Exotic freshwater clams (Corbicula)

We're working to prevent the spread

Biosecurity New Zealand has been working in partnership with iwi, councils, and other key stakeholders to protect the water we love by preventing the spread of exotic freshwater clams.

Legal controls are in place to contain the clams and prevent spread. Extensive surveillance of rivers and lakes in the Waikato and Bay of Plenty, as well as nationally, has shown no indication of freshwater clams outside the 2 known locations.

What are exotic freshwater clams and why are they a problem?

Two species of exotic freshwater clams have been found in the Waikato region.

Corbicula fluminea is native to eastern Asia and is widely established in North and South America and Europe.

Corbicula australis is native to Australia, where it is common and widely distributed.

Both are found in freshwater. They are found 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.

These 2 types of corbicula are prolific breeders, able to produce 400 juveniles a day and up to 70,000 juveniles a year, allowing them to reach extremely high densities in optimal conditions. This means they can clog water-based infrastructure, such as electricity generation plants, irrigation systems, and water treatment plants. They can also compete with native species for food and space.

Overseas, Corbicula fluminea has proved difficult to control, and eradication has never been achieved.

Top: Corbicula australis
Bottom: Corbicula fluminea
Photos credit: Jean-Claude Stahl and Kerry Walton, Te Papa.

Where the clams have been found

Exotic freshwater clams were first found in the Bob’s Landing area of the Waikato River near Lake Karāpiro in early May 2023, and were identified as Corbicula fluminea. Since then, Corbicula fluminea has been confirmed in the Waikato River from Lake Maraetai Landing to Tuakau.

In March 2024, Corbicula australis was found at the Lake Taupō Aqua Park in a small man-made lagoon.

Biosecurity response

After the discovery of Corbicula fluminea 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.

Surveillance

Surveillance has focused on detecting any spread of the clams outside the known locations.

Early detection allows a quick response and keeps open the possibility of local eradication of these newly detected populations if not well established yet.

Since May 2023, we have tested more than 1,650 water samples from 300 sites across New Zealand for corbicula DNA.

An initial delimiting survey of 20 sites on the Waikato River and 5 sites on Lake Taupō determined the spread of the incursion. This included visual surveys and water sampling for environmental DNA (eDNA) testing.

Wider surveillance of the Waipā River and tributaries to the Waikato River, was carried out in partnership with Waikato-Tainui, Te Nehenehenui and Ngāti Raukawa. This included providing training to iwi representatives to undertake water collection for eDNA testing. River iwi were involved in the selection of survey sites. The survey found no evidence of freshwater clams.

Following the detection of corbicula at the Lake Taupō Aqua Park, additional surveillance was carried out in the Waikato River and at multiple sites at Lake Taupō.

Extensive eDNA testing of high-risk water bodies across New Zealand, in partnership with regional councils and iwi, has found no indication of Corbicula.

To date, surveillance has not detected clams outside the 2 known locations – the Waikato River from Lake Maraetai Landing to Tuakau and the Lake Taupō Aqua Park.

Technical Advisory Group report

The response was informed by the scientific and technical expertise of a Technical Advisory Group (TAG).

The TAG included national and international scientists with extensive experience in freshwater invasion ecology, mollusc control, and experience with corbicula.

Representatives from Waikato iwi brought important local knowledge and perspectives.

The advice from the TAG provided a foundation for decisions about the management of the freshwater clam.

The TAG agreed that eradicating Corbicula fluminea in the Waikato River is not feasible with current technologies, but a strategy of containment and suppression is feasible.

Biosecurity Response to Corbicula fluminea in the Waikato River [PDF, 820 KB]

Biosecurity measures for the clam

Corbicula fluminea and Corbicula australis 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. To do so would be an offence under the Biosecurity Act. People can still use the Waikato River for recreation, including boating and fishing, or gathering of kai (food) if they follow the requirements of the controlled area notices, including clam-specific 'Check Clean Dry' procedures, before moving their equipment or craft.

Most consent holders with consent to take water from the Waikato River under the Resource Management Act (RMA) – such as for irrigation, municipal water supply or other commercial uses – will notice no impact.

Go to clam-specific 'Check Clean Dry' procedures

Controlled area notices

Biosecurity rules are in place to stop the spread of the clam through a mechanism under the Biosecurity Act called a Controlled Area Notice (CAN).

