ICES Journal of Marine Science: Journal du Conseil Advance Access originally published online on February 19, 2007
ICES Journal of Marine Science: Journal du Conseil 2007 64(3):453-463; doi:10.1093/icesjms/fsm001
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Human activities in UK offshore waters: an assessment of direct, physical pressure on the seabed
Centre for Environment, Fisheries and Aquaculture Science (Cefas), Lowestoft Laboratory, Pakefield Road, Lowestoft, Suffolk NR33 0HT, UK
Correspondence to P. D. Eastwood: tel: +44 1502 562244; fax: +44 1502 524546; e-mail: paul.eastwood{at}cefas.co.uk
Eastwood, P. D., Mills, C. M., Aldridge, J. N., Houghton, C. A., and Rogers, S. I. 2007. Human activities in UK offshore waters: an assessment of direct, physical pressure on the seabed. – ICES Journal of Marine Science, 64: 453–463.Integrated assessments (IA) and marine spatial planning (MSP) are becoming major drivers for the assessment and management of human activities at sea. To be successful, both require an understanding of the distribution of the pressures caused by human activities. We used spatial data for the major human activities operating in the England and Wales sector of UK waters in 2004 to provide an assessment of direct, physical pressure on the seabed from multiple human activities. Pressure was estimated as the spatial extent of each of the activities; the intensity, longevity, and impacts arising from the pressures were not considered. Estimates of spatial extent were assigned to three pressure categories, subdivided into six pressure types. We estimated that four of the six pressure types affected < 1% of the seabed of England and Wales in 2004, whereas selective extraction caused by demersal trawling affected a minimum of 5.4%, rising to a possible maximum of 21.4%, of the total area of the seabed. This was a greater area than all other pressure types combined. The assessment process described here can be used as the framework for reporting human pressures at regular time intervals and feed into both IA and MSP for regional seas.
Keywords: geospatial data, integrated assessment, marine spatial planning, offshore human activities, seabed pressure
Received 31 March 2006; accepted 3 January 2007; advance access publication 19 February 2007.
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Introduction |
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 Top Introduction Material and methodsResultsDiscussionReferences |
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UK marine waters are under increasing pressure from multiple human activities, and there are concerns about declines in the environmental status of many of the UK's regional seas (Defra, 2005). Integrated assessments (IA) in support of ecosystem-based management provide a means to assess the current ecological status and the identification of important ecosystem properties and threats (Link et al., 2002; Choi et al., 2005; ICES, 2005b). Key aspects of this process are: the identification of ecosystem components that are structurally and functionally important (e.g. Bremner et al., 2003); the quantification of the type and intensity of factors that influence the ecosystem components, including human activities (e.g. de Groot, 1996b; Jennings et al., 1999); and an understanding of the way ecosystem features and human activities interact (e.g. Kingston et al., 1987; de Groot, 1996a; Jennings and Kaiser, 1998; Newell et al., 1998; Hall, 1999; Kaiser et al., 2002; Breuer et al., 2004). Marine spatial planning (MSP) is also being promoted as a more integrated system of management for human activities at sea (RCEP, 2004; Tyldesley, 2006). Key to the success of MSP is accurate, timely, and representative spatial information on human activities and the pressures they cause (Defra, 2005).
Here, we describe an assessment of the spatial extent of human activities and the direct, physical pressures they exert on the seabed. Human activities can be grouped into several different types of pressure to describe the specific ways that ecosystems and their components are perturbed. Impacts are the changes in ecosystem components caused by the pressures, and these may vary within a single pressure type. Where pressure from human activities in offshore waters has been assessed previously, it is invariably on a sectoral basis and ad hoc rather than routine. For example, although assessments of the intensity and spatial extent of marine mineral dredging (aggregate extraction) in England and Wales (E&W) waters are published annually (Crown Estate, 2004), and similarly for broad-scale fisheries statistics at a UK and European level (Defra, 2004; ICES, 2004), assessments for other sectors such as the oil and gas industry are more difficult to obtain. The most recent pressure assessment covering multiple sectors was published almost a decade ago, using statistics compiled during the mid-1980s from unpublished and grey literature sources (de Groot, 1996a). Recent proposals under the EU marine strategy will require a comprehensive assessment of pressure from human activities on the state of the marine environment. An open and transparent pressure-assessment framework will allow such assessments to be standardized in all waters of European Member States (EC, 2005b).
