The damage anchoring can cause

NIWA's high-resolution mapping has produced the first ever global estimates of coastal habitat damage caused by anchoring.

With COVID-19 resulting in port congestion around the world leading to unprecedented bottlenecks in maritime traffic and intensive anchoring of high-tonnage ships, researchers from NIWA, University of Auckland, and Auckland University of Technology set out to characterise the footprint and extent of anchoring on the seafloor.
Using sonars and ship tracking data, they documented the morphology of anchoring outside Picton’s marine port on New Zealand’s South Island.

They found that every time a high-tonnage ship anchors, it can excavate the seafloor up to 80cm deep, and displace enough sediment to fill an Olympic-sized swimming pool, damage that remains for at least four years. Scaled-up globally, this equates to at least 6,000km2 of impacted seabed, which is probably a "substantial underestimate," said the study’s lead author Dr Sally Watson, a marine geophysicist at NIWA and lecturer in marine science at the University of Auckland.

"Picton is a low congestion port but in higher congestion ports, such as Wellington or Long Beach, USA, the footprint extends over a much wider region. "The anchor digs into the seabed and moves around as the ship drifts on the surface, dragging the chain and causing ‘broomstick-like’ abrasions, killing off marine species and releasing carbon stores from the sediment. This has far-reaching implications for already stressed ecosystems and carbon cycling,”Dr Watson said.

Dr Marta Ribó, a lecturer in marine geology at Auckland University of Technology and the study’s co-author said new solutions are needed to reduce seafloor destruction from anchors. "The impact of regular anchoring worldwide likely represents a major driver of shallow marine habitat degradation.

"We also don’t yet have a snapshot of cruise ships because our analysis was done during the height of the pandemic when most had ceased operating. Cruise ships are large and numerous, and they often drop anchor in pristine areas. Now knowing what we do, we must ask ourselves whether we want to encourage this kind of tourism," Dr Ribó said.

"To meet the climate goals outlined in COP26, countries have agreed to work towards a net-zero goal for global maritime shipping. As seaborne trade is projected to quadruple by 2050 and cruise ships begin to sail once more, new solutions are needed to avoid irreversible damage to marine habitats caused by this unreported problem," she said.

The study was published in Nature Scientific Reports,