They're flat oysters (Ostrea chilensis), a different species from the Pacific oysters farmed further north, and the Foveaux Strait beds have been dredged commercially since the 1860s.
Bluff oysters are dredged wild from the cold, fast-moving waters of Foveaux Strait and eaten raw, straight from the shell, prized for a meatier, more intense flavour than farmed oysters elsewhere in the country. Southland's fish shops and the annual Bluff Oyster Festival serve them by the dozen each autumn.