Stopping the spread of exotic caulerpa

If you came across exotic caulerpa on a dive, would you recognise it and know what to do?

Exotic caulerpa is the name for two closely-related species of fast-growing, invasive seaweed that's been found in parts of the Hauraki Gulf, Bay of Islands and the Coromandel.

In the right conditions, exotic caulerpa grows fast, blanketing the sea floor and smothering our native seaweeds and seagrass that are vital marine habitats for fish, shellfish and other marine life. This could lead to a significant decline in biodiversity, and threaten recreational fishing and diving - all the joys of the ocean we love.

What is exotic caulerpa?

Exotic caulerpa is a green seaweed that can appear similar to a number of common seaweeds, including native caulerpa, the sea lettuce Ulva sp., and native seagrass. It has leaves up to 10cm long, and long runners with roots known as stolons. 

Unlike similar species, it has distinctive leaves shaped like oars.

Exotic caulerpa can result in the loss of marine habitats that affect fish populations and overall marine health. However, the spread of this weed has a wider reach than the significant ecological impact. If it can't be controlled, consequences and measures could include restrictions on recreational areas for anchoring, fishing and diving, impact on our aquaculture and tourism industries, and financial consequences.

Where has exotic caulerpa been found so far?

Since surveillance on this pest began three years ago, exotic caulerpa has been found in eight regions of the Hauraki Gulf, including much of the western side of Aotea Great Barrier Island.

• Aotea Great Barrier Island
• Iris Shoal, Kawau Island
• Bostanque Bay, Kawau Island
• Rakino Island
• Waiheke Island

There has also been a small infiltration in Omaha Cove/Leigh Harbour. This was found early and successfully removed, but surveillance continues.

Outside the Auckland region, it has been found at Fantail Bay and Ahuahu Great Mercury Island in the Coromandel, and Te Rāwhiti in the Bay of Islands.

How is exotic caulerpa spread?

Exotic caulerpa is spread over long distances on vessels and equipment - it's been found around the Hauraki Gulf in areas where boaties often set anchor.

While exotic caulerpa can spread via currents and waves, vessels can take it much further and faster than any natural dispersion. For this reason, it's vital to check and clean all equipment, including anchors and anchor chains, fishing gear and dive equipment.

Can't exotic caulerpa just be removed once it's found?

It's not possible to remove well-established, large infestations of exotic caulerpa with current tools, which is why it's vital it isn't spread to more locations. Failure to contain this invasive weed could result in changes, and even total losses, of the recreational areas we love so much.

There is a vast body of work underway including councils, iwi and Biosecurity New Zealand. Some innovations since surveillance began three years ago include the use of wool benthic mats to smother the pest, removal with suction dredges, and even ultra-violet light. Projects have so far received more than $15m of government funding for caulerpa surveillance, management and research, as well as significant regional investment by Auckland Council, Northland Regional Council and Iwi.

How can I help?

The message to everyone is the same: make sure you know where you can safely anchor and fish before heading out. Look out for ambassadors and Auckland Council staff who are visiting boat ramps, marinas and the ocean to provide information and answer any questions.

Together, boaties, divers and beachgoers can help stop the spread of exotic caulerpa by following a few simple steps:

• Avoid anchoring or disturbing the sea floor in infested areas. Anchors, chains and fishing and diving gear can break up caulerpa - from there, pieces get snagged and accidentally carried to new locations.
• Always check and clean all your gear before moving to another location - especially if you've been in an area known to have exotic caulerpa.
• When at sea, always check your anchor and equipment for any attached seaweed. If you find any, bag it and bin it onshore.
• If you see exotic caulerpa - or any seaweed you don't recognise - take a photo, record where you've seen it and report it to MPI on 0800 809966.
• Respect Controlled Area Notices (CANs) and rāhui, which are in place to protect vulnerable environments. As much as possible, try to avoid areas exotic caulerpa has been found.

Protecting our paradise

The best way to stop the spread is to know the rules of each area before you head out.

Finding out more

For more information and guidance on how you can help, visit the Auckland Council website here, and the MIP website here.