Why a 2025 algal bloom killed almost 1,700 South Australian sea dragons

Nearly 1,700 sea dragons have been recorded dead during South Australia’s harmful algal bloom, including 1,524 weedy and about 159 leafy individuals, according to a new Dragon Search SA report.

The bloom first appeared in early 2025 and spread across Encounter Bay, St Vincent and Spencer gulfs, Investigator Strait and the waters off Kangaroo Island, creating dense patches of micro‑algae.

Sea dragons belong to the Syngnathidae family. Their gill pores are very small, so micro‑algae can easily block them. When the water is full of algae, the dragons cannot clear their gills quickly or swim away from the thickest patches.

Dragon Search SA analysed 1,540 citizen‑science reports from 253 people on iNaturalist covering 1 Jan 2025 – 31 May 2026. The project was led by local marine ecologist Janine Baker, who coordinated the data collection.

"The weedy and the leafy in particular occur in South Australia as a world hotspot," said sea dragon researcher David Booth from the University of Technology Sydney.

Observations show that pipefish and seahorses mostly washed up in winter and spring 2025, while dead sea dragons continued to be seen from summer 2025 through autumn 2026, indicating the bloom’s prolonged impact on them.

These deaths highlight how a microscopic bloom can quickly overwhelm species with delicate respiratory structures, underscoring the need for close monitoring of water quality and early warning systems.

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