old, discarding the inedible mussel in their search for pearls."

'Wildlife crime'

Chief Superintendent Mike Watson of Grampian Police said: "These mussels are a protected species and taking them is very much a crime.

"We would like everyone to realise this and to leave pearl mussels undisturbed. Wildlife crime is of very great concern to us.

"We would also like members of the public to come to us with any details of mussel disturbance they come across so we can catch the criminals."

Scotland's biggest-ever pearl ended up here

John Ralston of SNH said: "The populations in Scotland have become so endangered that they are now on a par with the tiger in Asia.

"If we don't do something about their decline they will become extinct in Scotland. Pollution, the decline of salmon and river engineering works have all played a part in their disappearance.

"But illegal removal, whether by members of the public unaware of the law or deliberately by criminals, is a big concern."

Freshwater pearl mussels can live for 80 to 100 years but they only mature after about 10 -15 years so young mussels cannot form a sustainable population.

They thrive in clean, fast running, low nutrient waters which are low in calcium and have a mix of stones and sand on the bottom.