Inside the lab racing to stop Waikato’s gold clam invasion
Friday, 5 June 2026
As concerns over the number of invasive and unwanted gold clams increase, staff at Earth Sciences New Zealand’s labs are suffocating, squishing and chopping them up to find out how they tick.
Following on from a site visit to Lake Karapiro where clam numbers are rapidly rising, the Waikato Times met with Earth Sciences NZ staff at their Ruakura laboratories to find out what they’re doing to prevent their spread.
Earth Sciences NZ freshwater ecologist Michele Melchior, and freshwater pollution mitigation scientist Harizah Hariz, are trialling the use of benthic barriers - basically fancy weed mats - to control clam populations.
The aim of their research is to create an impermeable barrier, “which means there is no water flow, gas exchange, or oxygen exchange through that barrier … creating conditions that are unfavourable for the clams,” Melchior said.
She said that information would be useful for preventing smaller, localised, and new clam incursions from spreading, and “to kill them in the field”.
They’re also finding out how much ammonia (which is not good for waterways) they release back into the river system when they die and decompose.
“We use tools like that to target their weaknesses,” she said.
“At different temperatures, their metabolism works in different ways too.
“High temperatures essentially mean that clams die more quickly … in lower temperatures, it takes a lot longer for them to die.
“The main aim is understanding how quickly can we kill those clams without impacting the environment.”
The team is also looking into how sound waves may be able to be used to control their spread.
“Sound vibrations are able to break up the byssal threads of juvenile marine bivalves, and we are in the early stages of testing that on clams,” she said.
Earth Sciences NZ environmental chemistry and toxicology principal technician Karen Thompson says the clams are an intriguing species, and can feed from both their ‘foot’, and from a protruding siphon, and is studying their reproductive ecology.
By dissecting them under a microscope she can figure out when, and at what stage of their life-cycle they produce the most offspring.
“By looking at the gills, you can basically see how many larvae each individual is holding, what proportion of individuals are reproductive at that time, and also at what size they are likely to be reproducing.”
She said clams under one centimetre don’t generally have any larvae “but as soon as they're getting to that sort of 1.5 centimetre stage or even like 1.1 centimetres, we've found larvae in those so they're able to reproduce”.
She said the clams were hardier than native kakahi (freshwater mussels).
“Their shell is a lot thicker. They clam shut a lot harder so you can't get any instrument in there to pry them open.
“Obviously these things are really horrible … but when you see them under the microscope they're still very cute … it's a love-hate relationship,” Thompson said.
“Finding anything that’s just going to harm them, and not harm anything else is the key, but it's not that simple.”
Earth Sciences NZ freshwater ecologist, and the person who first helped locate and identify the clams in May of 2023, is Brian Smith.
“I was probably one of the first people to see the clams. I went out with MPI at the time, and I was at Bob's Landing, which we pretty much consider ground zero.”
“They were definitely new to the system. I hadn't seen them before, and you recognise this pretty much straight away.”
“We're just really scratching the surface … the thing with invasive species is once they're here it's harder to get rid of them, especially in the numbers that we've been seeing.”
He’s studying the amount of juvenile clams in the water column with some “old school ecological sampling equipment”.
“That’s a useful way of measuring densities in the system,” he said.
The team are also figuring out what temperatures are best suited for clam reproduction, “and a bunch of other things that we don't quite know about yet”.
Earth Sciences NZ invasive clam programme lead and aquatic biosecurity chief scientist Deborah Hofstra said $10 million worth of Ministry of Business, Innovation & Employment Endeavour Programme funding was being put to good use.
“We're all about outcomes … and one of the outcomes we're wanting are some tools to actually manage the clams.
“That starts with things like how people clean up their gear … the decontamination phase, so there's practical things that people can do to prevent spreading them.”
She said other tools can be used the next time clams turn up somewhere.
“So we can go and assess that site and offer some evidence-based options.
“We are looking for ways in which we can manage it, control it, and better ways to predict where it's going end up and be a problem.
“It's not necessarily going to have the same impacts in this lake as it will in another one, so we want to be able to understand these things … and use that as part of a suite of information to start to better predict where it's going to be more of a problem.
“That's going to make more economic sense in the long run, as well as environmental sense, because the environmental impacts have economic impacts in terms of the costs of managing an invasive species.
“Less cost, the faster you can respond.”
She said their efforts were revealing new information, as well as building on previous scientific studies.
“Then we can have informed decision-making about the risk of the invasive species, and if we're using this control method, these are our potential off-target impacts”.
However, the one thing she is not seeing are any benefits the clams may have in New Zealand.
“I would suggest the waters of the Waikato River where the clams are occurring are sufficiently nutrient rich, and I can't see that’s going to be limiting them any time soon.”