The freshwater gold clam (Corbicula fluminea). Photo: Supplied/ NIWA
Special DNA tests show that the country's newest pest - the freshwater gold clam - has not spread into waterways outside a stretch of Waikato River where it was initially found.
Biosecurity New Zealand says since its discovery near Lake Karāpiro earlier this year regional and district councils as well as iwi have been working together to monitor the moullusc, and discuss preventing its spread.
The clams can clog up waterways, irrigation pipes and stock water troughs as well as negatively affect native freshwater species habitats.
Biosecurity deputy director general Stuart Anderson said the DNA tests were thorough.
"It's a highly sensitive test and can detect minute amounts of DNA such as clam secretions which can be picked up in the water samples."
It was more effective than relying on vision inspections and samples were being taken along the Waikato River both inside and further away from the stretch of water where the clams have been found, Anderson said.
He was urging anyone who uses the river for activities such as fishing, boating and rowing to clean and dry their gear when they take it out of the water as the gold clam larvae, that were too small to be seen with the naked eye, may be extra unwanted passengers.
"Checking all your equipment such as nets and outboard motors is important. Clean and wash it down - then dry it before taking it to another waterway."
Six other fresh water bodies (lakes and rivers) in the North Island were being regularly tested to see if the clam has migrated to other areas but so far there have been no positive DNA results, Anderson said.
This was good news, he said.
A big worry was that the clams breed very quickly, he said, describing them as dirty white, tan or yellow in colour with an obvious ribbed shell that grows up to about three centimetres in length.