Didymo is an invasive algae that can spread by a single drop of water. Once it’s established in a waterway, it’s virtually impossible to eradicate. It significantly impacts water quality and reduces native freshwater biodiversity.
Description
- Forms a thick pink-brown layer on rocks and submerged plants in freshwater.
- Sometimes forms flowing ‘rat tails’ which are woolly and white/cream at the end. This can resemble wet toilet paper in the water or when washed onto land.
- Appears slimy but feels like wet, gritty cotton wool.
What you need to know
Didymo can degrade the quality of water, restrict recreational activities such as fly fishing, and compete with native plants, invertebrates, fish, and some bird species. It can smother 100 per cent of stream/river beds and looks unsightly. Large blooms can block water intakes.
Management approach
Didymo is declared an Unwanted Organism by the Ministry for Primary Industries (MPI).
The South Island is a controlled area for didymo, which was first detected in 2004.
This makes it a legal requirement to clean all gear used in the water before going from one South Island waterway to another waterway. So far, no didymo has been found in the North Island.
Any species declared a pest cannot be sold or be in a place where plants are being sold. Pests cannot be propagated, bred, multiplied, communicated, released, cause to be released, or otherwise spread.
Control
Once didymo infests a waterway, it’s almost impossible to eradicate. It can spread through a single drop of water and cannot be seen with the naked eye until large blooms have formed.
If you find didymo, do not attempt to remove it as you could spread it into new waterways via your clothing or equipment.
If you've been in a waterway and plan to move to another within 48 hours, you must clean all your gear that has been wet using the 'Check, Clean, Dry' method.
It’s vital that everyone uses the Check, Clean, Dry method on all equipment and vessels to stop freshwater pests.
Biological control
Biological control is currently not available for this species.
Report
MPI hold a database with all known didymo sites across Aotearoa/New Zealand. Report your didymo sighting to MPI on aquabiosecurity@mpi.govt.nz
Didymo on rocks in Arthurs Pass
Didymo underwater
