DATE 1 November 2005
No further didymo finds - but no guarantees given
The national Didymo survey has all but been completed with no further finds of the invasive alga. The last find was
reported a week ago from the Von River near Queenstown, bringing the total known affected rivers to eight. However,
river users should not assume any river to be free of Didymo and should continue to clean items that have been in
contact with water before using in any other New Zealand waterway, Biosecurity New Zealand Team Manager, Surveillance
and Incursion Response Amelia Pascoe says.
Didymo was confirmed in seven rivers (the Mararoa, upper and lower Waiau, Oreti, Buller, Hawea and upper Clutha) before
the national survey started on October 2. Didymo has subsequently been confirmed in the Von River which flows into Lake
Miss Pascoe says the results are good news, but do not guarantee that other rivers are free of Didymo.
“The best we can say is that following intensive surveillance using the most effective and efficient methods currently
available, we haven’t found any sign of Didymo outside of the eight known affected rivers. As we’ve said from the
beginning of this response, river users throughout New Zealand need to clean their equipment between waterways or Didymo
“It’s a microscopic organism, so the risk of spread won’t always be obvious. The sample from the Von River was three
cells only, with no algae of any sort visible to the naked eye.
“This is a personal responsibility issue. New Zealanders need to treat rivers with more care and ensure that they
practice good river hygiene at all times. “A significant public awareness campaign as been expanded to form part of the
established National Aquatic Weeds Awareness Campaign.
“The future of the controlled areas currently in place will be decided within the next few weeks and the development of
a strategy for ongoing river monitoring is also well advanced.”
The national survey sites were selected based on a NIWA study which rates every New Zealand river on its ability to
provide a suitable habitat for Didymo. Information about river values and local knowledge on use rates were also
considered when selecting specific sample sites. 14 samples not collected due to weather and river conditions along with
samples taken from sites where the public think they may have seen Didymo, will be processed during the next few weeks.
The survey involved over 800 hundred samples from more than 500 sites across the South and North Islands, with field
work done by Biosecurity New Zealand, Fish and Game, AgriQuality, Department of Conservation and regional and local
- Eight rivers are known to be affected by Didymo – Southland’s Mararoa, upper and lower Waiau and Oreti Rivers, Otago’s
Von, Hawea and upper Clutha Rivers, and the Buller River in the Tasman District.
- Didymo is an invasive Northern Hemisphere alga. It was first reported in New Zealand in Southland’s Mararoa and lower
Waiau Rivers in October 2004. However we will never know exactly how, when or where Didymo first arrived to New Zealand.
- In bloom Didymo forms large, unsightly brown mats, sometimes with streamers, that smother the riverbed. This affects
sources of food for fish and makes recreation an unpleasant experience.
- Didymo is easy to spread. It could only take one drop of affected water or damp equipment to introduce it to another
- Didymo has no significant human health impact. Exposure to affected water may irritate the skin or eyes. This is from
the silica in the cell structure.
- Despite structured surveys, public awareness and many suspect samples being submitted by various agencies from around
the country, the first confirmed find outside of Southland was the Buller River on 28 September. This was followed by
Hawea and upper Clutha in Otago, then the Oreti and upper Waiau in Southland.
- Little formal scientific study has been done on Didymo. The work done in New Zealand since last October has made New
Zealand a world authority.
An extensive public awareness campaign has started around the “Check, Clean, Dry” message.
- Check for visible clumps of Didymo before you leave the waterway
- Clean your equipment as recommended by Biosecurity New Zealand
- Dry any equipment that can’t be cleaned for 48 hours after it is completely dry
The public awareness campaign also currently features:
- Signs on the affected rivers. Didymo will become part of the National Aquatic Weed Pest Management programme, which
will see Didymo included on signs elsewhere. This work is ongoing.
- Newspaper and radio advertising
- Information on a variety of websites
- Emailing information to a wide variety of stakeholders, such as DOC, Fish and Game, Maritime New Zealand, Regional
Councils and river user groups for further distribution through their networks. One recreational user database used has
- Articles about the risk of Didymo to various groups through appropriate agencies
- Mailing of information to every fishing licence holder
- Information handed out at the border, DOC and regional council offices. This will be expanded to Information Centres
- Media coverage