Giant cuttlefish mating numbers plummet 970% in South Australia amid toxic algal bloom

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."

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.