Gold clams, corbicula fluminea. Photo: Dr Michelle Melchior
At the end of the small jetty at Bob's Landing on Lake Karaapiro, what looks like a bed of silt-covered gravel turns out to be something else entirely.
Peering through an underwater viewer to look at the lake bed below, the many small holes dotted around reveal themselves as frill-lined siphons belonging to gold clams.
It should not be a surprise. The edge of the lake is littered with the shells of this invasive clam, and when she scoops a small handful of sediment in the shallows, Dr Michele Melchior collects five without even trying.
Further out, in deeper waters, the clam's preferred habitat here, the numbers are mind-boggling.
"What we've been finding in some spots is that the number of clams can exceed thousands per square metre," Melchior says.
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Michele is a freshwater scientist with Earth Sciences New Zealand in Kirikiriroa Hamilton. She is part of a large research programme involving mana whenua, council and local and international researchers, aiming to better understand the gold clam, predict where it might go to next and develop strategies to prevent its spread.
It was here, at Bob's Landing near Cambridge, just off State Highway 1, that the clam was first spotted in 2023. Freshwater ecologists stopping for lunch noticed their tan-brown shells and alerted the Ministry for Primary Industries (MPI).
A subsequent survey identified their geographical limit - from Lake Maraetai near Mangakino all the way to the where the river enters the ocean at Port Waikato.
Dr Michele Melchior has an MPI permit to collect gold clams for experiments in the lab. Photo: Claire Concannon
Gold clams are native to Southeast Asia, but have spread to many other parts of the world.
No one knows exactly how they arrived in Aotearoa, but the species' success overseas offers a warning. In North America and Europe gold clams have overrun lakes and slow-moving rivers, clogging irrigation and power plant infrastructure, and disrupting native ecosystems. Once established, they have never been eradicated.
They are robust - able to withstand a reasonably wide temperature range and diverse habitats. Their true secret weapon though, is their ability to multiply.
"We just need a single clam to reproduce," Melchior says. "It's a hermaphrodite, self-fertilising, self-reproducing. So that means that one clam can produce 70,000 to 100,000 babies annually."
They only live until about five years old, but they mature quickly, able to produce larvae within the first year, once the water warms up to over 15C or 16C.
Photo: Dr Michele Melchior
The larvae are microscope and can drift with the current. As small juveniles they make a sticky thread that they can use to attach to surfaces - driftwood, weeds, sediment, boating equipment. This helps the gold clam to spread rapidly into new areas.
A key question Melchior and the team would like to address is whether the clams are having a negative impact on native species, such as native freshwater mussels, kākahi.
Overseas research shows dense clam populations can out-compete native mussels for food and space, and even consume their larvae. For now, the researchers are still watching how things unfold in New Zealand waters.
"It's early days," Melchior says. "We're literally just witnessing the early stages of the invasion."
Comparison between the gold clam (on left of image) and kākahi. Photo: Dr Michele Melchior
One key question is how the clams might be reshaping the lake's ecology. At Earth Sciences New Zealand's campus in Kirikiriroa Hamilton, freshwater scientist Karl Safi has assembled a complex looking series of small chambers, tubes and water containers.
Each chamber contains a single clam, fed a set flow rate of lake water. The water coming out of the chamber is then collected for analysis, and Safi hopes to measure what the clams filter from the water and what they excrete.
"We can see what the clams graze on over a period of time and then we can back calculate how that will affect the populations in the lake," he explains.
Karl Safi is investigating how the clam's feeding habits change the concentration of tiny critters that are the base of the lake's food web. Photo: Claire Concannon
The clams feed by sucking in water through their siphons, filtering out certain microalgae and bacteria, and excreting out faeces. Through their sheer numbers they could potentially impact the entire ecosystem, Sofi says.
"The small little things provide food for everything, so it's that whole food chain process."
Quarterly monitoring is allowing the researchers to investigate changes in the tiny critters in Lake Karaapiro, as well as the chemistry of the water. Some boat users have noticed one visible change: clearer water, an effect of the filtration by thousands and thousands of clams.
But clearer water isn't necessarily better. High densities of clams overseas have been linked to altered nutrient cycling, leading to reduced oxygen levels and increased algal blooms.
Alongside these investigations to understand how the gold clam works in a New Zealand context, experiments into possible control measures have also begun.
One is the use of benthic barriers - rubber or natural fibre mats laid on the lakebed to suffocate the clams by cutting off oxygen. The method has had some success overseas.
"It currently takes around 35 days using benthic matting to kill clams," Melchior says.
She and the team are hoping that they can identify something that might reduce that timeframe.
The trials will first take place in the lab, where they will also check the effects on native species such as kākahi, kōura and pihaurau - lamprey. If the barriers prove too destructive, they might have to move the native species before laying the mats.
Gold clams are not the only invasive pests at Lake Karapiro. Photo: Claire Concannon
In the meantime, preventing the spread of the clams is crucial. The gold clam has been designated an 'unwanted organism' under the Biosecurity Act, which means people must not knowingly move live clams or water that might contain them.
MPI has specific 'check, clean, dry' protocols for the North Island that were developed following initial trials on how best to kill juvenile clams. Now Melchior and the team are further investigating ways to quickly and efficiently kill clams across all age ranges.
For now the gold clam, already in quite dense numbers, will continue to multiply. Eradication is not a realistic goal. Instead the research programme's aim is pragmatic: understand the invasion, predict where the clams may go next, and develop methods to limit their spread and impacts - while protecting the native species that remain.
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