Thousands of sea dragons, pipefish, and seahorses have died along South Australia’s coastline as a result of a harmful algal bloom, marking one of the most severe mass mortality events recorded for the Syngnathidae family, which includes these species. The bloom, caused by the rare and toxic algae Karenia cristata, began in March 2025 and continues to persist in parts of southern Spencer Gulf.

Data collected by citizen scientists and analysed by Dragon Search SA revealed that the die-off has affected at least 13 Syngnathidae species. More than 1,600 weedy and leafy sea dragons, 1,400 pipefish, and 160 seahorses were recorded as washed ashore or found dead amid the prolonged algal outbreak. The weedy and leafy sea dragons, species endemic to Australia, were often among the earliest casualties noted as the bloom spread along the coast.

Marine ecologist Professor David Booth from the University of Technology Sydney expressed significant concern about the future of South Australia’s sea dragon populations, particularly the leafy sea dragon, which serves as a marine emblem for the state. He noted that the event likely led to substantial population declines in what is considered the core range for these animals. Recently, the conservation status of weedy sea dragons was upgraded to vulnerable by the International Union for Conservation of Nature, and reassessments for the leafy sea dragon’s status are underway in light of these mortality figures.

The prolonged algal bloom has also caused widespread habitat destruction, including extensive losses of kelp, seagrasses, and sponges essential to the affected species. These habitats provide food sources such as small shrimps critical to the diet of these fish. Booth highlighted ongoing efforts towards habitat restoration but acknowledged the scale of damage and recovery work required.

Additional conservation strategies under consideration include captive rearing of juvenile sea dragons and the installation of artificial shelters, colloquially termed “seahorse hotels,” aimed at supporting pipefish populations.

Local dive instructor and chief executive of Divers for Climate, Dr Yolanda Waters, described the losses as deeply troubling. Divers, often among the first observers of underwater impacts, have documented dramatic environmental changes over the past 18 months, with uncertainty about the long-term repercussions.

Stefan Andrews, a marine biologist and co-founder of the Great Southern Reef Foundation, emphasized the unprecedented scale of the mortality event, particularly for rarely encountered species like sea dragons. He noted that over 800 marine species in the region have been affected by this first recorded bloom of Karenia cristata in Australia.

The involvement of citizen scientists and diving communities has been vital in collecting timely data, helping researchers quantify the ecological toll, such as the concurrent catastrophic decline of giant cuttlefish populations. Andrews stressed the importance of this collaborative approach, noting that shore-washed mortalities can disappear rapidly, making prompt reporting essential for scientific analysis and understanding.