Gold clams ‘never successfully eradicated elsewhere’

Thursday, 28 September 2023

Invasive freshwater gold clams were found in the Waikato River in May after being in the river for at least two years.

Eradicating the invasive golden clam from the Waikato River is “extremely unlikely” - and it’s never been achieved elsewhere either.

The clam, also known as the Asian clam, was first detected there in May and one iwi is already looking to close a lake to avoid it spreading if “check, clean and dry” advice is not followed.

According to an advisory report to Biosecurity New Zealand (BNZ) released this week, “eradicating freshwater gold clam in the Waikato River is extremely unlikely, but a strategy of containment and suppression is feasible”.

Waikato River iwi volunteers gathered 125kg of gold clams from a small stretch of beach at Lake Karapiro in August.

BNZ said much of what the Technical Advisory Group (TAG) report recommends was already in progress.

“This includes a national surveillance plan, a public awareness campaign promoting Check Clean Dry to prevent spread, and making Corbicula fluminea [gold clam] an unwanted organism under the Biosecurity Act, which triggers legal powers to control movement of the clam.

“Suppression and containment in the Waikato will minimise the risk of spread to the rest of Aotearoa New Zealand,” the TAG found.

Niwa and MPI staff sifting sand on the shores of Lake Karapiro in June while collecting samples of invasive clams.

The report recommends facilities and companies on the river, such as power generators and councils, start developing their own procedures to “mitigate potential impacts on their infrastructure”.

The TAG suggested “tools to suppress populations should be trialled”, such as benthic mats, which smother pests on the riverbed, and dredging, but said those were “likely to have significant impact on other species, including taonga natives”.

Another option was to transfer native species from infested areas to other rivers and lakes.

MPI's gold clam testing sites on the Waikato River, its tributaries and Lake Taupō.

Elsewhere, threats of an infestation have prompted the Ngāti Tarāwhai Iwi Trust to seek the temporary closure of Lake Ōkataina, near Rotorua.

“Our decision has been made with the best intentions, to maintain and safeguard the lake from this invasive threat,” the trust said in a statement.

The trust said gold clams “will cause irreparable and irreversible damage to our native flora and fauna … preserving the unique ecosystem of Lake Ōkataina is our duty as an iwi and this measure is essential to sustaining the well-being of our lake”.

The trust was hoping to close the lake by the end of this week.

When asked if authorities had given up trying to eradicate the clams or if they were likely to spread further, BNZ deputy director-general Stuart Anderson referred back to the TAG report and its recommendations.

“The clam has never been successfully eradicated elsewhere. Early detection of new populations keeps open the possibility of local elimination.”

He said a “clean, check, dry” coordinator to educate Waikato River users on how to help prevent their spread, and other pests, had been appointed, and a team of educators were present at events.

“Because of the way tiny juvenile clams drift on the current, the clam will naturally make its way downstream on the Waikato River, but they can’t make any significant progress upstream by themselves.”

Anderson said no other clam populations had been detected outside the river, or its tributaries, or in Lake Taupō.