Gold clams
Introduction
Gold clams are a species of freshwater clam native to eastern Asia which have become an unwanted invasive species around the world.Biosecurity NZ (BNZ) is the lead agency in the early stages of an incursion response to the discovery of gold clams. DOC and stakeholders – including river iwi, Waikato Regional Council, Waikato River Authority, Toitū Te Whenua Land Information New Zealand, Mercury Energy, and others – are supporting BNZ as part of the response.
What are they?
The highly invasive gold clam (Corbicula fluminea) was discovered in the Waikato River in May 2023. It is unknown how long the organism has been in NZ, the extent of its spread, nor how it was introduced to New Zealand.
Gold clams breed rapidly and can occur in dense populations in suitable habitats. The clams have been found in wading depth water, in moderately coarse sediments of the Waikato River. They can potentially exist in brackish habitats as well as fresh water. Based on current knowledge, the clams are most likely to spread between waterbodies by people boating and recreating (example: whitebaiting).
In mid-August 2023, the gold clam was given the legal status of an Unwanted Organism under the Biosecurity Act. As an Unwanted Organism, gold clam (and water potentially containing it), must not be knowingly moved or spread.
Why are they a problem?
Gold clam populations can clog water-based infrastructure such as:
- electric generation facilities,
- irrigation systems,
- water treatment plants.
They are filter feeders that can potentially compete with native species for food. It is unknown how this species will respond in New Zealand conditions.
Overseas, this clam has been difficult to control, and eradication has never been achieved.
It is currently unknown how gold clams might negatively impact the whitebait fishery. But, it is important to protect our native species, including native fish, from invasive species even if we do not have complete information.
DOC is taking a precautionary approach by encouraging whitebaiters to abide by the biosecurity measures recommended by Biosecurity NZ and reduce the potential spread of gold clams.
Where are they found?
Gold clams found in the Waikato River
Image: Ministry for Primary Industries
Currently, gold clams are only confirmed to be in the Waikato River. Since the initial discovery, surveillance has been underway to determine the extent of the population.
Significant numbers of the clam were found at several locations in Lake Karapiro, downstream at the Cambridge boat ramp, with a couple found as far downstream as Hamilton. This indicates a well-established breeding population potentially present for several years.
eDNA and visual surveys are continuing in the Waikato River. The currently known upper limit of the incursion in the Waikato River is in Lake Maraetai. Surveys of high-risk lakes across Aotearoa have not detected gold clam outside of the Waikato River catchment.
High risk locations include waterbodies where boats being used could have been at Karapiro, such as wakeboarding competitors and rowing coach boats. This is also why DOC is so concerned about whitebaiting. Whitebaiters could accidentally spread clam larvae by moving their nets between waterways.
How do they spread?
Gold clam larvae are exceptionally small, at just 200 microns or 0.2 mm, and are basically invisible. This means they can spread easily on boats or other recreational gear, including whitebaiting fishing gear, since they cannot be seen when visually checking gear.
Whitebaiting gear (nets, screens, spotterboards etc.) is very difficult to decontaminate, and so the biosecurity risk posed by whitebaiters is not the act of whitebaiting itself, but the movement of fishing gear from one waterway to another.
DOC’s work
DOC is supporting the Biosecurity New Zealand response to the gold clam through advocacy and sharing information.
It is too early to say what the effects of the clam may be on native species, but we are working to determine that.
DOC has a team of staff working on the gold clam response to understand what it means for us, and for conservation.
How can I help?
DOC supports Biosecurity NZ’s request for whitebaiters to fish in only one river. If fishers wish to move to another river, they should use different fishing gear (nets, screens, and spotterboards).
Fishing gear should be washed thoroughly then dried, and neoprene gear such as waders should be frozen to kill any larvae remaining in the residual water.
Check, Clean, Dry to help prevent the spread of freshwater pests
Check, Clean, Dry practices are more important than ever. The methodology has been updated for gold clam and is applicable around NZ. It is wise to stay within a single river system and work from upstream to downstream.
The updated practices are outlined below. Good decontamination is particularly important for people moving from:
- locations where gold clam has been detected (currently from Lake Maraetai downstream) to any area where presence of gold clam has not been confirmed on the Waikato River system or in another region, or
- the Waikato River system to any other freshwater environment such as a lake or river, or any brackish water such as an estuary.
| Action | Method |
|---|---|
| Check | Remove any visible matter, including any clams you can see, along with plant material or mud. Drain all river or lake water. |
| Clean |
Washdown your gear, vehicle, watercraft, and trailer that has been in contact with river or lake water with tap-water onto grass, beside the waterway or at home and not into a stormwater drain system. This will remove any remaining invisible material. For absorbent surfaces and materials that have been in contact with river or lake water (including carpet on trailers) use an appropriate treatment in the treatment options table (see table below). Treat residual water that always occurs when on-board ballast bladders or tanks have been pumped. |
| Dry |
Allow gear to dry to touch, inside and out, then leave it to dry for at least 48 hours (two days) before using again. Dry areas inside the watercraft where water has pooled, for example with an old towel, and then leave the craft to dry for at least 48 hours (two days). The hull of a watercraft will dry when towed. |
| Type of treatment | Method |
|---|---|
| Hot water | Above 60°C for at least one minute, or Between 50 to 54°C (hot household tap-water) for at least five minutes, or Above 45°C for at least 20 minutes. |
| Freezing | Until solid (i.e., freeze overnight). |
| Bleach | Mix household bleach in a 10% (1 in 10) ratio with water and immerse for 1 hour. |
| Isopropyl alcohol | 70% isopropyl alcohol, taking care as it is toxic and flammable, and there are requirements around storage and transport of isopropyl alcohol. |
Note, the “Check, Clean, Dry'” advice may be adjusted as further technical information becomes available or the need arises.
For locations outside of the Waikato River catchment, updated check, clean, dry practice applies and is more important than ever. Early clam infestations may not be easy to see, so we must assume each location may have clams, or other pests, and do excellent “check, clean, dry.”
Report a sighting
If you think you have seen the freshwater gold clam, note the location and take a photo if possible. Then report to Biosecurity NZ either on 0800 80 99 66, or complete their online reporting form.
Find out more on the Biosecurity NZ website.
