Didymo

The invasive exotic algae Didymosphenia geminata (didymo) has been confirmed in a number of South Island rivers.  All river users are urged to be on the lookout for the algae, and take care not to spread it from one waterway to another.

On this page

Why is Didymo a pest plant?
How do I identify Didymo?
Suggested ways to avoid spreading didymo
Read more about Didymo

Why is Didymo a pest plant?

Didymo - also known as 'rock snot' - is a freshwater diatom (a type of algae.) It can form massive blooms on the bottom of streams, rivers and lakes, and its spread is highly undesirable.

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.

Didymo

How do I identify Didymo?

The alga attaches itself to the streambed 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.

View of large mats from above

View of large mats from above

As the 'tails' of the algae get longer they become white in colour

As the 'tails' of the algae get longer they become white in colour

In Otago, didymo has been found in Lake Dunstan, the Clutha, Hawea and Von rivers, and possibly their tributaries. It has also been identified in a number of other rivers in the South Island.

Suggested ways to avoid spreading didymo

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.

Read more about didymo

Didymo in Otago: A Summary

This report has been written to provide a summary of the biology, ecology and impacts of Didymo in the Otago context. You can read this publication:

Download Section 1-3 (677KB)

Download Section 4-5 (1479KB)

Download Section 6-8 (143KB)

Download Appendix 1 (1714KB)

Download Appendix 2 (1607KB)

Download Appendix 3 (5149KB)

Didymo in Otago fact sheet 2008

Download Didymo in Otago - Surface water qualityfact sheet 2008 (513 KB)

Didymo Positive sites with map

 Didymo positive sites with map (57 KB)

Links

www.biosecurity.govt.nz/pests-diseases/plants/didymo/index.htm
www.envirosouth.govt.nz
www.fishandgame.org.nz

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