How much of the seabed is impacted by mobile fishing gear? Absolute estimates from Vessel Monitoring System (VMS) point data

Demersal trawling impacts extensively on the seabed, and the extent and frequency of this impact can be assessed using Vessel Monitoring System (VMS) data (positional data of fishing vessels). Existing approaches interpolate fishing tracks from consecutive VMS locations (track interpolation) and/or aggregate VMS point data in a spatial grid (point summation). Track interpolation can be quite inaccurate with the current 2-hour time interval between VMS records, leading to biased estimates. Point summation approaches currently only produce relative estimates of impact and are highly sensitive to the grid size chosen. We propose an approach that provides absolute estimates of trawling impact from point data and is not sensitive to an arbitrary choice of grid-cell size. The method involves applying a nested grid and estimating the swept area (area covered by fishing gear) for each VMS point. We show that the ratio of the swept area to the surface area of a cell can be related to the proportion of the seabed that was impacted by the gear a given number of times. We validate the accuracy of this swept-area ratio approach using known vessel tracks and apply the method to international VMS data in the Celtic Sea.