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

Abstract

Gerritsen, H. D., Minto, C., and Lordan, C. 2013. How much of the seabed is impacted by mobile fishing gear? Absolute estimates from Vessel Monitoring System (VMS) point data. – ICES Journal of Marine Science, 70: 523–531. 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.

The evolution of the industrial trawl fishery footprint off southeastern and southern Brazil

“…Because fishing tracks were not known, estimated swept areas were aggregated in a 30'x30'cell grid which generated uncertainty about the precise area impacted, particularly in areas of low fishing activity (Gerritsen et al, 2013). Yet the analysis provided a preliminary assessment of the area of the continental margin likely demanded (and impacted) by Brazil's largest industrial trawl fishing.…”

“…Delimiting the trawl fishery footprint off southeastern and southern Brazil and its main core areas comprised a first step towards assessing such marginal areas, for example, in the shelf-break and the lower slope below 500 m depths, providing preliminary information for future ecosystem-based fisheries management and marine spatial planning strategies. This process will largely benefit from the analysis of more precise georeferenced effort data, such as those produced by VMS Gerritsen et al, 2013).…”

“…Assessing the impact produced by a given bottom trawl fishery in the marine ecosystem initially requires delimiting its footprint, for example by merging the area swept by a number of trawl operations carried on in a fishing area during a certain period of time (e.g., Benn et al, 2010;Gerritsen et al, 2013;Penney & Guinotte, 2013). Within the delimited footprint, the actual impact of trawling vary according to fishing effort intensity and the sensitivity of the affected habitats to disturbance, usually assessed by the amount of change in benthic communities abundance, production, organisms size and diversity, and the amount of time required for ecosystem recovery (Kaiser et al, 2002(Kaiser et al, , 2006Lambert et al, 2011).…”

Record of anthropogenic impact on the Western Irish Sea mud belt

“…The system has been installed on all fishing vessels >24 m since January 2000 according to European Commission legislation, and on vessels >18 and >15 m since 2004and 2005, respectively (Gerritsen and Lordan, 2011. Gerritsen and Lordan (2011) and Gerritsen et al (2013) provide further information on VMS methods of analysis and processing. VMS data have been previously used to assess the extent and frequency of the impact by mobile fishing gear (Puig et al, 2012;Gerritsen et al, 2013).…”

“…Gerritsen and Lordan (2011) and Gerritsen et al (2013) provide further information on VMS methods of analysis and processing. VMS data have been previously used to assess the extent and frequency of the impact by mobile fishing gear (Puig et al, 2012;Gerritsen et al, 2013). Additionally, data from the Marine Institute have enabled estimates of fishing effort (in hours) for the Irish Sea.…”