Gold clam (Corbicula fluminea) was first discovered in the Waikato River in May 2023 at Bob's Landing, within Lake Karapiro. Following this discovery, Biosecurity New Zealand initiated a response, commissioning NIWA to conduct a delimiting survey to assess the extent of C. fluminea in the river.
Visual shoreline surveys detected C. fluminea not only upstream of Karapiro, at Lake Maraetai, but also as far as 40 kilometers downstream from the initial detection site.
Since that survey, populations have been visually identified from Lake Maraetai all the way downstream to Tuakau Bridge.
Gold clam is native to eastern Asia and is widely established in North and South America and Europe.
They are found in freshwater in highest abundance around the edges of lakes and rivers. They can survive a wide range of temperatures and can live in:
- lakes, and rivers down to where saltwater and freshwater mix
- on top of, or buried in, a variety of substrates (for example gravel, sand, mud)
- high oxygen waters.
Declared an unwanted organism (Biosecurity Act)
Biosecurity response
After the discovery of gold clam in the Waikato River, Biosecurity New Zealand immediately established a response in partnership with river iwi and other stakeholders, including the Waikato Regional Council, the Waikato River Authority, the Department of Conservation, Toitū Te Whenua Land Information New Zealand, and others.
The response has focused on identifying the spread of the clams and containing them to their known locations through Controlled Area Notices, research, wash stations and Check Clean Dry.
For more on the biosecurity response visit Exotic freshwater clams (Corbicula) | NZ Government
Why is the gold clam a problem?
Gold clam reproduces rapidly and in large numbers. As hermaphrodites a single clam can fertilise itself and produce thousands of juveniles leading to rapid population growth.
They can outcompete native species for food and habitat resources, disrupting ecosystems and leading to declines in native populations and so here they pose a potential threat to our freshwater mussels or kakahi.
They alter ecosystem dynamics including nutrient cycling and sediment composition which in turn affects water quality, and potentially leads to further ecological changes.
In addition to ecological concerns, gold clam can also have large economic impacts. Clogging water intake pipes and machinery, leading to increased maintenance costs for industries such as power plants and water treatment facilities of which along the Waikato River system there are many.
They could also impact recreational activities such as swimming and boating and cultural practices such as the migratory elver trap and transfer programme.
Gold clam has a high tolerance for a wide range of environmental conditions, allowing it to spread rapidly and establish populations in diverse habitats.
Controlling gold clam populations can be challenging and costly. Traditional control methods such as chemical treatments may have negative impacts on non-target species and the environment.
Gold clam lifecycle
The juvenile phase poses a particular biosecurity challenge, at approximately 0.2 mm in size their “invisibility” to the naked eye makes it difficult to determine their presence.
The below image shows only the siphons (the white patches) visible on the surface, while the clams are largely buried in the substrate and not obvious at first glance.
