Didymo set to spread by irrigation network

Didymo has been found in the Rangitata Diversion Race and will soon spread to the Rakaia, Ashburton and Hinds rivers.

Biosecurity New Zealand said yesterday there was no point in shutting down the RDR as the algae could already be in the other rivers.

The RDR takes Rangitata River water for power generation, stock water and irrigation, and discharges into the Ashburton, Rakaia and Hinds rivers.

The algae was discovered by a local Fish and Game representative when carrying out routine work.

Biosecurity New Zealand operations manager Jeff Donaldson said the biomass of didymo indicated the algae could have been present for up to three months.

A sample from just below Coal Creek, which is upstream of the race's intake, tested negative for didymo. Biosecurity would re-test Coal Creek and take further samples from the upper Rangitata to determine the extent of the spread.

Mr Donaldson believed it would have the biggest impact on irrigators with algae building up on equipment and blocking intakes.

Department of Conservation area manager George Hadler said the spread of didymo into braided rivers would have a significant impact on fisheries and their food sources.

The algae would have a big impact on galaxiid (whitebait) species.

The Peel Forest Enhancement Group was disappointed with the recent find in its area. However, it remained committed to protecting the upper Rangitata, including maintaining the vehicle washing stations already in place.

Fish and Game were disappointment at MAF's ongoing willingness to grant permits to irrigation companies to transport didymo.

Large irrigation schemes discharge water directly to other rivers and some district council stock water schemes transfer water between waterways.

Fish & Game have suggested that for smaller discharges, discharging to ground via soakpits would be an effective barrier to didymo spread.

Biosecurity didymo response manager Chris Bicknell said the find increased the potential for river users to spread didymo into other rivers in the region. It was a reminder of the importance of decontaminating all water equipment when moving between waterways.

He urged water users to check, clean and dry their gear between every waterway.

- © Fairfax NZ News