Australia usually hosts the world's biggest cuttlefish mating event - not this year

Australia, typically the site of the world's largest cuttlefish mating event, experienced a significant decline this year.

Each winter, a specific stretch of the South Australian coastline becomes the gathering point for the planet's most extensive mating ritual of giant cuttlefish. In some years, hundreds of thousands of these unique marine creatures, native to Australia, converge along the Cuttlefish Coast for their annual breeding season. Their vibrant, shape-shifting forms fill the shallow waters, drawing thousands of enthusiastic divers.

However, this year's turnout was the lowest ever recorded, raising fears of a "catastrophic decline" in the species. Official figures released this month estimate that only 1,811 cuttlefish arrived to mate, a stark contrast to last year's 63,000 – representing a 97% drop. The state government suggests that a toxic algal bloom, which has affected the state's coastline for over 18 months, might be responsible for this sharp decrease. The low numbers have also led to calls for the species to be classified as endangered.

Adult cuttlefish typically perish after breeding, making each mating season vital for the species' survival. Yet, in recent months, only 440 eggs have been counted, a drastic reduction from the millions usually laid.

Manny Katz, 28, was one of four divers who spent hours along the coast last month observing the annual breeding event, a task assigned by South Australia's Department of Environment and Water (DEW). "Because there have been so few cuttlefish, DEW thought it would be worth going out and trying to do an assessment of how many eggs there currently are around the coastline to understand what the population is going to be next year," he explained.

Over two days in July, his team conducted eight one-hour dives, meticulously examining the undersides of rocky reefs to count the eggs. "Along the eight kilometres (five miles) of coastline, there would normally be millions of eggs," he stated, noting that each female can lay dozens of eggs and tens of thousands usually participate in breeding. "[We] pretty much looked under every single rock that we could have and we only found about 440 eggs."

Crucially, adult cuttlefish have a lifespan of only about 12 to 18 months, Katz said, "so the success of one breeding season is going to determine the population next year." He added, "Under our current circumstances, with there being so few eggs along the coastline, we don't know if the population is ever going to be able to recover."

While it's premature to definitively predict the species' future, Katz commented, "we will know that answer probably in the next year or two, whether or not this year has been a catastrophic decline." His team also monitored adult cuttlefish numbers during the dive survey, observing only about a dozen – when tens of thousands should have been present – and noted unusual behavior. "This year, they were all really skittish... hiding under rocks. Because there's so few cuttlefish hanging out at the moment, they don't have strength in numbers."

Katz was not surprised by the low numbers this winter, given the severe impact of the toxic algal bloom in South Australia since its initial detection in March of last year. This algal bloom – a rapid increase in the population of algae in water systems – is a natural phenomenon but is exacerbated by ocean warming, marine heatwaves, and nutrient pollution, all linked to climate change. "Our marine habitats have been getting wiped out for the past year and a half now... this is an indicator that our marine ecosystems are in real big trouble - the canary in the coal mine, so to speak."

Last year's cuttlefish mating season was unaffected as the algal bloom's effects had not yet reached the Cuttlefish Coast, Katz explained. However, since late last year, small concentrations of the toxic algae have been detected in the area. This is particularly concerning as cuttlefish, along with other cephalopods like octopus and squid, are highly susceptible to the toxic algae.

Stefan Andrews, co-founder of the Great Southern Reef Foundation, is advocating for the giant Australian cuttlefish to be listed as a threatened species due to its "catastrophic decline" caused by the algal bloom and its potential impact on future generations. "The harmful algal bloom is a leading plausible explanation and appears likely to have contributed, but the available evidence does not yet allow us to say definitively that it caused the entire decline," he stated. He noted that with so few adults and minimal breeding activity observed, the number of young entering the population is likely to be very low. "That will make the next breeding season especially challenging."

A spokesperson for South Australia's Department of Primary Industries and Regions stated that cuttlefish populations "naturally fluctuate" based on environmental conditions. Previous surveys have reported lows of 13,500 in 2013 and highs of 247,000 in 2020. "The cuttlefish population estimate rebounded to more than 100,000 just two years following the low numbers in 2013," the spokesperson said. "While it appears likely the algal bloom has impacted cuttlefish numbers, researchers are working to determine whether this contributed to the reduced population or was just one of the potential impacts."

Prof Mike Steer, executive director of the South Australian Research and Development Institute, which assisted with the underwater surveys, described this year's unprecedented low estimate as "disheartening." However, he remained optimistic, stating, "we know from the past that this population has the capacity to recover when conditions are favourable." He pointed to "positive signs" as adult cuttlefish continue to lay eggs, suggesting the population may recover "over coming years."