Nearly 1,700 sea dragons have been recorded dead during South Australia's prolonged harmful algal bloom, adding to evidence of extraordinary losses across marine life along the state's coastline.
Leafy sea dragon in South Australian waters
A citizen-science analysis has documented 1,524 dead weedy sea dragons and 159 leafy sea dragons during the harmful algal bloom that has affected South Australian waters since early 2025.
The toll extends well beyond sea dragons. More than 1,400 pipefish and around 160 seahorses were also recorded, with at least 13 species from the Syngnathidae family affected.
Sea dragons appear particularly vulnerable. Their small gill openings can become clogged by microalgae, and their limited swimming ability makes it hard to escape dense patches of a bloom.
Divers help document the losses
The figures come from Dragon Search SA, which analysed 1,540 reports submitted by 253 people to the citizen-science platform iNaturalist between January 2025 and May 2026.
Divers, snorkellers and beachgoers have provided information that would otherwise have been difficult to obtain during an event spanning a large section of coastline. Dead sea dragons continued to be reported for more than a year after the bloom began.
The numbers do not, however, show precisely what proportion of South Australia's sea dragons has been lost. Researchers lack comparable systematic surveys from before the bloom, and populations farther offshore remain poorly documented. What is clear is that numbers in some previously monitored nearshore habitats are substantially lower.
An unprecedented bloom
The wider mortality event has been linked principally to Karenia cristata, a rare dinoflagellate now shown to produce brevetoxins. Peer-reviewed research estimates that the bloom affected at least 20,000 sq km and killed marine animals across hundreds of taxa.
A South Australian government investigation concluded that several ecological causes probably combined to create favourable conditions for the bloom: nutrient-rich water associated with the 2023 River Murray flood, unusually strong coastal upwelling and a marine heatwave.
Recovery may be difficult even after the bloom subsides. Kelp, seagrass and sponge habitats have also suffered, reducing habitat and food available to surviving sea dragons. Researchers are considering interventions, including captive rearing of young, but stress that restoring habitat will also be important.
