A major investigation has been launched by Scottish Natural Heritage (SNH) amid growing fears over the dwindling oyster population.

Once a common delicacy in restaurants throughout the country, the native or flat oyster (Ostrea edulis) is now a rarity and outnumbered by the farmed pacific oyster.

Factors blamed for their decline include overfishing, pollution, a lack of stock management and disease.

If oysters are sustainably managed they can be an important economic resource for Scotland

David Donnan Scottish Natural Heritage

Scientists say there is evidence of scattered oyster populations on the west coast of the country, but there is growing concern about the level of illegal fishing in the area.

David Donnan, maritime advisory officer at SNH, said it was vital the native oyster was preserved and its numbers successfully managed in the future.

He said: "This survey gives us the opportunity to find out what is happening to the oyster population in Scotland and what we can do to help it recover.

The Ostrea edulis was a delicacy

"If oysters are sustainably managed they can be an important economic resource for Scotland as well as a flourishing part of the ecosystem."

The numbers of native oyster were at their highest in the 18th and 19th centuries, before the days of overfishing and pollution.

The oyster survey is part of SNH's drive to boost biodiversity in Scotland and raise awareness of the country's animals and plants and their habitats.

Over 100 events are planned across the country ranging from walks, interactive games and exhibitions to mushroom hunting and wildflower planting.

Scottish Biodiversity Week runs from 31 August to 8 September.