‘Breeding machines’ set to swamp Waikato River

Friday, 21 November 2025

What looks like a white mist floating in the Waikato River is actually millions of gold clam larvae looking for a place to stay.

It’s breeding season for the region’s waterways worst enemy as millions of invasive freshwater gold clams begin their breeding cycle in the nutrient rich waters of the Waikato River.

“But this is no cause for celebration,” says Earth Science NZ (formerly NIWA) principal technician Karen Thompson who says freshwater gold clams (Corbicula fluminea) are “breeding machines” that can each produce around 70,000 larvae per season.

“Tiny clams have recently been seen in both adult clam and water column samples collected from Lake Karāpiro, meaning we are in a period of heightened risk of spread,” Thompson said.

Gold clam larvae are almost impossible to detect with the naked eye. Here is one under a microscope attached to a neoprene wetsuit used in the Waikato River.

The clams were first discovered at Bob’s Landing in Lake Karāpiro in May 2023 but were likely in the river for two or three years before being found.

Since then, they have made their way down river to Port Waikato, and up river to just outside Lake Taupō, and Lake Maraetai.

Earlier this month, they were detected for the first time outside of the Waikato, in Taranaki’s Lake Rotomanu.

The Ministry for Primary Industries gave up trying to eradicate them from the region’s waterways in March this year, and they are likely to be here to stay.

Gold clams have been breeding in the Waikato River, and river users being asked to ‘check, clean and dry’ even more thoroughly this summer.

“Our team have found the next generation is already maturing in adult clam brood pouches getting ready for release,” Thompson said.

She said gold clams are hermaphroditic, so they possess both male and female reproductive organs and self-fertilise - a significant advantage for their rapid population growth.

“Adult clams have a lifespan of one to five years and are known to spawn multiple times per breeding season, depending on environmental conditions.”

Karen Thompson is part of a $10 million, five-year project aimed at studying the ecology and impacts of gold clams in New Zealand.

Thompson says the clams’ juvenile phase presents a distinct biosecurity challenge as each clam releases larvae by the thousands into the water column, and are easily dispersed by water currents and human activities.

“At roughly 0.2 mm, which is smaller than a grain of sand, these juveniles are not visible to the naked eye, meaning they can easily go undetected on clothing, equipment, craft or in water removed from infested areas,” she said.

An Earth Sciences NZ-led five-year research programme aims to stop the invasive clam from spreading further and protect our important native species.

“To do this, we are working to find effective, culturally attuned ways to manage this invader,” said programme lead and principal scientist Dr Deborah Hofstra.

“We all have a role to play by following ‘check clean dry’ procedures any time we are in our lakes and rivers.

“This is particularly important during the heightened risk of the clams breeding season,” Hofstra said.

“Invasive gold clams have been given the legal status of an ‘Unwanted Organism’ under the Biosecurity Act. This means that people must not knowingly move them or water that may contain them.

“They are also not safe to eat.”

During clam population surveys, Earth Sciences NZ has found gold clams at all nine of its survey locations, and to depths of up to 15 metres.

The numbers of clams at the boat ramp at Maraetai and at Waipuke Park at Karāpiro have increased dramatically compared to 2024, while others showed little change or remained low.

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

Associate Professor Dr Ian Duggan, an invasion biologist at Waikato University specialising in biosecurity science and invasive species ecology, 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.

“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.

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”.