Whangārei Harbour – and others along Northland’s east coast – are at risk from new, superspreader invasive pest seaweeds that are devastating parts of the sea around Great Barrier Island, an ocean ecologist warns.

Tūtūkākā-based ocean ecologist Glenn Edney said it was now a case of when not if the seaweeds caulerpa brachypus and or caulerpa parvifolia arrived in Northland.

In the case of Whangārei Harbour, its arrival would devastate a harbour already under pressure from run-off, sediment and over-fishing, Edney said.

The two highly invasive exotic seafloor species were first found in New Zealand on Great Barrier Island by members of the public in 2021 and have only been confirmed there and about 65 kilometres by sea south on Great Mercury Island off Coromandel’s east coast.

Edney said Northland’s coastline including the internationally important Waipū River mouth wildlife refuge, Mangawhai, Tūtūkākā, Ngunguru, Pātaua, the Bay of Islands and Mangonui were among other at-risk harbours. Boats travelling to the Three Kings Islands often leave from Mangonui.

Ōpua marina is already displaying a notice warning of the pest.

“We need to have an official plan in place before its arrival,” Edney said.

He said caulerpa posed a greater risk to Northland’s east coast due to prevailing sea currents and boat traffic. It was unlikely to be able to get as far as Kaipara Harbour.

Edney has been working with community marine projects on Great Barrier Island, visiting there in early March and also with the community, and identified the spread of the new seaweed species to a new place on the island in October.

He said a piece of these caulerpa species not much bigger than a freckle, arriving at Whangārei Harbour’s seagrass beds could grow to become a thick mat covering the area of a rugby field within “a matter of weeks”, depending on the time of the year. The short bright green smothering seaweed grows up to 10 centimetres tall and spreads via creeping horizontal runners.

Last month Cyclone Gabrielle brought 40 tonnes of the pest seaweed ashore in huge bright green swathes, covering the beach in a deep thick mat at Great Barrier Island’s Okupu Beach at Blind Bay.

Edney said the seaweeds’ arrival in Whangārei Harbour would threaten the internationally-famous shellfish beds at Mair Bank, Marsden Bank and Snake Bank. It would also put scallop beds at places like Urquharts Bay at risk. Seagrass beds at One Tree Point would also be under threat.

This would in turn severely impact the local community’s recreational fishing on the harbour. It would also add further pressure for orca visiting the harbour, due to a reduction in the availability of food for stingray, their favourite prey.

Whangārei Harbour’s seagrass beds were critical juvenile fish habitats.

“Areas like the Urquharts Bay scallop beds and Mair Bank and Marsden Bank are already closed to try and help hugely under pressure stocks of scallops, cockles and pipis in these places recover,” Edney said.

“If the pest caulerpa species arrived, it would tip these spots over the edge.”

Whangārei Harbour is a major estuary habitat and food source for as many as 2000 shorebirds at a time, and Edney said the invasive weed would also impact their world.

The highly tenacious and invasive seaweed would cover an area like Snake Bank “within a matter of months”.

Key boating harbours and bays on Great Barrier Island’s south – including Tryphena and Whangaparapara – have been closed to boaties by a Ministry for Primary Industries (MPI) now five-month ban and a mana whenua rāhui is also in place to try to halt the pest’s spread.

Two species of the non-native caulerpa have been found – caulerpa brachypus and caulerpa parvifolia. The Ministry for Primary Industries website says they are a native to the Indo-Pacific from Africa to Australia, the Pacific and southern Japan. Caulerpa brachypus is considered an invasive pest in the US’s Florida and the Caribbean’s Martinique Island.

Edney said affected Great Barrier Island communities were highly distressed about what was happening, how the pest had been allowed to proliferate and what the official plan for dealing with it was going forward. He said nobody knew where the pest, which has also majorly infested areas such as the Mediterranean, had come from to Great Barrier Island.

Information about the two new caulerpa seaweeds can also be found via Northland Regional Council’s online pest hub.

Information about the new seaweed invaders and what to do if finding it can be found here.

■ Local Democracy Reporting is Public Interest Journalism funded through NZ On Air