Didymo and other threats

Important information

Felt-soled waders and wading boots may not be used by anglers. This restriction applies around the country. Felt and fabric soles are difficult to clean they are a high-risk potential vector for the invasive freshwater algae didymo. More information on size limit and wading boot rules.

In early growth stages didymo forms small
bubbly colonies and a thick brown layer on
rocks. Although it appears slimy, it feels like
wet cotton wool

What is didymo?

Didymo (Didymosphenia geminata), commonly known as "rock snot", is a freshwater alga native to the northern hemisphere. It was first discovered in New Zealand in 2004 and has now spread to a number of rivers in the South Island.

Didymo prefers to establish in cool, moderate flowing waterways that have stable riverbeds and water flows. Here it may exist as harmless colonies on submerged rocks, boulders and gravel, or it can "bloom", forming extensive underwater carpets over river beds.

Didymo can form thick dense finger-like
clumps

What does didymo look like?

Severe blooms have the potential to change the habitat for insects trout feed on by smothering the stones, displacing the large desirable insects (mayflies, caddis and stoneflies) and encouraging the proliferation of small, less nourishing species like midges and snails.

This could lead to reduced fish growth, smaller average size and condition and possibly fewer trout. It may similarly potentially cause adverse effects on indigenous river dwelling birds (such as blue duck), fish and insects.

In the advanced stage, didymo forms long
streaming filamentous carpets, several
centimetres long. Streamers turn white at
their ends and fragments float downstream,
similar to clumps of tissue paper

Intense blooms make angling very difficult, and some methods, like those practiced in Taupo rivers, maybe impossible due to gear becoming continuously clogged with didymo. Reduced water clarity means rivers are visually less attractive and fish are harder to spot.

Didymo is most readily spread on contaminated gear used by river users such as anglers and kayakers when they move from one river to another.

To help prevent didymo from spreading and establishing in high value freshwater sites, some waterways may have access restrictions placed upon them.

Clean, Check, Dry to help prevent the
spread of didymo and other freshwater
pests

What can you do?

• Do not move between catchments on the same day.
• Treat every river as potentially infected
• Remember to always Check, Clean, Dry all equipment that comes into contact with a waterway.

To Check, Clean, Dry:

• CHECK: 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.
  
• CLEAN: There are several ways to kill didymo. Choose the most practical treatment for your situation which will not adversely affect your gear.

Non-absorbent items

• Detergent: soak or spray all surfaces for at least one minute in 5% dishwashing detergent or nappy cleaner (two large cups or 500mls with water added to make 10 litres); OR
  
• Bleach: soak or spray all surfaces for at least one minute in 2% household bleach (one small cup or 200mls with water added to make 10 litres); OR
  
• 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:

• Hot water: soak for at least 40 minutes in hot water kept above 45 °C; OR
• Hot water plus detergent: soak for 30 minutes in hot water kept above 45 °containing 5% dishwashing detergent or nappy cleaner; OR
• Freezing any item until solid will also kill didymo.
• DRY: 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.

NOTE: The thicker and denser the material, the better it will be at holding moisture (and live cells), the slower it will be to dry out and the more difficult it will be to soak completely with cleaning solutions.

Who is responsible?

MAF Biosecurity NZ (MAFBNZ) is the lead agency for didymo. MAFBNZ have also led the development of the didymo Long-Term Management Programme which involved various partners working together to help prevent the spread of didymo.

To report a suspected didymo sighting, call the MAF Biosecurity NZ exotic pest hotline on 0800 80 99 66

The Taupo Fishery Area of the Department of Conservation actively supports MAF Biosecurity NZ in their efforts as part of our management of the Taupo fishery.

back to top