It's a wrap! - Smothering squirts on HMNZS Canterbury
Brendan Gould (left) and Peter Stratford,
Senior Advisers (Marine) with BNZ's
Post-clearance Directorate, prepare the
plastic cover for the hull of the
Canterbury to kill the sea squirt before
the decommissioned frigate is sunk in
the Bay of Islands.
Photo: Brendon O'Hagan.
It's not quite a case of klingons on the starboard bow, but Styela clava sea squirts on the stern of HMNZS Canterbury have seen Biosecurity New Zealand (BNZ) embark on a hull wrapping trial to deal with the pest before the decommissioned frigate is sunk as a dive attraction.
The trust hoping to sink the 113-metre, 3,000-tonne Leander class frigate gained resource consent to sink the vessel in Deep Water Cove, Cape Brett, in the Bay of Islands, provided there were no unwanted organisms on the hull. An inspection revealed Styela clava.
"The presence of Styela is not too surprising, given that the Canterbury has been tied up at Devonport Naval Base for two years, and Styela is well established in the Hauraki Gulf," says BNZ Senior Marine Adviser, Dr Peter Stratford.
Styela clava and other sea squirts can smother underwater structures, particularly marine farms. They have had a severe impact on the Canadian aquaculture industry.
"Obviously, the Bay of Islands is a high value area, and Styela populations have not been detected there. The situation gave BNZ an opportunity to trial a technique that we know has worked on smaller vessels and minimise the time the frigate has to spend in dry-dock, helping reduce costs for the Trust," Dr Stratford says.
The trial sees the underwater section of the hull shrouded in the same PVC plastic used on farms to wrap silage. The idea is to deprive organisms on the hull of oxygen, so they suffocate and drop off, being contained in the plastic wrap when it is removed after about two weeks. Levels of oxygen in the water are to be monitored during that period. As the wrapping technique is still in the trial phase, dry-docking will be required to ensure the unwanted sea squirt is not transported to the Bay of Islands.
"It's similar to a technique used to wrap wharf piles and underwater structures to deprive sea squirts of habitat," Dr Stratford says. "This works very well, but this trial is pushing the envelope in terms of boat size. The largest vessel we know of that has been subjected to the wrapping technique is about 60 metres long. The Canterbury is 113 metres, so it's an opportunity to expand the technology as well.
"Only time will tell whether it works, but it's a fairly large-scale example of the need for hulls of all sizes to be kept clean, particularly if they are going to high-value areas," he concludes.
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Page last updated: 20 June 2008