Ostrea edulis is found naturally from the Black Sea to the Norwegian coast and is the native oyster to the British Isles. It is a bivalve mollusc that has and oval or rounded shell, grows to around 100mm, is permanently attached to the substratum on which it settles, feeds by filtering nutrients from the water and will typically live five to ten years. Over the centuries Ostrea edulis has been harvested by humans to be used as a food source, once considered a paupers food it is now considered a delicacy. Once abundant in its native waters, the biomass of Ostrea edulis has dropped significantly due to a combination of over harvesting, disease, and the introduction of non-native species. Aquaculture is commonly used to produce the oysters in greater numbers, with other measures being taken to ensure the continued survival of established colonies. These include monitering and restricting the movement of the oyster world wide to stem the spread of disease and halt the introduction of non native predatory species or those competing with the oyster for resources. Less dense oyster beds inhibit the spread of disease and in the Fal estuary in Cornwall dredger men can only collect the oyster under sail or man power, limiting the catch and ensuring the continued survival of the local oyster population.