Anchor Scour Pressure on Seabed Ecosystems in a Busy Commercial Anchorage Calculated from Automated Identification System (AIS) Data.

Commercial shipping is a vital element of global trade, with large ships carrying nearly 90% of global cargo. As part of the routine navigation of these vessels time must be spent at anchor, often in busy anchorages with multiple other ships. The impact of the deployment of anchoring equipment and the subsequent scour caused by the length of heavy chain, proportional to vessel size, as it is dragged across the seabed, has been understudied as an impact of shipping. We present a new methodology for estimating the anchor scour pressure experienced on the seabed from Automated Identification System (AIS) data, producing a pressure estimate incorporating the size and weight of a vessels anchoring equipment. Using this method we estimate anchoring pressure in St Brides Bay, a busy commercial anchorage within the Pembrokeshire Marine Special Area of Conservation, Wales between 2013-2020. Our algorithm revealed over 6000 individual anchoring events across the 8 years impacting 60.84 km2 across the bay. Anchoring was found to be unevenly distributed, with areas of highest pressure located further from shore and closer to the bay’s centre, whilst areas of most frequent anchoring, found further south, were closer to the port of Milford Haven. Areas mapped as sandy and muddy substrate within the bay were also found to be favoured for anchor deployment over coarse sediment or rock. Overall, our results reveal a high and potentially biologically significant level of anchor chain scour disturbance within a protected area using a readily transferable method for handling AIS datasets.