Thousands of seadragons are dying in Australia’s toxic algal bloom

An algal bloom in South Australia has caused mass deaths of many species since March – now researchers warn that leafy and weedy seadragons could be facing the threat of extinction

By James Woodford

24 July 2025

Leafy seadragons rely on camouflage to avoid predation

Alastair Pollock Photography/Getty Images

One of the world’s most extraordinary fish could be in danger of extinction due to a massive bloom of toxic algae engulfing parts of the southern coast of Australia.

Leafy seadragons are in the same group of fish that includes seahorses and pipefish. They are covered in leaf-shaped protrusions that allow them to blend in perfectly amid forests of seaweed.

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Now they and their relative, the common weedy seadragon , are being found dead in their thousands along hundreds of kilometres of the South Australian coast.

It has already been a tough few years for the animals, with enormous storms driven by the La Niña weather system resulting in the deaths of hundreds of weedy seadragons in April 2022 in Sydney alone.

Then, beginning in March this year, a massive algal bloom of the species Karenia mikimotoi broke out in Gulf St Vincent near Adelaide, South Australia. This has resulted in mass deaths of fish and other marine life, with dolphins, sea lions and even great white sharks washing up on beaches.