A further complication in conducting an assessment of pressure from human activities is the different spatial resolutions used for reporting purposes. For example, marine mineral dredging statistics are reported at a resolution of 50 x 50 m, whereas national fisheries statistics are available only by ICES statistical rectangles, which are orders of magnitude larger. Such mismatches in spatial resolution make it difficult to quantify pressures of similar type across multiple human activities. Because of the differences in spatial resolution and problems of accessing accurate and high-resolution human activity spatial data, pressure assessments are typically reported as single metrics within broad sea regions (de Groot, 1996a; Link et al., 2002; Choi et al., 2005). This may be suitable for the purpose of an IA conducted at a regional sea level, but will be less useful for other policy needs, such as the implementation of MSP in UK waters (Defra, 2005). For MSP, assessments of pressure will be needed at a much higher spatial resolution if they are to be used for planning purposes.
Several frameworks have been proposed to categorize pressures from human activities and assess their likely impact on ecosystem components (Tyler-Walters et al., 2001; EC, 2005b; ICES, 2005b). Pressures in offshore waters can be categorized broadly into physical loss, physical damage, non-physical disturbance, biological disturbance, and contamination (Table 1). Defining generic pressures in this way recognizes that ecosystem components respond to the effects of individual sectors through the pressures they exert. It also allows the spatial distribution of several sectors which exert the same pressure to be combined.
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Our primary objective was to describe and quantify the major sources of direct, physical pressure (not chemical or biological) from human activities on seabed environments in E&W offshore waters and by regional sea reporting areas (RAs) defined by the UK Government (Defra, 2005) (Figure 1). Human activities operating in E&W waters and used in this analysis consist of oil and gas exploration and production, windfarm construction and operation, cable laying, extraction of marine aggregates, waste disposal, fishing with mobile seabed gear, and wrecks at sea arising from military activity and marine accidents. Likely and known effects of these activities can be broken down into their constituent parts and then assigned to pressure categories and types (Table 1).
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In order to perform a comprehensive assessment of direct, physical pressure, up-to-date, high-resolution, spatially resolved data for all major offshore human activities are required. Many of these data are available for use, although temporal coverage, data formats, and methods of spatial representation can vary considerably. We therefore assessed seabed pressure from human activities in 2004, the latest year for which a comprehensive set of data was available. We used estimates of the spatial extent or the "footprint" of each activity as a proxy for direct, physical pressure and did not quantify the pressure intensity (e.g. the number of times a pressure was superimposed, such as the number of passes of a trawl per m2). This was due to a lack of suitable data for some pressure types and the complications of deriving a common metric able to express intensity across all pressures types. We did not attempt to estimate the cumulative pressure leading up to 2004, as this would only have been possible for a limited number of pressures, given limitations on the availability and suitability of the data needed for this type of assessment. We also made no attempt to estimate whether any impact might have occurred as a result of the pressures because this would have required spatially resolved information on ecosystem attributes along with models linking pressure to impact across all attribute types. Inadequacies in the data also prevented us from attributing a time element to the pressures, which would have allowed an assessment of the duration of the pressures in 2004 (e.g. number of passes of a trawl per m2 y–1) Although our approach to defining direct, physical pressure on the seabed is relatively simplistic, the use of a common metric allowed us to make direct comparisons of pressure types. By clearly describing our methods for translating spatial data for human activities into quantitative estimates of pressure, our aim is also to promote discussion on their accuracy and validity and to ensure repeatability of the pressure assessment in future.
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Material and methods |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Spatially resolved data for human activities causing direct, physical pressure on the seabed were obtained from a number of different sources. The data were stored in different formats and were rarely provided in a form suitable for yielding precise descriptions of the spatial extent of the activity and associated pressures. The following sections describe the methods used to translate these data into uniform estimates of spatial extent. All data manipulations and analyses were performed in ArcGIS v.9® (Environmental Systems Research Institute) unless stated otherwise.
