Caulerpa invasive seaweed threat spreading - what you need to know

Northland is facing an at least 10-year-long battle to eradicate the new marine seaweed superspreader, caulerpa.

New Zealand's first mainland infestation of the exotic pest was found at Omākiwi, Te Rawhiti in the Bay of Islands on 3 May. It is still the only known confirmed mainland infestation.

It was first confirmed on Great Barrier Island/Aotea in June 2021, and this infestation is thought to have been on the island for several years before that official identification.

The new pest has created alarm among those who live, work and play along Northland and New Zealand's coast - including mana whenua, councils, boaties, divers, recreational and commercial fishers and tourist operators.

More than 1000 kilometres of New Zealand's north-eastern coastline from Cape Reinga to East Cape are at risk.

Local Democracy Reporting talked to New Zealand caulerpa expert, NIWA's national marine biosecurity research programme leader Graeme Inglis in Nelson, to find out more about the new pest seaweed arrival.

Inglis has led NIWA's marine research programme for 20 years.

He said globally there were about 100 different species of caulerpa. Their main range was around the tropical Indo Pacific central zone of the world. Many of these caulerpa were native to the areas in which they lived, meaning they had evolved naturally within their own complex local food web, with its own checks and balances. New Zealand has nine native caulerpas distributed throughout the country.

He said problems arose when caulerpa jumped to new places overseas where it was not native and became an invasive pest. This happened predominantly via human activity.

Inglis said New Zealand had evolved in isolation from the rest of the world for centuries, meaning much of its marine ecosystems were unique. That in turn made the new arrivals' presence an even-bigger threat.

New Zealand's native caulerpas were not a problem, Inglis said.

Local Democracy Reporting also talked to Tūtūkākā-based ocean ecologist Glenn Edney who worked with exotic caulerpa-affected communities on Great Barrier Island/Aotea.

Edney warned the exotic caulerpas are capable of spreading from a piece the size of a freckle, to cover the equivalent of a rugby field within a few weeks in the right conditions.

What is pest caulerpa?

Caulerpa is a kind of giant algae. The two types of exotic caulerpa in New Zealand form messy, thick short carpets marching across the sea floor. Depending on type, it is typically bright apple green as it grows, but can become yellowish if sunburnt in shallow water.

New Zealand's nine native caulerpas include three in the North Island. Another is found in the South Island including Stewart Island and the subantarctic Snares Islands, as well as on the Chatham Islands.

Aotearoa now also has two exotic pest caulerpas, caulerpa brachypus and caulerpa parvifolia.

These two caulerpas are found naturally in Lord Howe Island, Norfolk Island, central and western Pacific including Tonga, and in Australia.

In Indonesia and the Philippines they are also farmed and harvested.

Why is caulerpa a problem for Northland and New Zealand?

Caulerpa grows and changes the environment in which it lives by carpeting everything in its path and turning it into a monoculture. Inglis said some creatures benefit from this but many don't.

The Ministry for Primary Industries (MPI) said once established in Northland and New Zealand, it would potentially reduce the nursery habitat for fish by crowding out native marine plant species. This reduces the population size of recreationally and commercially important fish stocks. Caulerpa's dense clumps can block fish from finding their food such as bottom-dwelling crabs, clams and worms.

Where does Aotearoa's pest caulerpa grow?

Along the coast between just a couple of metres deep, up to about 40 metres deep.

How does pest caulerpa spread?

The pest caulerpa in New Zealand mostly spreads via fragments. These tiny pieces easily break off a plant in storms or through boat anchors, divers' actions or fishing equipment. It also spreads to a much lesser extent through its roots. Caulerpa grows about 10 centimetres tall and sends out horizontal creeping roots across the sea floor. Its spread is most significantly thought to be caused by people.

Aotea/Great Barrier Island's Tryphena Harbour is now shut to anchoring boats due to caulerpa infestation in the key anchorage.

Who does caulerpa most affect?

Those who live, work or play in or on the sea including mana whenua and others gathering kaimoana such as mana whenua and others. Boaties unable to anchor in the equivalent of more than 1550 rugby fields due to rāhui and Controlled Area Notice bans in the Bay of Islands and Great Barrier Island/Aotea and Great Mercury Island/Ahuahu. Tourist operators who rely on healthy coastal marine ecosystems for their business.

Does caulerpa really matter to me if I'm not a boatie, have a bach on Northland's east coast or go to the beach?

Yes, definitely.

Tūtūkākā-based ocean ecologist Glenn Edney said Northland's regional climate was already being affected by the breakdown in natural ecosystems, including the increased quantity of rain Northland had been experiencing over the last six months.

Ecosystem health was a key defence against human-induced climate breakdown.

"People might think it doesn't affect them, but it sure as hell does because everything is connected. The problem affects absolutely everybody," Edney said.

Is it dangerous for me to eat fish that have been eating caulerpa?

There appears to be a variety of positions on this.

