A company in New Zealand was contracted to develop and plan a management strategy for the eradication of Didemnum spp. from Shakespeare Bay, NZ. A unique removal system was created including a special cutter and vacuum that was used to remove Didemnum spp. from ships hulls and the ocean floor. Other treatments included dumping dredging on the seabed under the old barge site to suffocate the remaining Didemnum spp. that could not be manually removed; placing plastic wrappings around the wharf piles in the hopes of again suffocating the invasive; covering the seabed under the wharf with filter fabric; and inspecting all vessels in the harbor and treating them when it was found they carried Didemnum spp. Treatment costs were estimated to be around $300,000. These management actions were unsuccessful, and officials have abandoned further eradication efforts, although the situation will still be monitored continually (Vaughan, 2004).
Location Specific Management Information
Didemnum spp. was found on December 2001 on a heavily-fouled barge from Tauranga anchored at the entrance of Shakespeare Bay, adjoining the Port of Picton. The fouling contained two potentially harmful organisms, Undaria and a colonial sea squirt Didemnum spp. The weight of fouling organisms was estimated to be 25 tonnes, of which Didemnum spp. accounted for 3 tonnes. A further 0.5 tonnes of Didemnum spp. had been deposited on the seabed below. Cawthron Institute was contracted by Port Marlborough NZ Ltd to provide recommendations for management of the infestation (Vaughan, 2004).
The level of concern at the potential impact of this organism on the mussel industry led the stakeholders to select the option that aimed to achieve immediate eradication (Vaughan, 2004). Cawthron Institute created an innovative campaign for the removal of Didemnum spp. from ships hull using a specially-designed cutter that ensured debris and spores were retained in the filtering system. Further treatment included dumping dredgings on the seabed under the old barge site, placing plastic wrappings around the wharf piles, covering the seabed under the wharf with filter fabric, and the treatment of infested moorings and vessels. Treatment costs were estimated to be around $300,000 (Vaughan, 2004).
It was disappointing to find from a delimitation survey in July 2004 that eradication was unsuccessful. Didemnum spp. was found actively growing on the plastic wrappings, under the filter fabric, and on some vessels and moorings. The survey also found that it had also been transported to Arapawa Island in outer Queen Charlotte Sound. An updated report reviewed four management options, and considered estimated costs and benefits to the mussel farming industry. The active management options provided benefits that substantially exceeded costs, but estimates of success were less than 50%. The NZ Marine Farmers Association recommended an option involving monitoring and information gathering; this was adopted by the Council (Vaughan, 2004).
1. Coutts, A. D. M. 2002. The development of incursion response tools - underwater vacuum and filter system trials. Cawthron Report No. 755 Prepared for New Zealand Diving and Salvage Ltd.
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