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 found on a rockDidymo mats in a riverDidymo mats underwater

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. Biosecurity New Zealand has since declared the entire South Island an Outbreak Control Area for didymo. This means rivers and lakes remain open to angling, boating and other recreational activities, but all gear should be cleaned before going from one waterway to another.

In Otago, didymo is found in Lake Dunstan, and the Clutha, Hawea, Von, Fraser, Makarora, Wilkin, Motatapu and Matukituki rivers, and possibly their tributaries. The alga attaches itself to the stream bed by stalks and can form a thick brown layer that smothers rocks, submerged plants and other materials. It forms flowing 'rats tails' that can turn white at their ends and look similar to tissue paper.

Management and control 

All river users are urged to take care not to spread the alga from one waterway to another.

Before you leave any river or lake:

Check - remove all visible threads or clumps of weed and algae from gear and clothing.

Clean - soak or scrub all items of equipment for at least one minute with either:

  • Hot (60 degrees C) water, or
  • 2% solution of household bleach, or
  • 5% solution of salt or nappy cleaner, or
  • 5% solution of dishwashing detergent, or
  • 5% solution of antiseptic hand cleaner.

Dry - after cleaning equipment, or if cleaning is not practical (e.g. animals), dry until touch dry then leave for a further 48 hours before entering another waterway.

A number of cleaning stations have been set up in the Upper Clutha area for the summer, to help stop the spread of didymo. The unmanned stations will be identified with signage. People can pull in there to wash and disinfect boats and fishing gear that may have come into contact with infected waters.

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

  • Otago Regional Council 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.

View our factsheet on Didymo. 

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

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