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 ICES Journal of Marine Science (2013), 70(3), 523 –531. doi:10.1093/icesjms/fst017 How much of the seabed is impacted by mobile fishing gear? Absolute estimates from Vessel Monitoring System (VMS) point data Hans D. Gerritsen 1 *, Cóilı́n Minto 2, and Colm Lordan 1 1 Marine Institute, Rinville, Oranmore, Co Galway, Ireland Marine and Freshwater Research Centre, Galway-Mayo Institute of Technology, Dublin Road, Galway, Ireland 2 *Corresponding author: tel: +353 91 387 297; fax: +353 91 387 201; e-mail: 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. Received 1 November 2012; accepted 23 January 2013; advance access publication 18 February 2013. 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 gridcell 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. Keywords: Automatic Identification System (AIS), impact of trawling, Marine Strategy Framework Directive (MSFD), nested grid, swept-area ratio, Vessel Monitoring System (VMS). Introduction Demersal trawling is probably the most extensive human activity that impacts on the seabed (Eastwood et al., 2007; Foden et al., 2011). Trawling has both direct and indirect effects on benthic ecosystems, and the severity and longevity and complex interactions involved are receiving increasing attention (Hiddink et al., 2006; Lambert et al., 2011). Policy developments such as the ecosystem approach to fisheries management (FAO, 2008) and the EU Marine Strategy Framework Directive (EC, 2008) oblige member states to provide indicators that include the quantification of the impact of fishing on the seabed. These indicators should describe how much of the seabed is impacted by trawling and how often it is impacted. The widespread implementation of Vessel Monitoring Systems (VMSs) for surveillance purposes has, as a by-product, given scientists access to a rich dataset of fishing vessel positional data. These data have allowed major progress towards the goal of quantifying the distribution and intensity of trawling. Fishing vessels fitted with VMSs transmit their position and speed at regular time intervals; in EU waters the maximum time interval between transmissions is 2 h, and since 2005 all fishing vessels .15m are required to carry VMS (EC, 2003); since 2012 this has been extended to all fishing vessels .12m (EC, 2009). There are two existing approaches to estimating the area impacted by fishing gear: track interpolation and point summation methods. Track interpolation methods aim to re-construct vessel tracks between consecutive VMS points. This can be done using a straight line or a spline curve (Hintzen et al., 2010; Russo et al., 2011, and references therein). Skaar et al. (2011) found that straight line interpolations at 2-h intervals deviated . 3 km from the real track for the majority of hauls of two Norwegian demersal trawlers. Lambert et al. (2012) found that # 2013 International Council for the Exploration of the Sea. Published by Oxford University Press. All rights reserved. For Permissions, please email: 524 H. D. Gerritsen et al. both straight-line and spline interpolations deviated  2 km from the real track for Isle of Man scallop dredgers and trawlers. These findings show that there is considerable uncertainty in the vessel position during the 2-h interval between VMS records. So while interpolated tracks can be used to estimate the likelihood that a location is trawled, they cannot be used to accurately estimate the number of times a location is trawled (Hintzen et al., 2010). The other main existing approach, point summation, involves applying a grid to all VMS point locations where the vessels were deemed to be fishing (Lee et al., 2010); each grid cell that contains VMS points is then considered to be impacted by trawling. This approach is generally applied to the observed VMS points but can also be applied to interpolated VMS points, where interpolated fishing tracks are converted back into a sequence of point locations. An important problem with the point summation approach is that the proportion of impacted cells is strongly dependent on the grid size chosen (Dinmore et al., 2003; Piet and Quirijns, 2009; Hinz et al., 2012; Lambert et al., 2012; Piet and Hintzen, 2012), because larger grid cells are less likely to be completely free of trawling activity than smaller cells. Related to this is the issue that generally only a part of each cell is impacted by fishing gear; for large cells with a small amount of effort this may only be a small proportion of the cell area. We are not aware of any studies that take this into account. We modified the existing point summation approach by addressing its sensitivity to an arbitrary choice of grid-cell size and accounting for the fact that cells with fishing effort may be only partially impacted by fishing gear. We applied the proposed method to VMS data from 2011 in the Celtic Sea (south of Ireland) to illustrate the approach. Methods VMS data and study area VMS data were available for all fishing vessels .15m in total length inside the Irish Exclusive Economic Zone (EEZ). We used data from the most recent year available (2011) in the Celtic Sea (ICES Divisions VIIg and VIIj). We chose this region because it contains a broad range of habitats and it displays strong spatial structures in the distribution of fishing effort. Because no deepwater fishing takes place in this area, we excluded areas deeper than 800m. The study area covers an area of 90 367 km2 and is shown in Figure 4. Only mobile bottom-impacting gears were included in the analysis: demersal otter trawls, beam trawls and dredges. Seines were not considered to be bottom-impacting gears for the purpose o (...truncated)