Protect our Waterways from Invasive Clams

Invasive Clams - Waikato River - Bobs Landing - 25May2023 - Photo: T Burton

What are Invasive Clams?

Invasive freshwater clams are Corbicula species and are sometimes referred to as ‘Gold Clams.’ They are a small shellfish that has been introduced to Aotearoa accidentally and now threatens our ecosystems. We want to work together to keep it out of our biggest lake, Lake Taupō!

Where are the Invasive Clams?

They were found in New Zealand for the first time in May 2023 near Lake Karāpiro in the Waikato River. They are now present from Lake Maraetai (Mangakino) and downstream. They are an Invasive species, which means that they do not belong here and they can harm our environment.

Currently, we do NOT have invasive clams in Lake Taupō. You might have heard that invasive clams were found in 2024 at the lagoon used as a water park (The Aqua Park) near Taupō. This is true, but those clams were a different species, Corbicula australis, from Australia. There are plans underway to eradicate these ones as they are in a contained space. Scientists say they have been there at least 4 years.

The ones found in the Waikato river are a species that originally came from Asia. This species of clam has accidentally been introduced to many places in Europe and the USA. These places continue to battle with these clams, as they have had an effect on native wildlife and on infrastructure, such as power production and irrigation. Unfortunately, they can badly damage the environment that they are introduced to and are impossible to eradicate from a river. They will continue to spread downstream on the current.

Invasive clams are small. Photo: Ministry for Primary Industries

What do they look like?

A very small symmetrical shellfish (the ‘knob’ or join is in the middle!)
Rounded fan shape
Ribbed shell
Adults are 2 to 3 cm wide -babies are so small (0.2mm) that they are invisible to our eyes!
Shades of brown and khaki-gold
Sit on top of sand or mud, or may be slightly buried

Native kākahi such as this one can be mistaken for invasive clams. Notice that it is not as round, or symmetrical, and it does not have the deep ribs. Photo: Sarah Wraight

Don't get confused! Kākahi or Invasive Clam?

Invasive Clams in sediment at Bobs Landing on the Waikato River- 25 May 2023 - Photo: T Burton

We do have native shellfish in our lakes and rivers. Kākahi, our native freshwater mussels, can look very similar to the invasive clams and are often mistaken for them. However, they grow much bigger than the clams. They are also more of an oval shape, they do not have ribs, and they are NOT symmetrical.

Why are they so Invasive?

They are self fertile (they don’t need a mate to breed)
One adult clam can have over 400 larvae in a day, up to 70,000 per year per individual clam!
Larvae are 0.2 mm, basically invisible to our eyes! They could be in your clothes without you noticing.
They spread downstream with the current
They have a sticky mucous thread that acts like a sail, helping them move downstream, then like an anchor when it sticks them to rocks, boats, and other objects.
They are very difficult to kill and can stay alive for a long time in damp equipment, “If it’s wet, it’s a threat!”
They can easily be moved from one place to another in damp clothing, wetsuits, life jackets, or on boats, kayaks, paddleboards etc

Why are they a Problem?

Overseas, they undergo mass die-offs and leave masses of shells known as “shell banks” or “a concrete bottom” to lakes and rivers. This can impact native ecosystems.
People overseas talk about the smell when there is a mass die-off.
They can clog up water pipes – used for power generation, irrigation systems & town water supply.
They can compete with native species for food, light & space.
Species such as kākahi, tuna (eels), and kōura could be at risk if their ecosystem changes.