Data sources and processing
Oil and gas
Geospatial data for oil and gas infrastructure in 2004 were obtained from the UK Digital Energy Atlas and Library (UKDEAL) (www.ukdeal.co.uk). To estimate the spatial extent of this sector, we used geospatial data for platforms, sub-sea wells, and pipelines. Oil and gas platforms can take a number of forms, but in the UK, they are generally of the steel jacket type, whereby the platform sits on a number of steel supports known as a jacket (DTI, 2001). Fixed steel jackets are normally four- or six-leg structures and constructed of a welded steel tubular framework. Each leg is pinned to the seabed by steel piles, which can vary in number depending on the conditions, but are generally around 1–2 m in diameter (UKOOA, 2002a). Dimensions and construction specifications for each platform are difficult to obtain, so we attributed all platforms a nominal extent of 
180 m2, which we represented in GIS as a circle, or buffer, of 15-m diameter. This would allow for a platform comprising six legs, each with four piles of 2-m diameter, and associated drilling and production gear. Rather than representing the exact layout of the platform at the site, this area provides an approximation of the spatial distribution of those components of the platform in direct contact with the seabed. Dimensions of individual wells were also not available. Buffers of 50-m diameter (nominal size) were therefore applied to these features to act as an estimate of their direct spatial extent, including any protective structures built over them, to avoid accidents with bottom trawl gear and anchors (R. Pedley, British Geological Survey, pers. comm.).
In addition to the presence of fixed structures, direct pressure on the seabed is caused by the accumulation of drill cuttings. When a well is drilled, waste cuttings are carried to the platform. They then undergo separation, and rock cuttings are normally discharged to the seabed (UKOOA, 2002b). Cuttings piles typically contain a variety of metals and hydrocarbons at higher than background concentrations (Breuer et al., 2004). There is no single source of information to describe the spatial extent of cuttings piles. For the North Sea, estimates range between 400 and 1000-m radius from the installation, depending on whether cuttings are discharged close to the seabed or at the surface and on the type of cuttings discharged (de Groot, 1996b). Field studies have shown that, for the majority of North Sea installations, biological communities are largely unaffected beyond a 500-m radius (Kingston et al., 1987). We therefore applied buffers of 500-m radius to all platforms and wells to provide an estimate of the spatial area affected. Despite plugged and abandoned wells being filtered out of the data set, this is still likely an overestimate because many of the wells were drilled before 2004 and some dispersal of cuttings may have taken place.
Both the location and individual dimensions for oil and gas pipelines were available from UKDEAL, so their direct spatial extent in 2004 could be represented with a high degree of precision.
Cables
Geospatial data for all continental shelf cables were provided by the UK Hydrographic Office (UKHO) and the Seafish Industry Authority (Seafish) as one-dimensional line vectors. Offshore telecommunication cables, which represent the vast majority of cable types on the UK continental shelf, are typically double armour or rock armour and have outside diameters of 0.07 and 0.09 m, respectively (C. Fenn, UK Cable Protection Committee, pers. comm.). As we were unable to discern the cable type from the geospatial data provided, all cables were buffered to a 0.09-m diameter to represent the spatial extent of all marine cables.
Windfarms
Geographic coordinates describing the location of windfarm turbines in 2004 were obtained from environmental statements prepared by offshore developers. Turbines typically consist of monopiles 4 m in diameter and often include scour protection at the base, which increases the direct spatial extent at the seabed to 20-m diameter. Scour pits around monopiles occur in areas of mobile sediments and can cause an area of sediment abrasion of up to 100-m diameter (Rees, 2005). All monopole locations found on mobile sediments were therefore buffered to a 100-m diameter to estimate the spatial extent of abrasion.
Marine mineral dredging
Marine mineral dredging in E&W is routinely monitored by The Crown Estate via an electronic monitoring system (EMS), whereby onboard loggers automatically record dredging activity at regular 30-s intervals. Dredging locations are then spatially aggregated into 50 x 50 m blocks and categorized from low to high intensity, where intensity is expressed as hours dredged (Crown Estate, 2004). EMS data for 2004 were used to represent the direct spatial extent of aggregate dredging.
