Several dozen freshwater pearl mussels have been killed by poachers in Assynt in the north west Highlands.
Assynt Field Club said the shellfish were forced open in the "off chance" one or more might contain a pearl.
The group said the poaching had happened at some point during the last two months.
Freshwater pearl mussels, which are a protected species, play a part in healthy river ecosystems and are found in some of Assynt's rivers.
Assynt Field Club urged anyone who believed they were witnessing illegal pearl hunting to call police, and not to approach the suspected poachers.
Other incidents of poaching in recent years have included more than 100 being killed at a river south of Lochinver in the Highlands.
Scottish Natural Heritage has previously warned that freshwater pearl mussels may be extinct from several rivers in Scotland due to poaching.
Pearl mussels are similar in shape to common marine mussels, but can grow larger and live for up to 130 years in fast-flowing rivers.
Early in their lifecycle they live harmlessly on the gills of young salmon and trout.
Poaching, loss of habitat and pollution are among the reasons for dramatic declines in their numbers.
Scotland's Highlands and Islands are among Britain's last strongholds for the critically endangered species.
In history, Julius Caesar's admiration of pearl mussels is cited by his biographer as a motive for the first Roman invasion of Britain in 55BC.