The annual mass aggregation of giant Australian cuttlefish in South Australia’s Upper Spencer Gulf experienced a catastrophic collapse this year, with official surveys counting just 1,811 cuttlefish—a 97% drop from last year and the lowest number ever recorded.
Environment Pulse Desk: Every winter for decades, one of the strangest and most beautiful natural spectacles on Earth has played out in the shallow waters of the Upper Spencer Gulf. Tens of thousands of giant Australian cuttlefish — sometimes as many as 200,000 — converge on the rocky reefs off Whyalla to mate, turning the seafloor into a shifting canvas of colour as the animals flash and pulse their way through courtship. It’s a phenomenon found nowhere else on the planet, and it draws divers from every corner of the globe.
This year, almost none of them came.
Official surveys this season counted just 1,811 cuttlefish along the aptly named Cuttlefish Coast — a 97% collapse from the roughly 63,000 seen last year, and the lowest number ever recorded. Fewer than a few hundred eggs have turned up so far, in a place that normally hosts millions.
“Something We Haven’t Seen Before”
Researchers are still piecing together what went wrong, but the leading theory points to the ocean itself. South Australia’s coastline has been gripped by a toxic algal bloom for more than a year and a half now, and scientists suspect it’s playing a major role in the crash. Unusually warm water earlier in the season may have compounded the problem, potentially scrambling the migration and breeding signals the cuttlefish rely on.
“This is something we haven’t seen before,” says Associate Professor Zoe Doubleday, a marine ecologist at the University of Adelaide who has studied the aggregation for years.
The scale of the absence has stunned even people who know these reefs intimately. Local dive operators describe stretches of seafloor that should be alive with colour and movement now sitting almost silent — divers combing wide areas and turning up fewer than 50 animals in total.
A Season Cancelled
The human toll has been immediate. Tourism operators scrapped the entire 2026 season outright. One of Whyalla’s biggest dive operators turned away thousands of would-be visitors and issued refunds across the board once surveys made it clear there simply weren’t enough cuttlefish to see.
In response, the South Australian government moved to protect what’s left of the population, declaring a 90-day exclusion zone around the key breeding sites. Anyone caught disturbing the animals during that window faces fines of up to $100,000.
Why Every Season Counts
The stakes here are higher than they might first appear. Giant cuttlefish live fast — just 12 to 18 months from hatching to death — and adults typically die shortly after breeding. That means there’s no margin for error: a single failed aggregation doesn’t just mean a disappointing season for tourists, it potentially means a badly weakened generation to come.
The Cuttlefish Coast earned a place on Australia’s National Heritage List back in 2023, a recognition of just how singular this event is in the natural world. Now, scientists and conservationists are calling for far more intensive monitoring and research, hoping to understand — and ideally head off — whatever combination of forces conspired to nearly empty these reefs in a single season.
