Didymo (Rock Snot)

​​​​​Hand holding up didymo (rock snot)Didymo (Didymosphenia geminata), also called Rock Snot, is a freshwater algae that is widespread in the Northern Hemisphere. It is highly invasive and is considered a significant biosecurity pest in Australia. Didymo is is categorised as Prohibited Matter under the Biosecurity Act 2019 and its entry into Tasmania is prohibited. Its primary spread pathway is via contaminated fishing equipment.

Where is didymo present?

Didymo occurs naturally in the cool, low nutrient waters of northern Europe and the northern parts of America. Since the mid 1980s, however, it has taken on characteristics of an invasive species in its original range and colonised many new locations throughout North America, Canada, Europe, parts of Asia and Great Britain. In 2004 it was discovered in New Zealand, invading waterways in the South Island.


Didymo is a diatom (type of algae) invisible to the human eye until cell colonies form. Didymo is made up on millions of microscopic cells that can't be seen until a large colony has formed - by which stage it's almost impossible to eradicate. It attaches to rocks and submerged plants by stalks and can multiply rapidly to form a thick brown layer completely smothering the stream or lake bed. These massive blooms grow flowing 'rat's tails' that can turn white at their ends and appear similar to tissue paper. It can spread quickly, forming massive blooms in waterways or lake edges and can have a significant impact on the insect life that many species of fish rely on for food. It also affects water quality and can pose a risk to waders and swimmers by making riverbeds slippery. Once established, didymo blooms can adversely affect water quality, aquatic invertebrates and fish stocks and are a hazard for hydroelectric generation, agricultural irrigations and recreational pursuits. Currently there is no 'cure' for didymo. Preventing further spread relies on freshwater users cleaning aquatic equipment between waterways.

Didymo may be confused with Tasmanian native species of algae but can be distinguished by;

  • Touch - although it looks slimy, it doesn't feel slimy, but rather spongy and scratchy like cotton wool

  • Strength - didymo attaches very securely to river stones and does not fall apart when rubbed between your fingers

  • Colour - didymo is beige/brown/white but not green

  • Odour - live didymo has no distinctive odour

  • Microscope - definitive identification requires microscopic analysis.

How does it spread?

It takes only one didymo cell in a single drop of water for the algae to spread between waterways. Its primary spread pathway is via contaminated aquatic and fishing equipment between waterways.

Reducing the risk of invasion into Australia

Hand holding up didymo (rock snot)The Government is working to prevent the introduction of didymo to Australia at Biosecurity entry points. Anglers who are visiting Australia or returning home from a fishing trip overseas are now required to declare all used fishing equipment for inspection. Any potentially contaminated fishing or other freshwater recreational equipment will be confiscated by the Department of Agriculture and Water Resources and treated at the traveller's expense.

The following flyer contains important information on how to avoid bringing didymo back into Australia from New Zealand. All anglers returning with their fishing gear from New Zealand are asked to pay particular attention to clean-                                    down procedures.

Going Fishing in New Zealand? (360 KB)

Biosecurity form: Checklist for Boaters, Fishers and Divers Travelling to Tasmania


Biosecurity Enquiries