Invasive oysters in KZN

By: Dr David Pearton
ORI Senior Scientist

South Africans love their seafood, particularly around the festive season where champagne and oysters are a particular indulgence. But the roots of our relationship go back a great deal further than that; the prehistoric inhabitants of the Cape were gathering various shellfish over 160,000 years ago.  Our methods have obviously evolved since then and cultivation of various shellfish is now a massive global industry.  In South Africa we rely on both cultivated and wild-gathered oysters to supply our local demand.  Non-native Pacific oysters (Crassostrea gigas) are cultivated in the Western and Eastern Cape (e.g. Saldana Bay and Knysna) while the native Cape Rock Oyster (Striostrea margaritacea) is picked by licenced small-scale gatherers along the East Coast, particularly in KZN.

A potential issue with the cultivation of non-native species is the potential for them to develop into invasive species that can have negative impacts on native species and environments.  The Pacific Oyster (C. gigas) has become established as an invasive species in a number of places around the world, such as North America, Australia and Europe.  In South Africa, previous studies showed that there were only a few, very limited non-self-sustaining populations in the Eastern and Western Cape.

Recent genetic studies on the oyster populations along the shoreline and in the estuaries of the Eastern Cape and KZN by ORI and the University of Stellenbosch have, however, discovered thriving populations of oysters on the Wild Coast and in KZN, particularly in Durban and Richards Bay harbours and even in protected areas such as the Umhlatuze estuary.  This dramatically extends the known range of this alien species and suggests that is has become established along the whole of the east coast of South Africa.  The potential impacts of this on native oysters and ecosystems is currently unknown but will be actively monitored by ORI researchers in the future.