Caulerpa taxifolia (as distinct from the exotic caulerpa brachypus and caulerpa parvifolia found here) that now covers the Mediterranean's sea floor produces a toxin to stop fish eating it. Some fish are still able to, but the toxin stays in their flesh. The toxin accumulates up through the food chain. Some people who have eaten fish from the Mediterranean have become sick.

Edney said the potential exists for that to happen in New Zealand and research was needed into this area.

Inglis said all caulerpas contained a chemical known as caulerpryne, aimed at making them less palatable to fish. They contain this chemical to varying degrees, the quantity increasing where the caulerpa was grazed.

The Ministry for Primary Industries' (MPI) website said the two exotic species of caulerpa in New Zealand do not contain the toxin therefore do not create any food safety issue with consuming fish.

Will I still be able to buy local fish and shellfish if caulerpa takes over?

The local fishery has the potential to be very seriously affected by major caulerpa infestation, Tūtūkākā's Edney said.

Quantity and variety of fish declined in heavily caulerpa-infested areas.

Overseas research showed the quantity of life in caulerpa-infested locations drops by as much as half. Dense clumps of caulerpa can block fish from feeding on bottom-dwelling small creatures like crabs, clams and worms.

Northland Regional Council (NRC) biodiversity and biosecurity working party chair Geoff Crawford represented New Zealand at the 2014 world spearfishing championships, held in the heavily caulerpa-infested Mediterranean. He said there were "bugger all" fish there and that spearfishers had needed to dive as deep as 45 metres to find fish.

Commercial shellfish harvesting has long been a feature of Whangārei Harbour life but could be under threat.

Traditional cockle-harvesting areas on Great Barrier Island have already died off under smothering exotic caulerpa carpets.

What is the prognosis for New Zealand's caulerpa infestation?

Inglis said it was hard to give a prognosis.

There were so many variables, he said.

On the one hand northern New Zealand's weather conditions were changing over time.

"It seems that the conditions are getting more suitable for some of these subtropical species (such as caulerpa)," Inglis said.

Caulerpa likes sea temperatures averaging above 15 degrees.

On the other hand, the warm water East Auckland current that sweeps down Northland's east coast at certain times of the year brought many tropical fish with it. Then they disappear as the current retreats.

"Maybe caulerpa will disappear in the same way," Inglis said.

New Zealand's ecosystems were different from other places globally, therefore making it challenging to know how either of the pest caulerpas would behave here.

"It could be that their growth becomes seasonal in New Zealand. We don't know how that's going to play out with caulerpa here in New Zealand," Inglis said.

New Zealand is the first and only country on the globe to have a pest caulerpa brachypus infestation. There was no research that might help with understanding how it might behave here, he said.

Inglis said New Zealand's marine heatwave of recent years had brought conditions likely to be favouring caulerpa.

However, Aotearoa was coming out of the prolonged heat associated La Nina conditions.

"It we get a cold winter with El Nino conditions, perhaps we me have success in getting rid of it

The pest had previously been expected to show a reduction in biomass and area on (Great Barrier Island/)Aotea over the winter, but that didn't happen.

Can caulerpa be eradicated?

There appears to be a variety of opinion on this.

Inglis said some locations globally have had a lot of success in eradicating invasive caulerpa, where they have the right tools to apply to the local situation, are organised and resourced to do the job.

This was the case where caulerpa taxifolia had been successfully eradicated in the Newport and San Diego marinas in California.

MPI said no country in the world has fully eradicated caulerpa.

That was not stopping NRC, which is solely focused on the pest's eradication in Northland.

How will I know if I see caulerpa?

Caulerpa spreads by breaking off from larger clumps, as can be seen with this small piece of the marine pest on the beach at Omakiwi in the Bay of Islands. Its lost some of its original bright green colouring due to being out of the water for a while.

Underwater: clumps or a fully engulfing carpet of typically bright green short seaweed that looks a bit like a bad hair day. Sometimes looks slightly yellowed where it gets sunburned.

On the land: What you will see from caulerpa washed up on the beach varies with how much of the pest is growing in adjacent sea area and how long the seaweed has been out of the water. On Great Barrier Island/Aotea several metres-long giant sausages of the bright green seaweed washed up in Blind Bay during Cyclone Gabrielle. The infestation there may have been present for about five years. In the Bay of Islands, the infestation is not usually as dense. Thinner sausages washed up on the beach may be only 10-20cm long. Some pieces are tiny, not much bigger than an oversized bean sprout. The washed-up seaweed changes to dark green in colour after it has been out of the water for a few days. Caulerpa looks innocuous.

What can I do?

Keep your eyes peeled when you're in or on the water or walking on the beach or rocks.

Anybody can search, on land or on sea.

"The quicker we are able to work out where it is in northern New Zealand the better - to help us get a picture of where it is and whether it's expanding," Inglis said.

It is illegal to shift caulerpa from the beach.

If you find it, take a photo, note exactly where you are even with GPS. Complete MPI's online reporting form at ]or contact Biosecurity New Zealand on 0800 80 9966.

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