Gold clams could be the end of native species in the Waikato

Wednesday, 13 May 2026

Native species are under threat from more than one invasive pest species in the Waikato but a new study has found that gold clams could be the final nail in their coffins.

Invasive gold clams have the potential to change entire ecosystems, wiping out native species while providing food for other unwanted organisms, a new research paper states.

It was three years ago this month when freshwater gold clams were first detected in the Waikato River, and had likely been there for at least three years before that time.

Since then, they have made the river their home from Lake Maraetai at Mangakino down to Port Waikato, and into Lake Rotomanu in Taranaki.

Whitebait could also fall foul of the invasive gold clam, a new study has found.

The research paper, published in the New Zealand Journal of Marine and Freshwater Research this week, was authored by The Cawthron Institute’s Calum MacNeil who said the “little clam invader is an ‘ecosystem engineer’ changing conditions in rivers and lakes to suit itself”.

On top of the fact that the gold clam “is one of the most ecologically and economically damaging freshwater invaders worldwide”, MacNeil said it can also “filter water at such a rate, that it can alter the food webs, chemical processes, and water clarity of the places it invades.”

Freshwater gold clams are prolific breeders, and overseas studies have found they can take over entire waterways.

MacNeil says the pest also favours conditions linked to toxic algal blooms, which were a massive problem in the river last summer, and the clams could affect freshwater ecosystems “more than any previous invader”.

'All of this will affect fish species, some of which may benefit from this ‘engineering’ but many will suffer.

'Small native fish who need particular types of river flow, habitat and plankton may lose out as the clam could swamp their habitat and compete for the same food,“ he said.

'Trout and salmon may benefit from clearer waters for feeding but lose out as spawning grounds are lost.

'Pest fish such as koi [carp] and catfish may be overall ‘winners’, as the clam may be a great new food source for them.“

He said only time will tell what will happen to New Zealand’s waterways if they continue to spread, but overseas research has found mass clam die-offs have been linked to low oxygen and ammonia spikes.

“Careful monitoring and proactive research are needed now, while the invasion remains in its early stages, both to anticipate and mitigate risks to Aotearoa-New Zealand’s unique and culturally significant freshwater fishes.

“Opportunistic generalist fish, many of them introduced, may exploit clam-altered systems, while native species with narrower niches may be at the greatest risk.

“Fish frequenting backwaters or slower-flowing reaches could be even more vulnerable, as clam densities are often highest here.”

International studies cited in the paper show the clam can filter large amounts of phytoplankton from the water, shift food production from the water column to the riverbed, promote conditions that favour toxic cyanobacteria, and change sediment structure by forming dense clam beds.

“Native galaxiids (such as whitebait and mudfish), already threatened by habitat fragmentation/degradation, may be at further risk from additional pressures of a clam invasion,” MacNeil wrote.

He said introduced fish could be affected differently.

“Among introduced fish, salmonids may lose spawning habitats, whereas generalist ‘pests’ such as catfish and koi may benefit from clam-derived food resources.”

The Ministry for Primary Industries gave up on any clam eradication plans in March last year, instead turning to “containment and suppression” options.

It said eradication from the Waikato River was not feasible with current technology.

– The study was supported by, and conducted through, the Fish Futures Programme for the Ministry of Business, Innovation and Employment (MBIE).