The population of giant Australian cuttlefish gathering along South Australia’s coast for their annual breeding season has fallen to its lowest recorded level, triggering concerns over the future of the species and prompting calls for it to be given threatened-species protection.
Official figures released this month estimated that only 1,811 giant cuttlefish arrived along the Cuttlefish Coast this year for the winter mating season, a dramatic 97% decline from the 63,000 recorded last year. The sharp fall has raised fears that the species, which is found only in Australian waters, could be facing a serious population crisis.
The annual gathering is considered one of the world’s largest mating events involving giant cuttlefish. During the breeding season, the marine animals congregate in the shallow coastal waters, where their ability to rapidly change colour and shape makes them a major attraction for divers and marine enthusiasts. In some years, hundreds of thousands of cuttlefish have gathered in the region.
The latest figures have also raised concerns about the next generation of cuttlefish. Adult giant cuttlefish generally live for only about 12 to 18 months and typically die after breeding, making each annual mating season critical to maintaining the population. However, underwater surveys conducted this year found only about 440 eggs along an eight-kilometre stretch of coastline, compared with the millions normally expected.
Manny Katz, a diver who participated in the survey for South Australia’s Department of Environment and Water, said the extremely low number of eggs was particularly worrying. His team of four divers conducted eight one-hour dives over two days in July, examining rocky reefs and searching beneath rocks where cuttlefish typically lay their eggs.
Katz said that millions of eggs would normally be found along the surveyed coastline, but the team discovered only around 440. He said the small number of eggs could have serious implications because the success of a single breeding season has a direct impact on the population in the following year.
The unusually low number of adult cuttlefish was also evident during the survey. Divers reportedly saw only around a dozen adults in areas where tens of thousands would normally be expected. Katz said the animals appeared more nervous and were frequently hiding beneath rocks, possibly because they no longer had the protection that comes from gathering in large numbers.
Researchers believe the prolonged toxic algal bloom affecting South Australia’s coastline could be one of the major factors behind the decline. The algal bloom, which was first detected in the state in March last year, has persisted for more than 18 months and has affected marine ecosystems across the region.
Although algal blooms can occur naturally, their frequency and intensity can be influenced by factors including rising ocean temperatures, marine heatwaves and nutrient pollution. Climate change can worsen several of these conditions. Cephalopods such as cuttlefish, squid and octopus are considered particularly vulnerable to the effects of toxic algae.
The bloom had not reached the Cuttlefish Coast during last year’s breeding season, meaning the 2025 population was largely unaffected. However, smaller concentrations of toxic algae were detected in the area from late last year, raising concerns that exposure may have contributed to this year’s collapse in numbers.
Stefan Andrews, co-founder of the Great Southern Reef Foundation, has called for the giant Australian cuttlefish to be considered for threatened-species protection. He said the dramatic decline in adult numbers and the extremely low breeding activity could mean that very few young cuttlefish enter the population next year.
However, scientists have cautioned that it is too early to establish a direct link between the algal bloom and the entire population decline. A spokesperson for South Australia’s Department of Primary Industries and Regions said cuttlefish numbers naturally fluctuate in response to environmental conditions.
Historical surveys show that the population has experienced significant variations before. Numbers fell to about 13,500 in 2013 before rising to a record high of around 247,000 in 2020. Authorities noted that the population had recovered to more than 100,000 within two years of the 2013 low.
Professor Mike Steer, executive director of the South Australian Research and Development Institute, which assisted with the underwater surveys, described this year’s estimate as disheartening but said there was reason for cautious optimism. He pointed to the species’ previous ability to recover when environmental conditions improve and noted that some adult cuttlefish were still laying eggs.
Researchers are now expected to closely monitor the population and breeding activity over the coming years. The next one or two breeding seasons could prove crucial in determining whether this year’s unprecedented decline represents a temporary setback caused by unfavourable environmental conditions or the beginning of a much more serious population collapse.
