A clam-filled future for the Waikato River

Saturday, 12 April 2025

With eradication of gold clams in the Waikato River ruled out, it’s now unclear what will happen to the health of the country’s largest river.

Removing invasive gold clams from the Waikato River is impossible, says a Waikato University scientist, but their containment is now critical for the rest of the country’s vulnerable waterways.

Biosecurity New Zealand said will no longer pursue the possibility of removing gold clams from the river after they were first found at Bob’s Landing in Lake Karapiro in May 2023.

Freshwater gold clams have joined koi carp, gambusia (mosquito fish), catfish, rudd, and alligator weed as unwanted residents of the country’s largest waterway.

Waikato University’s Dr Ian Duggan said eradication of gold clams was ‘impossible’ and the best we could hope for is their continued containment.

Associate Professor Dr Ian Duggan is an invasion biologist at Waikato University, specialising in biosecurity science and invasive species ecology, and said eradication of gold clams from the river was impossible.

He said in the coming years, millions of dead clam shells would be found in the river.

“We can’t get rid of possums or deer, and these will be far more difficult than that.

“There has never been a successful eradication of gold clams anywhere where it has invaded globally.

“The best that can be done is suppression to reduce its effects on native species, and to try as much as possible to reduce the chances of it moving to other drainage basins.

Karen Thompson is one of the NIWA scientists taking part in a $10m project to study gold clams in the Waikato River.

“Unfortunately, by the time it was detected, eradication was already out of the question.”

Duggan said previously that gold clams would thrive in the mild climate of the Waikato River and “conditions in Lake Taupō would be ideal for them as well”.

The invasive freshwater clam has the potential to destroy native habitats, clog water supplies and hydroelectric dams, and could infest Lake Taupō if not properly contained.

Waikato River Authority co-chairperson Danny Loughlin said the organisation “remains concerned about the potential spread of invasive freshwater gold clams within the Waikato River catchment and beyond”.

Gold clams are very easy to find at Lake Karapiro. A study is underway to find out what will happen to the rest of the Waikato River after authorities gave up trying to remove them.

“We are particularly worried about the risk of gold clams moving into areas that are currently unaffected, including lakes, upstream rivers and streams, and the Waipā River,” he said.

“There is also a serious risk of this pest spreading to other parts of the country.”

He said research would be critical in making decisions about clams in the future.

“At this stage, we are not aware that eradication is impossible in all parts of the river, so we will continue to investigate every opportunity.”

NIWA freshwater ecology principal scientist Dr Deborah Hofstra is leading a Ministry of Business, Innovation, and Employment-funded $10m research programme on the invasive gold clam.

Clams have been found at Mercury’s Karapiro power station but have not caused any issues so far, the company says.

The five-year research programme has three aims.

“First to understand the ecological, economic, and cultural impacts of the gold clam.

“Second, to predict dispersal and future establishment, enabling the prioritisation of locations for monitoring and early intervention.

“The third aim of the research programme is to develop a suite of effective tools to reduce the risk of transfer and to manage the gold clam.”

She said in researching their potential impacts, the programme is looking at the habitats most at risk, as well as native species that may be threatened by competition for resources.

“Controlling gold clam populations can be challenging and costly.

“This programme also includes research to find the control methods best suited to natural environmental conditions in New Zealand.”

Hofstra said that research would focus on developing the knowledge needed to predict the further spread of the clams in the Waikato River and other waterways.

“The importance of predicting the clams’ spread and knowing where to look for surveillance cannot be underestimated, because early detection provides the best opportunity to take action.

“Early detection allows a quick response with the potential of local eradication of newly detected, not well-established populations.”

Mercury runs nine hydro-electric power stations on the Waikato River, and its generation head of operations Matt Kedian said regular maintenance checks at its Karāpiro, Maraetai and Waipapa hydro stations had turned up gold clam populations.

“Our checks show that while the clams are present, they have not caused any damage or increased frequency of maintenance activities to the hydro station infrastructure.

“All of our hydro stations continue to operate normally … however, we share the ecological concerns of others over the impact of the gold clam on the Waikato River and are taking it seriously.”

Kedian said he was looking forward to the outcome of NIWA’s research.

“There are a lot of organisations and groups who depend on the river, and we are committed to working alongside them on this … to share information to help all of us effectively monitor and track the clam.”