Two CANs are in force – one for the Waikato River and another for the Te Arawa lakes with additional protections for Lake Ōkataina.

Waikato River Controlled Area Notice and amendments

A CAN with specific rules for all wake boats applies to the Waikato River from the Whakamaru Dam to the river mouth at Port Waikato. Wake boats are defined as any craft with an internal tank or bladder that cannot be completely drained. This came into effect at 11.59pm on 24 November 2023.

Amendments to the CAN came into effect on 1 December 2024 and will be in place until further notice. The key change from the original CAN is to the 'Check Clean Dry' treatment table (on this page below)

Waikato River Controlled Area Notice [PDF, 866 KB]

Notice of Amendment to Controlled Area Notice [PDF, 225 KB]

Waikato River CAN requirements

All wake boats that have been in the controlled stretch of the river must not be used in any other waterways, including parts of the river outside the controlled area.
Recreational river users that have been in the controlled stretch of the river must continue to follow 'Check Clean Dry' requirements for any craft, gear (such as fishing and other equipment used to gather kai) and equipment (including water skis, wakeboards, and inflatable pool toys).

Map showing the stretch of the Waikato River from Whakamaru Dam to the river mouth at Port Waikato, covered by the Waikato River CAN.

Te Arawa lakes Controlled Area Notice

A CAN applies to the 14 Te Arawa lakes in the Bay of Plenty with additional protections for Lake Ōkataina. This came into effect from noon on Friday 10 November 2023 and will be in place until further notice.

Te Arawa lakes and Lake Ōkataina Controlled Area Notice [PDF, 225 KB]

For Lake Ōkataina

All boats must be cleaned at a designated wash station in Rotorua before entering the lake. This is in addition to the 'Check Clean Dry' requirements

For all 14 Te Arawa lakes

Any boat that has been in the Waikato River in the previous 30 days must be cleaned at a designated wash station before entering the water. This is in addition to the 'Check Clean Dry' requirements

Local lake updates, including events and closures, are on the Rotorua Te Arawa Lakes website.

Lake updates (including special controls, events, and closures independent of Biosecurity New Zealand) – Rotorua Te Arawa Lakes Programme

Designated wash station

The designated wash station is The Wash Place, 338 Te Ngae Road, Rotorua

At the wash station, boaties will need to:

wash down their boat and trailer, and drain any remaining water
scan the QR code shown on signage and complete the online form to prove they've completed the wash-down.

The Wash Place is a commercial facility and there will be a cost for washing your boat and trailer depending on the length of the cycle chosen. You can pay with cards or coins.

The station is open 24/7 and detailed instructions about how to complete the wash are provided.

Special permissions

Special permissions are considered for the taking of water likely to contain the freshwater clam. This could include, for example, research, sampling, or taking of water not previously consented under the RMA. Consent holders who want to carry out movements that differ from their usual business should also seek a special permit.

To apply for a special permit, email corbiculalongterm@mpi.govt.nz

Section 52 permissions of the Biosecurity Act 1993 [PDF, 191 KB]

Check Clean Dry

When you move from the controlled stretch of the Waikato River (which is from Whakamaru Dam to the river mouth at Port Waikato) to another part of the river or its lakes, you must follow the 'Check Clean Dry' procedures for the exotic freshwater clam for all watercraft, gear, or clothing that has come into contact with river water.

Some North Island regions are also requiring water users in their area to follow these 'Check Clean Dry' procedures. Check with your local regional council for their specific requirements.

Note, 'Check Clean Dry' procedures to prevent human spread of the exotic freshwater clam will also manage other freshwater pest species.

Check

Remove any visible matter, including any clams you can see, along with plant material or mud. Drain all river or lake water.

Clean

Wash down 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 materials that have been in contact with river or lake water use an appropriate treatment in the treatment options table on this page
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 (2 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 (2 days). The hull of a watercraft will dry when towed.

Treatment options for gear made of absorbent material

Type of treatment	Method	Details
Hot water	70°C, soak for 2 minutes, or 50°C to 55°C, soak for at least 5 minutes, or Above 45°C, soak for at least 20 minutes.	Most homes have hot water set at 50°C to 55°C. Caution is advised, especially with the higher temperatures.
Freezing	Until solid, usually overnight.
Bleach solution	5% bleach solution, soak for 60 minutes, or 10% bleach solution, soak for 30 minutes.	5% bleach solution: 5-litre mix = 250 ml bleach in 4.75 litres water. 10% bleach solution: 5-litre mix = 500 ml bleach in 4.5 litres water. Take care of how you dispose of the used bleach solution.
Salt	100 parts per thousand (ppt) household salt in warm water, soak for 4 hours.	100ppt salt solution: 5-litre mix = 500 g salt in 5 litres warm water. Seawater is not a suitable alternative, as the salt content is too low (on average 35ppt ratio) to be effective against clams.

