Dragging the chain: Quantifying continued losses of seagrasses from boat moorings

Globally, seagrasses and other estuarine habitats are under threat from a range of human disturbances, including boating-related activities. The fixed, block-and-chain swing moorings used to secure boats in many countries can create visible scars in seagrass meadows and remove other benthic organisms. 2Losses of seagrasses in two estuaries in New South Wales (NSW), Australia, were quantified using aerial imagery and used to estimate estuarine habitat damage across the remainder of the state. The rate of mooring scar development was approximated and factors influencing the size of mooring scars were identified. 3Individually mapped mooring scars in meadows of the endangered Posidonia australis ranged from 55 to 706m(2), and were generally larger than those in the more opportunistic seagrass Zostera muelleri subsp. capricorni (22-342m(2)). Scar size in both species of seagrass increased significantly with depth and boat length. 4The rate of increase in the size of mooring scars amongst P. australis was roughly linear over 4.8years, averaging 6.3m(2)/month up to a maximum of 16.7m(2)/month for one 16-m yacht. 5Across all of NSW, it was estimated that currently leased moorings were causing losses of 129 884m(2) of P. australis and 93 940m(2) of Z. capricorni, and disturbing 2 790 907m(2) of non-vegetated soft sediments. NSW seagrass loss estimates relate only to damage from current moorings and could be underestimated by similar to 41% for P. australis if estuaries contain many old scars that remain after the relocation of moorings (as has occurred in Lake Macquarie). 6Mandating the use of moorings that do not rely on a chain dragging on the sea bed would greatly reduce future physical damage of benthic habitats and enable some recovery from legacy effects.