Didemnum

An aquaculture industry Working Group is pitting Kiwi ingenuity against an invasive sea squirt called Didemnum.  Introduced to Shakespeare Bay in 2001, this pest is primarily a problem for the aquaculture industry.  Over the course of three months if left to its own devices, it can smother mussel lines, dragging the shellfish to the seabed.

Didemnum is also seen as posing a major threat to the undersea environment, including marine reserves. Following its discovery, the marine farming industry rallied together to discover ways of controlling this underwater pest then tried a solution; suffocating infected areas by wrapping them in plastic.

The aquaculture industry-backed Didemnum project was the supreme winner of the 2007 Marlborough Environment Awards, recognising this as an excellent example of a primary industry combining ideas and resources to tackle a biosecurity threat.

Control efforts benefit the NZ$166 million aquaculture industry and the New Zealand/Marlborough economy while protecting the Marlborough Sounds environment.  If the organism is not controlled/eradicated, it has the potential to spread to wharves, beaches and recreational boating facilities, as well as areas of high conservation value such as marine reserves.

Didemnum vexillum is a sea-squirt that thrives on underwater surfaces such as wharf piles, boat bottoms, submerged trees, mussel lines and salmon cages. It has the appearance of poured yellow wax and the feel of wet leather.

The organism arrived in New Zealand in 1992 on a logging barge, originally from the Philippines. It was brought to Shakespeare Bay, Picton in December 2001. It has also been found in the Bay of Plenty near Whangamata and Tauranga.

Left to multiply, Didemnum can cover mussel lines in three months, eventually dragging the farm to the sea floor.  It smothers areas of sea-bed as well as paua, seaweed, crabs etc and badly fouls salmon cages. It is spread by current/water flows, which assist spawning and pieces drift to new sites starting new populations.

The economic and environmental implications of this pest is significant. Marlborough's $180 million marine farming industry, which provides 2500 jobs, is feeling the marine pests effect. It could become an environmental problem, spreading onto pebbly beaches and slow-moving species like crabs and crayfish, as it has in other parts of the world. It has already been found on seaweed and rocks.

Didemnum was first identified in 2005 as a potential threat to the aquaculture industry when Queen Charlotte College aquaculture students found it; on a mussel line they'd located near Picton. The mussels were dragged to the sea bottom and unable to be harvested. In 2003 the infested logging barge was scuttled in Cook Strait and a clean-up of Shakespeare Bay was attempted, but was unsuccessful.

In March 2006, Marine Farming New Zealand set up a Didemnum Working Group including representatives of the Marlborough District Council, Port Marlborough, Cawthron Institute, Marlborough Marine Farmers, Bio Security New Zealand, the Department of Conservation, Queen Charlotte College and NZ King Salmon. Its first priority is to contain the sea squirt to reduce potential for spawning, but local eradication is the ultimate goal. A full survey of Pelorus, Kenepuru and Queen Charlotte Sounds identified many infected areas. 

After some trial and error, in June 2006 an ingenious method of controlling Didemnum, by wrapping affected surfaces with baleage wrap, was developed and successfully applied. The wrap works by starving the area of oxygen/water flow and effectively suffocating the noxious pest.

Sheets of plastic work well on wharf piles and vessels and plastic tubing is used to treat mooring lines. Low concentration household vinegar is sometimes used to aid the process.

The 2007 clean-up involved wrapping 12 jetties, 27 vessels, 47 moorings and 190 wharf piles in baleage wrap and covering 11,396m2 of sea-floor. Professional divers were contracted to do the job, also treating over 100 mussel lines (wrapped in plastic sleeves) and two submerged trees (removed from the water).

The wrap is put in place during the winter/spring months and removed at the end of the spawning season, in May.

Every three months, divers check that the organism has not reappeared in treated areas.

The aquaculture industry has spent a million dollars to date in cash and kind, on Didemnum control with contributions from Marlborough Marine Farmers, NZ King Salmon the Cawthron Institute in Nelson and the Marlborough District Council.

Biosecurity New Zealand has contributed $295,000 to Didemnum control; a relatively small amount due to the pest being discovered before the Government organisation was established. This means that like gorse and rabbits Didemnum is not seen as a new pest so isn't given such a high priority.

Biosecurity NZ help has included help with building awareness of Didemnum plus contributions to research and surveillance. A hotline support has been set up whereby anyone can phone a 0800 number to report suspicious organisms including Didemnum.

The Department of Conservation has donated some vessel time.

Currently only isolated areas of Didemnum remain. Two people are now working part-time on Didemnum control; Aaron on operations management and another person on communications liaison work with stakeholder groups. For the last eight months, a commercial dive team has been contracted to work almost full time on eradication.

The Nelson-based Cawthron Institute, in association with the Didemnum Working Group, is conducting research on how this sea squirt reproduces and spreads. Specifically it is looking at when, where and how it can relocate onto natural and artificial surfaces including mussel spat lines. A number of private aquaculture companies are studying Didemnum's effects on their businesses and how to treat structures and mussel seed to eradicate or prevent its spread.