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Mussel farmers through the Marlborough Sounds have been put on alert for any spread of brown mussels from Tasman Bay after an oil drilling company cleaned its rig late last year.
Biosecurity officials last night said that the semi-submersible drilling ship Ocean Patriot apparently brought the potentially-invasive brown
mussels from South Africa, the last place it was cleaned.
They were found when it was defouled in December - 22km offshore, but still in shallow waters - to rid it of New Zealand green-lipped mussels before being towed to Australia.
The Ministry of Agriculture and Forestry (MAF) is arranging to have the seafloor dredged at the site. It has asked oyster fishers for a 2km "exclusion zone" around the site until then, and has warned mussel farmers of the possibility of the brown mussel establishing.
"We have asked for any immediate reports," MAF's biosecurity incursion response manager David Yard said last night.
"I'm reasonably confident that we will get all of them," Mr Yard told NZPA. "But if they have already spawned ... then we have much bigger problems".
"This (rig) may have been contaminated for some time and everywhere it has been it has spread the mussels ... but we have had no reported cases of brown mussels occurring in New Zealand to date".
The brown mussel is difficult to tell from native green-lipped mussels.
MAF's biosecurity incursion response manager David Yard said the initial response was focused on the seabed where the rig was cleaned in December.
The rig has been moored off Napier, the Wairarapa coast, off the Canterbury coast and in Taranaki waters, and Mr Yard said it was possible other site might be surveyed for brown mussels.
It was not known exactly what effects there would be from establishment of the brown mussel on the doorstep of New Zealand's $200 million mussel industry.
The species has recently invaded North America around the Gulf of Mexico and is reported to have become a nuisance at water-cooling intakes for power stations.
The pest was left on the seabed by workers cleaning the Ocean Patriot at the order of the Victorian state government, before a tow across the Tasman.
Ironically, the Australians were worried about it carrying New Zealand's green-lipped mussels into their waters.
The rig owners, Diamond Offshore Drilling Inc, complained to financial analysts on February 7 that hiring divers and blasters to remove green-lipped NZ mussels cost it US$5 million ($6.2 million) and put the rig out of commission for 23 days.
The company is now also largely funding next week's clean-up, MAF said.
Diamond Offshore claimed that rough weather meant the defouling could not be done in the open sea. MAF gave the go-ahead to do it in Tasman Bay, as far offshore as possible, based on survey information indicating there were no unwanted organisms on the rig, and the fact that the weather was posing a threat to human safety.
A spokesman for Aquaculture NZ, Chris Choat, said it was not known how the brown mussel would adapt to life in NZ waters, and he said mussel farmers were not blaming MAF.
"They have been operating within their legislation," he said.
The defouling site was about 12 nautical miles offshore - outside the territorial waters controlled by New Zealand - and Mr Yard said there are "huge difficulties" caused by a gap in the laws covering waters beyond territorial waters.
"We are looking at regulating bio-fouling," he said. NZ had asked the International Maritime Organisation to pursue the issue.
Mr Yard said even if the defouling was done beyond 12 nautical miles, spawn from mussels on the rig could have survived for weeks: "If they had spawned, its quite likely the spawn would still have hit New Zealand".
- NZPA
