It's breeding season for invasive clam

Earth Science NZ researchers are seeing signs that breeding season for the invasive freshwater gold clam (Corbicula fluminea) has begun.

Earth Sciences NZ Principal Technician Karen Thompson says gold clams are breeding machines.

"It's breeding time in the Waikato River, but this is no cause for celebration. Tiny clams have recently been seen in both adult clam and water column samples collected from Lake Karāpiro, meaning we are in a period of heightened risk of spread. Our team have found the next generation is already maturing in adult clam brood pouches getting ready for release," says Karen.

Invasive gold clams are hermaphroditic, so they possess both male and female reproductive organs and self‑fertilise, a significant advantage for their rapid population growth.

Adult clams have a lifespan of one to five years and are known to spawn multiple times per breeding season, depending on environmental conditions. They have high reproductive rates, with an average individual producing around 70,000 larvae per season.

Karen says that the juvenile phase presents a distinct biosecurity challenge.

Each clam releases larvae by the thousands into the water column that are easily dispersed by water currents and human activities. During this stage, the clams are fully formed, allowing them to spread rapidly within waterbodies and establish in new areas.

"At roughly 0.2 mm, which is smaller than a grain of sand, these juveniles are not visible to the naked eye, meaning they can easily go undetected on clothing, equipment, craft or in water removed from infested areas," she says.

Invasive gold clams were first confirmed in Aotearoa‑New Zealand in May 2023 at Lake Karāpiro, and by the end of that year had been visually identified throughout much of the Waikato River downstream of Lake Maraetai. They were recently detected in Lake Rotomanu, Taranaki.

An Earth Sciences NZ‑led five‑year research programme aims to stop the invasive clam from spreading further and protect our important native species.

Programme lead and Principal Scientist – Freshwater Ecology Dr Deborah Hofstra says they are working to find effective, culturally attuned ways to manage this invader.

"We all have a role to play by following 'Check Clean Dry' procedures any time we are in our lakes and rivers. This is particularly important during the heightened risk of the clams breeding season," says Deborah.

Invasive gold clams have been given the legal status of an Unwanted Organism under the Biosecurity Act. This means that people must not knowingly move them or water that may contain them. They are also not safe to eat.

People can still use the Waikato River for recreation, including boating and fishing, or gathering of kai if they follow the requirements of the controlled area notices 'Check Clean Dry' procedures before moving their equipment or craft.

When using or moving between waterways you must follow 'Check Clean Dry' procedures.