Further warning of a so-called red tide in the Knysna estuary on the Cape south coast was issued by SA National Parks on Monday.
"Authorities – including SANParks, Knysna Municipality and the Knysna Basin Project – are cautioning against the collection of shellfish from the Knysna estuary until further notice," it said in a statement.
What is a red tide?
A red tide is the common name for an algal bloom involving large concentrations of red or brown-coloured micro-organisms. These can become concentrated in filter feeders such as mussels and oysters, making them poisonous to humans.
Other types of shellfish, including abalone, are also affected.
SANParks said possible evidence of a red tide was seen from the Knysna Heads on Sunday afternoon.
The warning follows a similar one issued for the area two weeks ago.