Every winter, a sliver of the South Australian coast hosts the world’s largest mating event for giant cuttlefish.

Some years, hundreds of thousands of the unique marine species endemic to Australia gather along the Cuttlefish Coast for the annual mating season, bringing their colourful, shape-shifting figures to the shallow waters and attracting thousands of keen divers.

But this year, the turnout was the lowest on record, sparking concerns of a “catastrophic decline” of the species.

Official figures released this month estimate just 1,811 cuttlefish turned up to mate this year, down from 63,000 last year – a 97% drop.

The state government says a toxic algal bloom that has ravaged the state’s coastline for more than 18 months may be to blame for the sharp decline.

The low figures have also prompted calls for the species to be listed as endangered.

Adult cuttlefish typically die after breeding so each mating season is crucial for the species’ future, but in recent months, just 440 eggs have been counted, instead of the millions that are usually laid.

Manny Katz, 28, was one of four divers who spent hours along the coast last month to observe the annual breeding event, having been tasked 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.”

Over two days in July, his team did eight one-hour dives, exploring 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 said, as each female can lay dozens of egg and tens of thousands usually turn up to breed.

“[We] pretty much looked under every single rock that we could have and we only found about 440 eggs.”

Crucially, adult cuttlefish only live about 12 to 18 months, Katz said, “so the success of one breeding season is going to determine the population next year”.

“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 too early to tell what this means for the species’ future, Katz said: “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 tracked adult cuttlefish numbers during the dive survey, and said the dozen or so they saw – when there should have been tens of thousands – appeared to be behaving differently.

“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 said he wasn’t shocked by the low numbers this winter given the devastating impact of the toxic algal bloom in South Australia since it was first detected in March last year.

The algal bloom – a rapid increase in the population of algae in water systems – is naturally occurring but is caused by ocean warming, marine heatwaves, and nutrient pollution – all exacerbated by 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 not impacted as the effects of the algal bloom had not yet reached the Cuttlefish Coast, Katz said, but since late last year, small concentrations of the toxic algae have been detected in the area.

This was particularly worrying as cuttlefish, along with other cephalopods like octopus and squid, are particularly vulnerable to the toxic algae.

Stefan Andrews, co-founder of the Great Southern Reef Foundation, is seeking to have the giant Australian cuttlefish listed as a threatened species given its “catastrophic decline” due to the algal bloom and the impact on the next generation.

“The harmful algal blooma is a leading plausible explanation and appears likely to have contributed, but the vailable evidence does not yet allow us to say definitively that it caused the entire decline,” he said.

He said with so few adults and so little 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 said cuttlefish populations “naturally fluctuate” depending on environmental conditions.

Previous surveys have reported record lows of 13,500 in 2013 to 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 helped carry out the underwater surveys, said the unprecedented low estimate this year was “disheartening”.

But he remained hopeful as “we know from the past that this population has the capacity to recover when conditions are favourable”.

He said there were “positive signs” as adult cuttlefish were continuing to lay eggs and the population may recover “over coming years”.