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Ship Anchors May Cause Extensive And Long-lasting Damage To The Seafloor, According To New NIWA Research

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NIWA anchored their research vessel in Wellington Harbour and observed in real-time how its anchor changed the surrounding environment.

Study leader NIWA geophysicist Dr Sally Watson says they saw broomstick-like scarring on the seabed.

"Our preliminary findings showed marks on the seabed typical of an anchor being moved around due to wind and currents. The damage caused is often compared to that of trawling, but anchoring differs in that it tends to happen in shallow waters and repeatedly over the same locations, with potentially more intense and deeper seabed penetration," said Dr Watson.

This work is part of a multi-year MBIE Smart Ideas project to redesign anchoring practices for a more sustainable shipping industry. Little research is available on how anchoring damages the environment, particularly in relation to larger vessels.

"Currently, most research is conducted on small recreational boats in shallow water, so this gave us a great insight into how bigger ships impact the seafloor at different depths. The findings will eventually be used to develop alternative solutions to current anchoring practices, including introducing new disturbance criteria and using designated drop points," said Dr Watson.

During the study, Watson and her team anchored NIWA’s 73m long research vessel Tangaroa in Wellington Harbour, collecting over 1,000 water samples, taking before and after surveys of the seafloor, and capturing video footage from drones, a GoPro, and underwater cameras.

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They battled several disturbances, including gales, shifts in wind direction, and changes in location due to other ships coming into port. Serendipitously, another vessel was anchored nearby and gave permission for NIWA to study their impact too, giving scientists a great comparison opportunity.

"These challenges were all blessings in disguise because we now have a rich set of data. The drone and GoPro footage was particularly fascinating, as they captured striking visuals of plumes of sediment being dredged from the seafloor as the anchors were raised," said Dr Watson.

Globally, it is predicted that there will be a four-fold increase in shipping by 2050. Wellington Harbour is considered a moderate congestion port, handling approximately 13,000, 3.8 million tonnes of cargo, and 1.5 million passengers annually.

"This year, New Zealand saw its biggest ever summer cruise season, with 1,000 port visits and 300,000 passengers. Cruise ships often visit pristine locations and are a massive and a constant presence, so they likely put huge pressure on marine ecosystems," said Dr Watson.

NIWA researchers were also able to compare their findings to underwater maps taken of the same area in 2009. They discovered that some features were still visible on the seabed over 15 years later.

"Until this point, we knew that damage could last up to four years, but it turns out the legacy lasts a lot longer - our marine activities can leave their mark over decades. Anchoring may need to be rethought to mitigate potential impacts to the marine environment, while balancing this against the projected growth in shipping," said Dr Watson.

CentrePort, which has been a key contributor to the study, says the work is aligned with its efforts to continuously improve the management of environmental risks and impacts to the harbour.

"We’re eager to support efforts that improve the sustainability of the shipping industry and continue to improve the management of environmental risks and impacts, such as those that may prove to be associated with ship anchors. While most of the ships that come to Wellington don’t anchor, studies like this offer a great opportunity to learn more and gather vital information that will inform what the shipping industry does to protect the precious marine floor," Environmental Manager Thomas Marchant said.

The next stages in this project involve case studies from around Aotearoa New Zealand using ship tracking technology, as well as analysing biological data.

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