Gold clams unlikely to be eradicated and are 'already well established'

Wednesday, 7 June 2023

Gold clams have probably been living in the Waikato River for more than two years, and efforts to eradicate them would be “highly unlikely” to succeed, according to a Waikato University invasive species expert.

On May 25, the Ministry for Primary Industries (MPI) announced the discovery of freshwater gold clams in the Waikato River, near Lake Karāpiro, to the public.

The clams were first discovered by researchers on May 1, and four days later BNZ investigators found both live and dead clams in the river, “indicating an established breeding population”.

Biosecurity New Zealand (BNZ) has confirmed another eight sightings of the invasive pest in the river, but they have not yet found their way into Lake Taupō.

Associate Professor Dr Ian Duggan is an invasion biologist at Waikato University, specialising in biosecurity science and invasive species ecology.

Dr Duggan said he visited Bob’s Landing, the site of the first gold clam discovery, a few weeks ago.

“There are shells everywhere, and based on their size may have been there up to two to three years, but probably not much longer than that.

“They are already very well established there.”

He said gold clams would thrive in the mild climate of the Waikato River and “conditions in Lake Taupō would be ideal for them as well.”

The invasive freshwater clam has the potential to destroy native habitats, clog water supplies and hydroelectric dams, and could infest Lake Taupō if not properly contained.

So far, the clams have been detected at Bob’s Landing, Horahora Domain, Moana Roa Reserve, Keeley Reserve, Cambridge Boat Ramp, and at Wellington Beach in Hamilton.

Dr Duggan holds great fears for the river’s freshwater mussel (kākahi) population, which is already under threat.

“An adult [gold clam] can produce 400 young per day, or about 70,000 per year, so it won't take long for them to build a large population.

“The main thing we can do is reduce the rate of spread to other catchments, such as the lakes around Rotorua … and getting onto that very quickly, so they don’t spread.”

Dr Duggan said it was “highly unlikely” to eradicate them and the best thing to do was “to slow down its spread to other waterways”.

He said any answers to how the clams got there would be pure conjecture until MPI had completed its analysis.

“They’re a pretty tolerant little critter … and can survive up to 36 days out of water as long as they are moist.

“Overseas, a lot of the invasions have come from ballast water, but that’s unlikely in this case.

“Their larvae can attach to things like boats and anchors or other aquatic plants, or equipment like waders.

“There is a possibility it could have been introduced, or through intentional smuggling. Some have said it was through the aquarium trade, but I’m not so sure about that either.”

He said there was overseas evidence of them being introduced as a food item by immigrants, or they could have been introduced by recreational boats coming in from overseas for competitions on Lake Karāpiro.

“If you can work out where it has come from … we can work out how to cut off that pathway.”

The Waikato Regional Council declined to answer questions directed to its Maritime Services team and referred questions back to BNZ.

BNZ response controller David Yard said 14 gold clam notifications had so far been received – all within a similar stretch of river it had already surveyed.

“Of those 14 notifications, eight have been positively identified as the gold clam.

“We are also carrying out surveillance at a further 20 sites on the river and sites at Lake Taupō.”