Jellyfish blooms in Cape Town send swimmers scrambling

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On Sunday morning, triathletes participating in a training event off Granger Bay were forced to abandon their swim due to thick fog and an influx of stinging jellyfish.

All week blooms of starlight or “night light” jellyfish have been washing up in the shallows along Cape Town’s Atlantic Seaboard.

Regular bathers at Saunders Rocks beach—known for its tidal pool and swimming cove—have also been steering clear due to sudden blooms of sea jellies.

“Unfortunately the sea was full of jellyfish, so I had to get over to the pool,” said one regular at Saunders on Friday.

Another Saunders swimmer shared photos of jellies that had infested the tidal pool.

An image taken last Thursday of jellyfish in the Saunders Rocks tidal pool. Image: Supplied

Starlight or “night light” jellyfish usually inhabit warmer coastal waters. Image: Supplied

A popular local marine conservationist, who goes by the name Dave The Dolphin Whisperer, shared a photo in the Bay of Sewage community WhatsApp group on Sunday.

The image, recently taken on the Atlantic Seaboard, showed a large bloom of the pinkish-purple jellies, along with the following caption:

“Warning! These “night light “ jellyfish sting! A good few “tri-athletes” got stung whilst swimming this morning off Granger Bay (race was cancelled due to fog and jellies) … saw some nasty welts round necks and faces.”

There was also debate on the group about whether the species is capable of stinging, with casual swimmers and divers in Cape Town claiming to have been stung.

Speaking to The South African, Dave noted that this particular species is an unusual visitor to the Atlantic side of the city.

“These starlight jellyfish, we don’t see them often but we’ve got a lot in at the moment,” he said.

“Today there were loads, I’ve never seen this many. They also don’t usually come into this side of Cape Town as they are a warm water species.

“Normally we get box jellies and moon jellies.”

A sign of ocean decline?

Blooms of these marine animals are often an indication of unhealthy seas.

Given the water quality concerns along the Atlantic Seaboard over recent years, it has reignited conversations about how the City of Cape Town discharges its sewage and wastewater.

Why jellyfish blooms may signal ocean decline:

Overfishingreduces predators and competitors, allowing jellyfish populations to grow.
Climate change accelerates jellyfish reproduction, especially in warming waters.
Nutrient pollution from agriculture triggers algal blooms, creating low-oxygen zones where jellyfish thrive.
Coastal development provides more surfaces for jellyfish polyps to settle and grow.
Ocean acidification affects many marine species but appears to have little impact on jellyfish.

Why jellyfish blooms aren’t always a bad sign:

Natural cycles cause periodic jellyfish population spikes, even in healthy ecosystems.
Ecosystem role—jellyfish contribute to marine food chains, especially when they die and sink to the deep sea.
Not all species are thriving—some jellyfish populations are actually in decline.
Resilience—jellyfish have survived mass extinctions, making them highly adaptable rather than just indicators of ecosystem failure.