Thousands of sea dragons, pipefishes and seahorses died during South Australia’s devastating algal bloom, in what ecologists say is the worst mass mortality event for the fish family globally.
More than 1,600 weedy and leafy sea dragons, 1,400 pipefishes and 160 seahorses washed up as the harmful algae spread along the state’s coastline, according to analysis of citizen scientist records by Dragon Search SA.
The state is home to more than 30 species of Syngnathidae, a family that includes sea dragons, seahorses, pipefishes and more. At least 13 were affected by the toxic bloom, which began in March 2025 and still lingers at one site in southern Spencer gulf.
Weedy and leafy sea dragons, found only in Australia, were often some of the first mortalities observed, as the algal bloom spread up and down the coast.
“Pipefishes and seahorses washed up mainly over winter and spring of 2025, but records of dead sea dragons continued for more than a year,” the report said.
Prof David Booth, a marine ecologist specialising in sea dragons at the University of Technology Sydney, said scientists, governments and citizens were all “very concerned” about the future of the state’s marine emblem, known as “leafies”. “South Australian populations have probably been very much decimated, which is really sad, that’s the centre of their world distribution.”
Weedy sea dragons, which dominated the tally with 1,524 specimens recorded, recently had their conservation status upgraded to vulnerable by the International Union for the Conservation of Nature. The leafy sea dragon’s status was also being reassessed in light of the bloom.
The large and sustained mortalities and the loss of habitat, including kelp and seagrasses, would probably have long-term consequences, the report said.
“There’s so much habitat destruction, huge loss of sponges, kelps, seagrasses, all those sorts of things that these animals need, and their food, little shrimps, that we really need to be working on,” Booth said. “There’s habitat restoration going on but it is a big job.”
Scientists were also looking at other solutions, including rearing the sea dragon young in aquariums, and even artificial structures like seahorse hotels, which might help pipefish.
Dr Yolanda Waters, a dive instructor and chief executive of Divers for Climate, said the losses were “extremely concerning”, and divers were often among the first to observe the bloom’s impacts underwater, she said.
“It’s been a really up and down journey for the last 18 months, watching the ocean change, and we’ve all honestly still got our fingers crossed because we don’t know what’s going to happen long-term.”
Stefan Andrews, a marine biologist and co-founder of the Great Southern Reef Foundation, said the scale of the mortalities was “mind blowing”, especially for animals like sea dragons, which were rare to see and found in only a handful of locations along the coast.
More than 800 species have been affected in Australia’s first bloom of Karenia cristata, a rare and toxic type of algae deadly to marine life.
Surveys and reports were beginning to put numbers on the devastating toll, such as the catastrophic decline in giant cuttlefish this year, research made possible through the contributions of citizen scientists and divers.
“We saw so many people just step up and do their bit to report observations,” he said. “Sometimes mortalities that wash up on the beach could have disappeared in a matter of hours. So to have all of this data and information that scientists can analyse deeply is so useful to get an understanding of what’s going on.”
