They can be smaller than a $2 coin, but the damage gold clams do when introduced to ecosystems is a threat looming large.

A year on from the closure of a popular Rotorua fishing lake to install protections against the prolific invader, Local Democracy Reporting’s Laura Smith looks at how the risk is being managed.

On the eve of the fishing season in 2023, a Rotorua iwi trust made a controversial public call to close one of New Zealand’s best trout lakes to guard against the threat of invasion by gold clams, which had been found in the Waikato River.

Biosecurity NZ’s six-week closure of Lake Ōkataina upset some anglers and prompted threats of environmental terrorism.

But a year on, Ngāti Tarāwhai Iwi Trust chairman Cyrus Hingston does not regret insisting on protections for the lake.

“We’re going to do our best to ensure the danger to our lake and whenua doesn’t get here,” he said.

Exotic gold clams, also known as Asian clams or corbicula, eat the plankton on which most native species survive. They reproduce rapidly and can clog infrastructure in the water. The impact on native species if they become well-established is unknown.

They have never been eradicated overseas but there is an effort to do just that in New Zealand.

Last month Niwa was awarded $10.2 million to eradicate the clams.

They were first found in May last year in the Waikato River, then in the manmade Lake Taupō Aqua Park in March this year. It has been estimated they could have been in both sites for up to three years.

The prolific freshwater breeders can spread between waterways by attaching themselves to boats or equipment. They have not yet been found outside Waikato.

Special protections are in place at Te Arawa lakes, with extras for Ōkataina, to prevent clams from reaching the region, where about 20% of visitors come from Waikato.

Any boat that has been in the Waikato River in the previous 30 days must be cleaned at a designated wash station before entering any Te Arawa lake. Photo / Laura Smith.

A controlled area notice for all 14 Te Arawa lakes requires boats that have visited the Waikato River in the past 30 days to be cleaned at Te Ngae Rd wash station before entering the water, in addition to check-clean-dry requirements for cleaning boats and gear.

To visit Lake Ōkataina, any boat – whether it has been in Waikato or not – must go through that process.

‘Looking after our legacy’

Hingston said the iwi trust was exercising kaitiakitanga (guardianship) and believed its early response had helped make others take notice of the threat.

“We’re looking after our legacy.

“We’ve seen a great increase in understanding of the impacts of gold clams on the lakes and on the waterways and how it’s transmitted.”

Security guards were hired when Lake Ōkataina was closed last year. Photo / Andrew Warner.

The initial closure prompted threats to drop clams into the lake – which Hingston considered environmental terrorism – and to remove the boulders blocking boat access.

Hingston said feedback was 90% positive now, apart from the occasional comment he called “mostly bluster”.

The lake was open under restricted hours and Hingston estimated compliance with the biosecurity rules had increased from 20% to 40%.

“Still not that great, but it’s a lot better than it used to be.”

Continued vigilance was needed.

“Because the way it spread, it took two or three years before it was found that it was established. The testing methods used aren’t that great.”

Ngāti Tarāwhai Iwi Trust chairman Cyrus Hingston at Lake Ōkataina. Photo / Andrew Warner.

He said Lake Ōkataina’s rural location and single access point worked in its favour.

Installing a wash station closer to the lake, potentially at Rotoiti, to be more convenient for boaties coming from the east was under discussion with other agencies.

A Bay of Plenty Regional Council spokesman said agencies and iwi were also considering a gate for the Lake Ōkataina boat ramp, restricting access to boaties who had completed the required steps.

Controlled area notice under review

Biosecurity New Zealand pest management director John Walsh said the Te Arawa lakes were vulnerable because many visiting fishers and boaties travelled from Waikato.

“Biosecurity New Zealand understands that Ngāti Tarāwhai, mana whenua at Ōkataina, has been working hard to ensure access to Lake Ōkataina for boaties this season.”

The controlled area notice was under review and a decision was expected shortly.

Walsh said the clam response, aided by the notices and check-clean-dry rules, had been largely successful at containing it.

“These measures wouldn’t have been as effective without the contribution and support of iwi, who have gone to immense effort to protect lakes and rivers, as well as our other local partners.”

The main testing method was finding DNA in water samples. Iwi representatives had been trained in this as part of the surveillance programme in the Waikato, Te Arawa and Taupō regions.

Education overtakes emotion

Fish & Game previously called last year’s initial lake closure a “blow to anglers”.

Eastern Fish & Game chief executive Arash Alaeinia said this month that last year there was emotion on all sides about potential clam impacts to the environment and fishing access to all lakes.

“This was a new situation, so nobody had an awareness and it all happened two weeks prior to the opening.”

There was now more knowledge and understanding.

Lake Ōkataina is open under restricted hours following the discovery of gold clams in the Waikato River last year. Photo / Andrew Warner.

“We’ve got processes in place now, so anglers and users are a lot more aware of what’s required.”

The trout fishing season opened on October 1 and there had been “no issues”.

Alaeinia said education could help everyone take personal responsibility.

“None of us want to be that person that, you know, opened up the floodgates for the species to come in.”

The check-clean-dry system worked on a high-trust model but he called it a small inconvenience for the greater good.

Catfish are another unwanted species. Photo / Laura Smith.

He said Fish & Game already worked with the regional council and Te Arawa Lakes Trust to manage the spread of unwanted lake species such as catfish. It was the same with keeping gold clams out.

He encouraged lake users to know the rules and be kind to biosecurity officers checking that everyone was doing their part to safeguard the environment for future generations.

Biosecurity officers from Te Arawa Lakes Trust and regional council staff will be at the lakes daily throughout the peak summer period and every weekend until April, to continue to educate and check compliance.

Regional council biosecurity officer Te Wakaunua Te Kurapa said it was more important than ever that boaties checked, cleaned, dried and certified their vessels and gear “to protect the lakes we all love”.