Among the world’s most remarkable fish, the leafy seadragon may soon face extinction due to widespread blooms of toxic algae affecting parts of Australia’s southern coast.

The elegant leafy seadragons (Phycodurus eques) belong to the same family as seahorses and pipefish and are adorned with leafy appendages that allow them to seamlessly blend into underwater kelp forests.

Unfortunately, both they and their relatives, the common weed seadragon (Phyllopteryx taeniolatus), have been discovered dead along extensive stretches of South Australia’s coastline.

A major storm, propelled by La Niña, has already led to a challenging year for marine wildlife. In April 2022, hundreds of dead weed seadragons washed ashore in Sydney alone.

This year, large blooms of algae from the species Karenia Mikimotoi emerged in March, affecting the Gulf St. Vincent near Adelaide, killing numerous fish and other marine organisms while even washing ashore dolphins, sea lions, and great white sharks.

Surfers and swimmers have experienced illness due to these algae blooms, and multiple oyster harvesting areas have been shut down for several months due to health concerns. There are fears that the harmful algae will spread along the southern coast of the continent.

The South Australian Government has indicated that the source of these blooms is a marine heatwave that began in September 2024, leading to average temperature increases of over 2.5°C that persisted even into winter. Floods on the Murray River in 2022 and 2023 washed excess nutrients into the Gulf St. Vincent near Adelaide, compounded by unprecedented upwellings of nutrient-rich waters in subsequent years.

This week, both state and federal governments have allocated $28 million AUD to address the ongoing crisis.

Janine Baker, an independent marine ecologist leading citizen science initiatives to gauge the scale of this disaster, states that thousands of seadragons have perished since the bloom began.

While no toxicological results have been confirmed from the seadragon carcasses, Baker notes that the die-off aligns with areas affected by harmful algae blooms. “Seadragons have small gill openings and side head openings that can easily become clogged with algae. Additionally, they struggle to swim efficiently in dense algae masses,” she explains.

The significant mortality of adult seadragons means that the population available for breeding from late 2025 to early 2026 will be severely diminished. “This encompasses not only the loss of adult individuals essential for reproduction each year but also the elimination of next-generation seadragons that could reproduce for up to 10 years,” comments Baker. “This leads to a significant reduction in the number of young seadragons that could be born in areas afflicted by the intense blooms during this time frame.”

David Booth from Sydney’s Institute of Technology states that the potential risk of extinction resulting from the ongoing event remains uncertain. “Nonetheless, the scale of the die-off across a crucial section of the seadragon habitat is profoundly concerning.”

Weedy sea dragons had recently been classified as vulnerable due to declining populations in Tasmania. “Now, with the suspected mortality events in South Australia, I fear there are serious implications for the species in New South Wales as well, but we won’t have a clear picture until thorough investigations are conducted at key seadragon locations following the bloom,” says Booth.

Christopher Keneally from the University of Adelaide remarks that the environmental catastrophe at hand is glaringly apparent, with toxic foam on the beaches, algae aerosols making surfers and swimmers ill, and marine life washing ashore. He warns that as climate change persists and ocean temperatures rise, we can anticipate a surge in such environmental crises.

Many algal bloom cells may settle in sediments, setting the stage for a continuation or resurgence of the blooms next summer, Keneally adds.