Gold clam numbers explode in Waikato River

Tuesday, 2 June 2026

Gold clams are thriving in the Waikato River. Teams from Earth Sciences NZ and the regional council are trying to find out why. From left to right are Fern Kumeroa, Mashanta Mohsin, Elizabeth Graham, Michele Melchior and Gordon Tieman.

They are literally everywhere - millions upon millions of invasive gold clams within touching distance from the banks of the Waikato River - it’s something that needs to be seen to be believed.

Since their discovery in Lake Karapiro at Bob’s Landing in May of 2023, gold clams - Corbicula fluminea - have made the river their home, from Port Waikato to Lake Maraetai.

According to Earth Sciences New Zealand and Waikato Regional Council staff and researchers, the river seems to be the perfect breeding ground for gold clams as they pull bucket after bucket of the invasive fresh water pest species from the river.

Divers aboard a boat on Lake Karapiro are pulling clams from depths of up to 30 metres.

It’s little wonder New Zealand’s scientific community have been continually warning of its risks with recent papers suggesting it will, at some stage, become a major problem for Hamilton and Auckland’s drinking water supplies.

The Waikato Times met with researchers at one of their multiple test sites at Waipuke Park on the southern side of the lake where Earth Sciences NZ survey lead and freshwater ecologist Elizabeth Graham said they were trying to find out why clams loved the river so much, and what they were doing to the ecosystem.

Gold clams are easy to find in Lake Karapiro but it’s illegal to move them.

At the edge of the lake, trays of sand, rocks and shell fragments are spread across tables as researchers sort through samples where it looked like there were more clams present than anything else.

Samples are taken by divers at different depths of the river and brought back to be sized and counted. Sieves are used to separate juvenile, young, and adult clams.

The freshwater New Zealand mussel, kakahi, is larger and darker than gold clams. Their existence in the Waikato River is under threat from clams.

Samples are taken back to Earth Sciences NZ labs at Ruakura in Hamilton for detailed analysis.

“At the moment, in terms of the Waikato River system, it’s all about containment because, as you can see, the numbers are huge,” Graham said.

She said the point of the research was to find ways to stop them spreading to the rest of the country’s waterways, and if they did, how they could be managed or destroyed - something that has not happened anywhere else in the world.

Earth Sciences NZ’s Elizabeth Graham and Mashanta Mohsin don’t need to go far to find thousands of gold clams in Lake Karapiro.

“We are not able to eradicate them from the river … at the moment.”

Researchers say efforts are now focused on identifying and responding to new incursions.

Earth Sciences NZ freshwater ecologist Michele Melchior said they had been conducting their surveys for the last three years, and building up a significant database.

Gold clams can grow up to 5cm across. Adults can produce up to 70,000 offspring annually.

At Horahora, near Bob’s Landing, she said the number of clams present had increased from 27 clams per square metre to a “very concerning” 1360 clams per square metre - a 50-fold increase.

“In some areas we're seeing a dramatic increase, and really rapid establishment, it's something I didn't expect, but it's happening.”

At one monitoring site, researchers found up to 4000 clams per square metre.

“Essentially, what we're doing is trying to establish what they like in terms of the types of habitats they are favouring.”

She said their size and condition “will tell you that they are enjoying it here”.

“They look very healthy, and it's concerning because we're worried about spread, and because they are so healthy that means that they're likely actively recruiting, actively reproducing.

“Those larger sized ones are able to produce around 70,000, larvae per year.”

Graham said one of the more interesting results was an improvement in water clarity where clams were present in high numbers.

“But that’s not really good for the environment … because they’re basically taking food away from the native species, and not only food but calcium, which our natives also rely on.

“These guys are generalists. They thrive in a lot of different environmental conditions, and we're starting to really see that here as well.”

Melchior said it was also important to find out what they're feeding on, and understanding the make-up of the water they're living in and how they may be changing it.

WRC invasive clam programme communications lead Meg Taylor-Silva said the fact that a child was the person who found gold clams in Lake Rotomanu in Taranaki was telling.

“We don't want them to spread any further, so the right messaging is really important. Talking to kids and students is a great way to get the message across.”

She said he only knew what they looked like after seeing one in a small resin block shown to students.

“People need to be able to identify these pretty easily.”

She said the “check, clean, dry” mantra had never been so important.