Other treatment options

Type of treatment	Method	Details
2 steps: vinegar + detergent	20% household vinegar solution, soak for 10 minutes and 10% dishwashing detergent solution, spray and leave item wet for 10 minutes.	20% vinegar solution: 5-litre mix = 1 litre vinegar in 4 litres water. 10% detergent solution: 5-litre mix = 500 ml detergent in 4.5 litres water. Vinegar and detergent must both be used as part of a 2-step treatment to be effective against a range of invasive freshwater species (including clams and didymo).
Steam	Apply steam at 100°C or higher for a minimum of 30 seconds, at 3 cm to 4 cm from the material surface.	For safety reasons, ensure you follow the manufacturer’s instructions when operating the equipment.
Virkon Aquatic solution (a disinfectant typically used in marine farming)	1% Virkon Aquatic solution, soak for 5 minutes, or 2% Virkon Aquatic solution, soak for 2 minutes.	1% Virkon Aquatic solution: 5-litre mix = 50 ml Virkon-Aquatic in 4.95 litres water. 2% Virkon Aquatic solution: 5-litre mix = 100 ml Virkon-Aquatic in 4.9 litres water. Virkon–S (a disinfectant used in on-land situations) is not a treatment option as the efficacy for managing all invasive aquatic organisms has not been proven.
Isopropyl alcohol (70%) For hard surfaces only. This is not a viable option for absorbent materials.	Dilute isopropyl alcohol (70% strength) in a 50:50 mix with water, spray surface and leave for 10 minutes, or Dilute isopropyl alcohol (70% strength) to an 80:20 mix with water, spray surface and leave 5 minutes.	50% isopropyl alcohol solution: 1-litre mix = 500 ml isopropyl alcohol in 500 ml water. 80% isopropyl alcohol solution: 1-litre mix = 800 ml isopropyl alcohol in 200 ml water. For safety reasons follow the manufacturers warnings.

Note, the 'Check Clean Dry' advice may be adjusted as further technical information becomes available or the need arises. Refer to the manufacturer’s instructions for gear and any commercial treatments.

Research reports

To inform the response, Biosecurity New Zealand commissioned NIWA to research the effectiveness of a variety of treatment methods for killing juvenile Corbicula fluminea (less than 2 days old). We also commissioned NIWA to conduct a survey of Corbicula fluminea to collect data on population density, habitats occupied and depth profiles in which they are found. This will ensure future suppression trials are designed appropriately.

Treatment Trials for Killing Juvenile Corbicula Fluminea [PDF, 1.8 MB]

Corbicula Fluminea Suppression Trial, Step One Baseline [PDF, 5.4 MB]

Independent research on Corbicula fluminea

The Cawthron Institute has recently published research on habitats in Aotearoa New Zealand that may be suitable locations for Corbicula fluminea to establish.

Although not funded by Biosecurity New Zealand, this research will be useful to both Biosecurity New Zealand and our partners to inform on-going surveillance of the clam.

Cathrown research

How you can help and resources

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Help us by reporting suspected sightings

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How you can help us

You can help us build a picture of where this clam is present by reporting suspected sightings to Biosecurity New Zealand.

If you think you've seen the freshwater gold clam:

note the location
take a photo if possible.

You can then either:

freephone Biosecurity New Zealand on 0800 80 99 66, or
complete the online reporting form at report.mpi.govt.nz

If you are using the online reporting form, you'll need to start by saying you are a member of the public, that you want to make a report about 'a marine or freshwater issue' and then select 'freshwater fish or crustacean' from the dropdown box that will appear. Then you will be taken through some further questions including contact information and asked to load your images.

Example of how to complete the first 3 fields of the online reporting form

The image shows the Biosecurity New Zealand online reporting tool, with the first 3 fields filled in:
(1) "I am a member of the public"
(2) "wanting to make a report about a marine or freshwater issue"
(3) "freshwater fish or crustaceans".

Resources – updates and fact sheets