Didymo - also known as 'rock snot' - is a type of algae. It can form massive blooms on the bottom of streams, rivers and lakes, and its spread is highly undesirable. Didymo impacts on water quality, biodiversity and economic and social values.

Why is it a pest?

Didymo has the ability to form large blooms in continuous mats often confused with soggy toilet paper. It grows on river and lake rocks as pinkish-brown blogs at the surface and woolly white-cream blobs in the water. This microscopic pest can be spread by a single drop of water. Even if you can't see it, you could be spreading it.

The alga is a native of northern Europe and North America, and was first reported in New Zealand in 2004.

Didymo has been confirmed in various lakes and rivers in Otago however there are 600 waterways in NZ that still require safeguarding. It is not yet present in the North Island of New Zealand.

The South Island is a controlled area for didymo. This makes it a legal requirement to clean all gear used in the water before going from one waterway to another.


Management and control 

To prevent the spread of freshwater pests, including didymo, whenever you move between waterways you must check, clean and dry any equipment that comes into contact with water. 

Before you leave any river or lake:

Check - remove any plant matter from your gear and clothing and leave it at the site. Don’t wash plant material down the drain. 

Clean - soak or scrub all items using one of the below treatments:

Cleaning option Amount Duration
Dishwashing detergent or nappy cleaner  10% mix (1 litre to 10 litres of water)  Soak or spray all surfaces and leave wet for at least 10 minutes 
Bleach 2% mix (200mls to 10 litres of water)  Soak or spray all surfaces for at least 1 minute 
Hot water above 60°C 

Soak entire item


Soak for at least 1 minute


Hot water above 45°C 

Soak entire item

Soak for at least 20 minutes 

Freezing   Until solid 

Note: 60° C is hotter than most tap water; 45°C is uncomfortable to touch. Allow longer times for absorbent items. 

Dry - ensure your gear is completely dry to touch, inside and out, then leave to dry for at least another 48 hours before you use it. Didymo can survive for months on moist gear.  

If it’s wet, it’s a threat. 

If you come across any suspicious looking algae in Otago rivers, please contact:

  • Otago Regional Council’s Pollution Hotline on 0800 800 033, or
  • Ministry of Agriculture and Fisheries on 0800 809 966.

Remember to check, clean and dry all equipment, before you leave any lake, stream or river.

Read our technical report on didymo in Otago

Sections 1-3
Sections 4-5
Sections 6-8

Appendix 1
Appendix 2
Appendix 3

External Links

Unwanted organism

A number of species have been declared nationally as Unwanted Organisms. For the most up-to-date list of Unwanted Organisms, visit the MPI website.

Unwanted Organism status means that it is prohibited from sale, propagation and distribution in accordance with sections 52 and 53 of the Biosecurity Act. Didymo is not a pest to be managed under our Pest Plan, however, unwanted organisms may be controlled in other ways in accordance with our Biosecurity Strategy.


Didymo caught on fishing line

Didymo caught on fishing line


Didymo on rocks

Didymo on rocks


Didymo on rocks

Didymo on rocks

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