Each winter, the shallow waters off South Australia host the world's largest gathering of giant cuttlefish — a brief, singular act of reproduction for animals that live only long enough to breed once before dying. This year, only 1,811 arrived where 63,000 came before, and the 440 eggs found across eight kilometres of seafloor stand in place of the millions that would normally seed the next generation. Scientists suspect an 18-month toxic algal bloom, fed by warming oceans and human-driven pollution, has quietly dismantled the conditions that made this ancient ritual possible. What unfolds nex
Australia's giant cuttlefish mating event hits record low amid toxic algal bloom
The canary in the coal mine for our marine ecosystems
Mark
So we're talking about 1,811 cuttlefish instead of 63,000. That's a number that's hard to wrap your head around. What does that actually mean for the species?
Mimi
It means the population has essentially vanished from the breeding grounds in a single year. And because these animals only live 12 to 18 months and breed once before dying, this year's breeding season is the entire future of next year's population.
Mark
So if there are only 440 eggs instead of millions, what happens next?
Mimi
We won't know for a year or two. Those 440 eggs—if they survive and hatch and make it back to the coast—will determine whether the population can recover or whether we're looking at something much worse.
Luke
But here's the thing: the government says populations naturally fluctuate. They rebounded from 13,500 in 2013 to over 100,000 by 2015. Why is this different?
Mimi
Because the toxic algal bloom is still there. It's been ravaging the coast for 18 months. Last time there was a low, the environment wasn't actively hostile to the species.
Mark
Is the algal bloom definitely the cause, though?
Luke
No. The researchers are careful about this. Stefan Andrews said the bloom is "a leading plausible explanation," but they can't say definitively it caused the entire decline. There could be other factors.
Mimi
True. But cuttlefish are particularly vulnerable to the toxins these algae produce, and the bloom appeared in the breeding grounds right after last year's successful season. The timing is hard to ignore.
Mark
What would recovery actually look like?
Mimi
If conditions improve—if the algal bloom clears and ocean temperatures stabilize—the population has shown it can bounce back quickly. But that's a big if.
Luke
And we're talking about a species that only breeds once in its lifetime. There's no safety net. One bad year cascades into the next.
Mark
So we're waiting to see what happens next winter?
Mimi
Exactly. Next breeding season will tell us whether this is a temporary crash or the beginning of something we can't reverse.
Le Pouls
- A 97% collapse in cuttlefish numbers — from 63,000 to just 1,811 — has turned one of the ocean's most spectacular mating events into an eerie near-absence.
- With only 440 eggs counted where millions should lie, the next generation of the species has effectively been erased before it began.
- An 18-month toxic algal bloom along South Australia's coast, intensified by climate change and marine heatwaves, is the prime suspect behind the catastrophic decline.
- Divers describe the few cuttlefish present as skittish and withdrawn — animals behaving like survivors rather than participants in a mass gathering.
- Conservationists are pushing for the species to receive threatened status, while officials note past recoveries and urge cautious hope pending next year's count.
- The next breeding season is now a critical threshold: if too few juveniles return, the population may cross a line from which recovery becomes impossible.
Each winter, the shallow waters off South Australia host the world's largest gathering of giant cuttlefish — a brief, singular act of reproduction for animals that live only long enough to breed once before dying. This year, only 1,811 arrived where 63,000 came before, and the 440 eggs found across eight kilometres of seafloor stand in place of the millions that would normally seed the next generation. Scientists suspect an 18-month toxic algal bloom, fed by warming oceans and human-driven pollution, has quietly dismantled the conditions that made this ancient ritual possible. What unfolds next breeding season will tell us whether we are witnessing a temporary wound or the beginning of an irreversible unraveling.
Every winter, the Cuttlefish Coast of South Australia transforms into the stage for the world's largest giant cuttlefish mating event — a spectacle that draws divers from across the globe. This year, only 1,811 of these colour-shifting animals arrived, down 97 percent from the 63,000 counted just twelve months prior, marking the lowest number ever recorded.
The stakes are unusually high for this species. Giant cuttlefish live only 12 to 18 months, breed once, and die — meaning each mating season carries the entire weight of the population's future. Diver Manny Katz, surveying eight kilometres of reef on behalf of South Australia's Department of Environment and Water, spent two days methodically checking the undersides of rocks. His team found roughly 440 eggs. In a normal year, millions would be there. The handful of adults they encountered seemed unsettled, hiding rather than displaying — a behavioural shadow of what the gathering should be.
