Red tide returns to SA’s West Coast: What it is, why it happens and why seafood warnings matter

Red tide sightings along South Africa’s West Coast have raised concern among beachgoers and seafood lovers this holiday season, with discoloured water, dead shellfish on beaches and glowing waves at night drawing attention to the phenomenon.

Musson says red tide is a natural but potentially dangerous marine event that occurs most often during summer.

What is red tide?

Red tide is caused by a rapid bloom of microscopic algae known as dinoflagellates. These single-celled organisms can multiply to extremely high concentrations in nutrient-rich water, particularly along the West Coast, which is influenced by the Benguela Current.

When deep, cold, nutrient-rich water rises to the surface through upwelling, it creates ideal conditions for these blooms to explode in number.

“At very high concentrations, some species produce toxins that can harm marine life and pose risks to humans,” Musson explains.

Why shellfish are a concern

Shellfish such as mussels and oysters feed by filtering water. During a harmful algal bloom, toxins can accumulate in their bodies, even if the shellfish appear fresh.

Eating contaminated shellfish can lead to illnesses such as paralytic shellfish poisoning (PSP), which can cause tingling, numbness and, in severe cases, breathing difficulties. Less severe exposure may result in intense gastrointestinal illness like food poisoning.

Cooking or freezing shellfish does not destroy these toxins.

“If you find shellfish washed up on the beach, don’t eat them. Heat and freezing won’t make them safe,” Musson warns.

Impact on marine life

High concentrations of algae can also reduce oxygen levels in the water as blooms die off and decompose. This oxygen depletion can cause fish deaths and force species like West Coast rock lobster to crawl out of the water in search of air.

Shellfish and other small marine animals may suffocate when their gills become clogged with algae, leading to the dead organisms often seen along affected shorelines.

Is it always dangerous?

Not all algal blooms are harmful.

Algae form the base of the ocean food chain, and lower concentrations area normal and healthy part of marine ecosystems.

Problems arise when blooms become extremely dense or toxic. Authorities monitor conditions closely and issue public warnings when shellfish harvesting or swimming becomes unsafe.

Musson notes that South Africa has extensive shellfish testing programmes along the West Coast, with samples checked frequently to protect consumers.

What about bioluminescent waves?

The glowing waves seen at night are also caused by dinoflagellates, but these are usually harmless. The light is produced through a chemical reaction when the water is disturbed.

“Bioluminescence is beautiful and often safe to swim in, as long as there aren’t warnings, strong smells, or large numbers of dead fish,” says Musson.

Heed official warnings

Current advisories apply to parts of the West Coast between Hondeklip Bay and the Olifants River estuary, where red tide activity has been most concentrated.

Authorities urge the public to avoid harvesting or eating wild shellfish in affected areas and to stay clear of beaches if strong odours or widespread fish deaths are present.

“When in doubt, stay out – and definitely don’t eat anything you’ve picked up yourself,” Musson says.

For more information, listen to Musson using the audio player below: