Didymo (rock snot)

Whitebaiters, anglers, kayakers, trampers and other freshwater users need to know how they can help to prevent the spread of didymo.

Didymo, or rock snot, is an invasive Northern Hemisphere alga that was first discovered in South Island waters in 2004. It is not yet known to be in the North Island.

Didymo in the South Island. It attaches itself to streambeds by stalks and can form large mats on the bottom of streams, rivers and even lake edges. Thick growths can adversely affect freshwater fish, plant and invertebrate species.

It can be spread easily, with a single cell in a drop of water able to start a new colony when released into a different waterway.

The Taranaki Regional Council is part of a Taranaki Regional Didymo Group that also includes the Department of Conservation, Fish and Game Taranaki, Venture Taranaki, Trust Power and Taranaki iwi.


Check Clean Dry 

When moving between waterways, whitebaiters, anglers, kayakers, trampers and other freshwater users are urged to:

  • Check equpment for any debris and leave it at the site.
  • Clean all equpment with detergent, bleach or hot water.
  • Dry equipment and do not use it for 48 hours.

All items should be cleaned for at least one minute with a 5% solution of dishwashing liquid or nappy cleaner, dried to the touch and left for another 48 hours before use in another waterway.

Felt-soled waders require soaking for at least 40 minutes in hot water kept above 45C or soaking for 30 minutes in hot water kept above 45C containing 5% dishwashing detergent or nappy cleaner. Rubber-soled footwear is easier to clean and dry.

If cleaning an item is not practical, it should be allowed to dry. Once touch dry, it should be left dry for another 48 hours.

Freezing any item until solid will also kill didymo. If cleaning or drying is not practical, restrict equipment to a single waterway.

Absorbent items require extra treatment, either with prolonged soaking in water heated to 45 degrees or above, or by being frozen solid. People who hold fishing licences are banned from using felt-soled waders or wading boots when angling for sports fish such as trout. The ban applies throughout New Zealand, including the Taupo fishery.

Whitebaiters should note that although salt water will also kill didymo cells, seawater in whitebaiting areas is ineffectual because it is too diluted. Whitebaiters can prepare a 2% salt solution (saltier than seawater) by adding a small cup (200 ml) of table salt to water to maek 10 litres. Soak equipment for as long as it takes to thoroughly saturate the item, plus at least an additional 10 minutes.