State officials believe the cause is a toxic algal bloom that has been spreading along South Australia's coastline for more than 18 months, driven by ocean warming, marine heatwaves, and nutrient pollution. Cuttlefish, along with octopus and squid, are especially sensitive to the toxins such blooms produce. Last year's season was unaffected because the bloom had not yet reached the breeding grounds; by late last year, it had.
Katz describes the collapse as a canary in the coal mine for the broader health of marine ecosystems. Stefan Andrews of the Great Southern Reef Foundation is calling for the species to be formally listed as threatened, though he acknowledges the algal bloom remains a leading hypothesis rather than a confirmed cause.
Historical records offer some grounds for cautious hope — the population has rebounded before, climbing from a low of 13,500 in 2013 to over 100,000 within two years. Mike Steer of the South Australian Research and Development Institute called this year's results disheartening but noted that eggs are still being laid. The true reckoning will come next breeding season, when the number of juveniles returning to the coast will reveal whether this year was a temporary crash — or the beginning of something the species cannot survive.
Every winter, a stretch of South Australian coastline becomes the stage for the world's largest mating gathering of giant cuttlefish. In good years, hundreds of thousands of these colour-shifting marine animals converge in shallow waters, drawing divers from around the world to witness the spectacle. This year, only 1,811 showed up—a collapse so severe it marks the lowest count on record, down 97 percent from the 63,000 that arrived just twelve months earlier.
The numbers are stark enough on their own. But the real measure of the crisis lies in what happens after mating. Adult cuttlefish live only 12 to 18 months, which means they breed once and then die. Every breeding season is therefore the entire future of the species compressed into a few weeks. This year, divers surveying eight kilometres of the Cuttlefish Coast in July found approximately 440 eggs laid across the seafloor. In a normal season, millions would be scattered beneath the rocks.
Manny Katz, a 28-year-old diver tasked by South Australia's Department of Environment and Water to assess the breeding grounds, spent two days in July conducting eight one-hour dives with his team. They examined the undersides of rocky reefs methodically, looking under nearly every stone they could reach. The scarcity was immediate and unmistakable. "Along the eight kilometres of coastline, there would normally be millions of eggs," Katz said. "We only found about 440." The dozen or so adult cuttlefish they encountered behaved differently from previous years—skittish, hiding under rocks, lacking the confidence that comes from being part of a massive gathering.
State officials point to a toxic algal bloom that has ravaged South Australia's coastline for more than 18 months as the likely culprit. The bloom—a rapid, uncontrolled multiplication of algae in seawater—occurs naturally but is driven by ocean warming, marine heatwaves, and nutrient pollution, all intensified by climate change. Cuttlefish, along with octopus and squid, are particularly vulnerable to the toxins these algae produce. Last year's breeding season proceeded normally because the bloom had not yet reached the Cuttlefish Coast. By late last year, however, small concentrations of toxic algae began appearing in the area.
Katz frames the collapse as a warning signal. "Our marine habitats have been getting wiped out for the past year and a half now," he said. "This is an indicator that our marine ecosystems are in real big trouble—the canary in the coal mine, so to speak." Stefan Andrews, co-founder of the Great Southern Reef Foundation, is pushing for the giant Australian cuttlefish to be formally listed as a threatened species. He acknowledges that while the algal bloom is "a leading plausible explanation," the available evidence does not yet allow researchers to say with absolute certainty that it caused the entire decline. What is certain is that with so few adults breeding and so little egg-laying observed, the number of young cuttlefish entering the population next year will be extremely low, making the next breeding season especially difficult.
The state government's Department of Primary Industries and Regions notes that cuttlefish populations naturally fluctuate with environmental conditions. Historical records show the population has ranged from a low of 13,500 in 2013 to a high of 247,000 in 2020. After the 2013 low, numbers rebounded to over 100,000 within two years. Mike Steer, executive director of the South Australian Research and Development Institute, which helped conduct the underwater surveys, called this year's estimate "disheartening" but expressed cautious optimism. "We know from the past that this population has the capacity to recover when conditions are favourable," he said. He pointed to the fact that adult cuttlefish are still laying eggs as a positive sign, suggesting recovery could unfold over coming years if environmental conditions improve. The critical test will come next breeding season, when the number of young cuttlefish returning to the coast will reveal whether this year represents a temporary crash or the beginning of a collapse from which the species cannot recover.
Citations marquantes
Along the eight kilometres of coastline, there would normally be millions of eggs. We only found about 440.— Manny Katz, diver conducting the breeding survey
Our marine ecosystems are in real big trouble—the canary in the coal mine, so to speak.— Manny Katz, on what the cuttlefish collapse signals about ocean health
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