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Photograph of didymo

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About didymo

Didymo (Didymosphenia geminata), also known as rock snot, was first detected in New Zealand in Southland in 2004. It has since been detected in several South Island rivers. The North Island is didymo free, and Waikato Regional Council is working with a number of other agencies to help keep it that way.

Didymo is a freshwater alga which attaches itself to streambeds by stalks. The stalks form a thick brown mat on rocks, plants and other materials in the water. Thick growths can adversely affect freshwater fish, plant and invertebrate species by reducing the number of suitable habitats. It has no human health risk.

Didymo has been confused with New Zealand native alga. The main difference between didymo and native species is the way it feels. Native alga feels slimy and will break apart in your fingers, whereas didymo is strong and feels like wet cotton wool.

As part of the Central North Island Didymo Regional Partner Group, Waikato Regional Council is working with DOC, iwi, power generation companies, Fish and Game New Zealand, fly fisherman and canoeists to develop a long-term management action plan for didymo. It will include publicity and other measures asimed at preventing infection of our region. A surveillance programme is underway for high risk sites to ensure early detection if didymo is brought to the region.

Currently, there is no 'cure' for didymo, and preventing further spread relies on freshwater users cleaning aquatic equipment between use in different waterways – regardless of location and perceived didymo risk.

We must remain vigilant to stop didymo infecting the region's waterways.To help prevent any possible spread, the following 'Check, Clean, Dry' procedures should be used.

Tips to prevent the spread

Photograph of didymo

We all need to remain vigilant to help stop didymo - the nasty brown and cotton wool-like 'rock snot' - from infecting our region's waterways. When travelling between waterways treat them as if they are contaminated with didymo - all equipment that comes into contact with water should be decontaminated.

Use the following 'Check, Clean, Dry' procedures.


Before you leave a river or lake, check items and leave debris at site. If you find any later, treat and put in rubbish. Do not wash down drains.


There are several ways to kill didymo. Choose the most practical treatment for your situation which will not adversely affect your gear. For non-absorbent items you can use the following methods.

  1. Detergent - soak or spray all surfaces for at least one minute in a solution that is five per cent dishwashing detergent or nappy cleaner.
  2. Bleach - soak or spray all surfaces for at least one minute in a solution that is two per cent household bleach.
  3. Snot Off - soak or spray all surfaces for at least one minute in a solution that is 0.5 per cent Snot Off commercial cleaner.
  4. Hot water - soak for at least one minute in very hot water kept above 60°C (hotter than most tap water) or for at least 20 minutes in hot water kept above 45°C (uncomfortable to touch).

Absorbent items require longer soaking times to allow thorough saturation. For example, felt-soled waders require soaking for at least 40 minutes in water kept above 45°C, or 30 minutes if the water contains five per cent detergent, nappy cleaner or Snot Off.

Freezing any item will kill didymo.


Drying will kill didymo but slightly moist didymo can survive for months. To ensure didymo cells are dead by drying, the item must be completely dry to the touch, inside and out, then left dry for at least another 48 hours before use.

If cleaning or drying is not practical, restrict equipment to a single waterway.

Cleaning equipment

When cleaning equipment, we recommend that you:

  • soak porous materials for longer than the specified decontamination times to ensure cleaning solution has soaked right through the item before soaking for the required decontamination time
  • choose a decontamination solution that will not adversely affect your equipment
  • follow manufacturer's safety instructions when using products
  • dispose of cleaning waste well away from waterways

Swimmers and other water users should wash with soap or shampoo before entering another waterway. Alternatively, ensure hair and skin is thoroughly dry before entering another waterway.

All boats and trailers should be cleaned thoroughly both inside and out for at least one minute with decontamination solution.

Outboard motor cooling systems should be flushed out with decontamination solution for the specified minimum of one minute's time. It may then be flushed again with clean fresh water that has come from a town water supply.

Excess water should be removed from the interior, including the anchor recess, by removing bungs and then washing interiors with a decontamination solution.

Flushing the interior of your boat with a decontamination solution and then use the bilge pump to expel residual water before bungs are opened. This ensures that the bilge pump is flushed with the solution, and that residual water within the pump will be free of live didymo cells.

Mats, carpet (including carpet on the trailer), the anchor rope and other absorbent materials should be thoroughly soaked with decontamination solution, allowing extra time for the solution to fully soak through the item.

Drying is an acceptable alternative method, provided that all components are completely dry to the touch, inside and out, and then left dry for at least another 48 hours before entering a different waterway.

If you do not want to decontaminate your gear, you should restrict use to a single waterway.

Further information

Information on this page sourced from the Ministry for Primary Industries (MPI).

Possible didymo sightings should be reported directly to MPI on 0800 80 99 66. More didymo information is available on the Ministry for Primary Industries website.

Related publications

Visit our Waikato Regional Pest Management Strategy.

'What makes a pest a pest? - A guide to Waikato's pest management future' download, order or pick up for free from our offices.

Pick up or order 'Plant me instead - Plants to use in place of common pest plants' for free from our offices.

Download the National Pest Plant Accord.

Pick up or order 'Poisonous plants and fungi in New Zealand - A guide for parents, schools and child minders' for $15 from our offices.