Wildlife campaigners have launched an operation to protect endangered freshwater pearl mussel sites.
A Scottish Natural Heritage (SNH) study found that 75% of the internationally important sites surveyed have suffered significant and lasting criminal damage.
This ranged from illegal pearl fishing to unauthorised river works which destroyed whole pearl mussel populations.
SNH said that in one river system in the west Highlands, a bed of at least 600 pearl mussels has disappeared in the past five years, with pearl fishing the only explanation.
Police and SNH today launched Operation Caesar, urging members of the public to report any suspicious activity in a bid to protect the species.
Scotland has 21 designated sites for pearl mussels, with Scottish rivers holding around half of the world's population of the species.
Of the nine Special Areas of Conservation (SACs) and Sites of Special Scientific Interest (SSSIs), surveyed by SNH in 2008/2009, damage was found at seven of them.
Iain Sime of SNH said: "Our findings have been shocking, appalling even, and it is clear that we must undertake urgent work to save this species from almost certain extinction in some areas.
"Unauthorised engineering in one river seems to have destroyed the most significant pearl mussel bed of around 900 animals in the lower reaches of the river.
"Another site has seen the killing, in less than a day, of 50% of the mussels in the burn - making any recovery from the remaining animals extremely difficult. Operation Caesar has been launched to enlist the help of people in trying to stop these crimes."
The mussels are protected under the Wildlife and Countryside Act 1981 and crimes against them carry the possibility of prison sentences.
Freshwater pearl mussels can grow to over 15cm long and live for more than a century. They are dark brown to black in colour and live at the bottom of clean, fast-flowing rivers.
Police urged people to be on the lookout for any suspicious activity in and around rivers, from people searching in rivers and shells on banks to vehicles parked early in the day.
Environment Minister Roseanna Cunningham said: "It is clear that the pearl mussel continues to suffer from indiscriminate and illegal ransacking by determined criminals. It is a UK wildlife crime priority and SNH has been tasked with co-ordinating action to try and halt this decline."