Didymo on march across South Island

Didymo is spreading at a fast rate in the South Island.

Researchers say there are now 132 rivers and six lakes infected with the pest and have projected it to spread through 1800 rivers by 2024.

Waikato University scientists claim 70 per cent of freshwater ways in the South Island are ideal for didymo populations.

It is costly. A 2006 report assessed the potential value impact of didymo on New Zealand to be between $58 million and $285m over the eight years 2004-05 to 2011-12.

The economic analysis quantified the cost in financial terms of the potential impacts identified by the didymo MAF Technical Advisory Group, taking into account the potential range, rate of spread and impacts on several financial sectors.

Waikato University biodiversity researcher professor Craig Carry, said the long-term effect of didymo on freshwater fish was still unknown.

"It fouls the rivers and lakes, and looks awful. It blocks water intakes and streams. The economic impact has to be considered as well."

Prof Carry said didymo was introduced to New Zealand from Canada and the United States, where it is reported to be a major concern. It has spread to Mexico.

In New Zealand, the microscopic pest was first discovered in Southland in 2004 and quickly spread through Otago and the rest of the South Island.

There have been no reported cases of it in the North Island.

Mr Carry said the containment of didymo in the South Island could be attributed to a well organised outreach programme and early warning systems.

Ministry of Agriculture and Fisheries (MAF) Biosecurity New Zealand has declared the entire South Island a controlled area for didymo.

This means waterways remain open to angling and other recreational activities, but people are legally obliged to prevent the spreading of the plant.

Mr Carry said cost effective ways of controlling the pest had been implemented.

"We can test up to 100 samples per week and will be working in collaboration with the National Institute of Water & Atmospheric Research Search should there be any reports of it in the North Island."

Biosecurity New Zealand has commissioned a number of research reports into the effects of didymo.

The objective of the science and technical programme is to collate available information and reduce uncertainties underlying MAF Biosecurity New Zealand decision-making on response options and long-term management options for didymo.

- © Fairfax NZ News