The invasive algae didymo has been confirmed in the Kaipo River in northwest Fiordland, one of the region's most remote catchments, prompting a renewed plea to anglers, hunters, trampers and packrafters to check, clean and dry their gear.

The Department of Conservation said a recent survey had confirmed the pest in the river, which it described as pristine, important for freshwater biodiversity and popular with recreational users. DOC and Southland Fish & Game announced the find on 6 October.

Southland Fish & Game senior ranger Josh Tabak said the vast majority of Fiordland's rivers and tributaries remained free of didymo and the agencies wanted to keep it that way. Didymo significantly affected freshwater insect populations, he said, and because native fish and birds relied on those insects the effects moved up the food chain. The algae also detracted from the Fiordland experience that drew so many visitors to the region.

He said the Kaipo discovery was a reminder to follow the check, clean, dry routine every time people moved between waterways.

Cleaning kills it

DOC senior freshwater adviser Susan Emmitt said thoroughly cleaning and drying equipment, including when moving between waterways, was proven to kill didymo, and the same habit helped stop other freshwater pests such as lagarosiphon, hornwort and freshwater clams from spreading.

Packrafters, trampers, hunters and anglers all shared responsibility for cleaning their gear before entering Fiordland and when moving between catchments within it, she said.

The advice comes in three steps:

  • Check: before leaving a waterway, remove any weed, mud or pests and leave them on the bank, and drain all river or lake water from gear.
  • Clean: rinse gear with tap water and treat absorbent materials with an approved treatment; Biosecurity New Zealand lists the options.
  • Dry: once gear feels dry to the touch, leave it for a further two days before using it in another waterway.

Didymo is classed as an unwanted organism under the Biosecurity Act. It is already present in Lake Te Anau, Lake Manapōuri and the Waiau River, and has been found in creeks along the Routeburn Track between Harris Saddle and Lake Mackenzie, according to DOC's track information. The Irisburn River on the Kepler Track is among the waterways that remain clear of it.

The Kaipo find is a setback precisely because of where it is. Rivers in the remote northwest of the national park are reached mainly by air or on foot, which is why the agencies are putting the emphasis on the people most likely to carry the pest in: those who fly or walk into the back country and move between catchments.

Fish & Game and DOC will keep monitoring Fiordland waterways for didymo and other pests. DOC's website sets out special conditions for freshwater users in Fiordland.