What’s causing the mass fish deaths along the Western Cape coast?

Authorities have received extensive reports of pelagic fish mortality stretching along the Western Cape coastline, from Saldanha Bay on the West Coast to Gansbaai near Danger Point.

The die-off involves multiple species, though sardine remains the most frequently reported. Observations indicate the event spans a significant range, from immediate inshore waters to approximately 80 kilometres offshore.

Pelagic fish live in the open ocean rather than near the seabed.

In a message circulated to members of the commercial pelagic fishing industry, Mike Copeland, said investigations are underway. 

“As a precautionary measure, all purse seine operators are strongly advised to cease fishing activities immediately. This suspension will remain in effect until the underlying cause of the mortality is better understood,” said Copeland. 

A harmful algal bloom (HAB) has been confirmed offshore from Saldanha, which is known to cause paralytic shellfish poisoning (PSP). 

What is HAB, PSP and why does it matter?

Harmful algal blooms, sometimes referred to as “red tides” although they are not always red, occur when certain microscopic algae multiply rapidly.

Some species produce toxins that can affect fish, shellfish, seabirds, marine mammals and humans. Others can deplete oxygen in the water, causing marine animals to suffocate.

Blooms can be often triggered by a combination of favourable sea temperatures, sunlight and nutrient-rich waters. 

The algal bloom detected offshore from Saldanha is known to produce toxins that can accumulate in shellfish such as mussels, oysters and clams. If contaminated shellfish are eaten, the toxins can cause PSP, a potentially serious illness that affects the nervous system. Cooking or freezing does not destroy these toxins.

While a definitive link between the bloom and the mass mortality has yet to be established, officials are investigating the potential connection.

Government efforts

Efforts are currently underway to collect a comprehensive set of samples for laboratory analysis. These tests aim to identify the specific factors responsible for the event and to ensure the safety of the marine ecosystem.

Earlier this week, the Department of Forestry, Fisheries and the Environment (DFFE) said it is investigating reports of dead and dying sardines at several locations along the Western Cape coast. 

Department spokesperson Zolile Nqayi said fish have been reported washing ashore, while dead and dying sardines have also been observed drifting at the surface.

Sardines form one of South Africa’s most important commercial fish species, supporting the purse seine industry and providing food for larger fish, seabirds, seals and dolphins. A large-scale mortality event could therefore have ecological as well as economic implications if it continues.

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