Further pressures from marine mineral dredging arise from overspill and the discharge of unwanted sediments back into the water column in a process known as screening. Field measurements of the spatial extent of sediment plumes have been limited to the vicinity of the dredger (Newell et al., 1998, and references therein). These studies indicate a relatively localized impact of the dredger plume attributable to the effect of the initial dynamic plume phase, although fine material can be tracked over distances of 3 km from the source. In order to consider a worst case scenario for the direct pressure from sediment plumes, we used a Lagrangian particle-tracking model without the dynamic plume phase to estimate the dispersal of material arising from aggregate dredging in 2004. The model (EUROSPILL) was originally designed for simulating oil spills (Elliott, 1991), but it has been modified subsequently to deal with more general transport problems (Aldridge, 2001; Periáñez and Elliott, 2002).
For a given dredging licence area, EMS data were used to define a grid of particle start locations covering the license area at 0.5-km intervals. At each start position, the movement of 1000 tracer particles, released continuously over a 12.4 h tidal cycle, were simulated in the model. Separate calculations were undertaken at spring and neap tides to ensure a representative sampling of tidal states. The composition of dredger plumes is generally unknown, so a broad range of particle sizes was considered, and simulations assumed a uniform particle size distribution in the range 20–200 µm (fine silt to medium sand). Particles were tracked for a maximum of 48 h while they moved horizontally with the local tidal velocity and vertically under the influence of turbulence and a particle-size-dependent fall velocity. The position at which each particle hits the bottom was used to represent the direct, physical pressure on the seabed from the dredging plume. Settled plume material could be re-suspended and moved to other locations on subsequent tides. However, because of the difficulty in defining the end of such a process, the dilution in the quantity of material being moved each time, and the increasing uncertainty in predictions, no attempt was made to follow this longer term fate. The extent of dredging pressure for a given licence area was therefore estimated from the area occupied by the initial settling locations from all start positions. To do this, we first estimated the spatial density distribution of the particles associated with each licence area, using a kernel density estimator with a search radius of 0.5 km (comparable with the resolution of the release positions). This type of density estimator is widely used to predict animal movement home ranges from location data (Worton, 1987; Seaman and Powell, 1996). From this, we selected the area that encompassed 95% of the estimated spatial density distribution, using functions implemented with the Animal Movement Extension of ArcView 3 (Hooge and Eichenlaub, 2000). Boundaries between overlapping plume regions were dissolved to arrive at an estimate of the total area of seabed affected by dredging plumes.
Waste disposal
Disposal of waste material at sea is licensed under the UK Food and Environment Protection Act 1985. In 2004, all disposed material consisted of seabed sediments dredged as part of coastal developments, such as port construction and channel maintenance. Digital data for areas licensed for waste disposal are maintained by the UK's Centre for Environment, Fisheries and Aquaculture Science (Cefas). Information on the precise location of waste disposal within licensed areas is not available, so licence area boundaries were used to approximate the spatial extent of waste disposal, although this is likely to represent an overestimate in some cases and an underestimate in others because of plume dispersal (C. Vivian, Cefas, pers. comm.). Licensed areas actively used in 2004 were used to estimate the total spatial extent of waste disposal.
Fishing
For the assessment of fishing pressure, we concerned ourselves only with those vessels deploying mobile seabed gear, namely beam trawlers, otter trawlers, and shellfish dredgers. The most reliable source of positional data for fishing vessels is the EC vessel monitoring system (VMS). This system has been in operation since 2000 and requires all vessels > 24-m length operating in EC waters to transmit automatically their location at a minimum of 2-h intervals (EC, 1997). In 2004, coverage was extended to all vessels over 18 m (EC, 2003). Smaller vessels, which typically operate inshore, were therefore not represented. Vessel gear types were often missing from the VMS database. Gear-type information for UK vessels was therefore obtained from corresponding logbook records, whereas for non-UK vessels, gear types were obtained from the EC vessel registration database (EC, 2005a). As the VMS is unable to discriminate between different types of activity (e.g. fishing, steaming, in port), we used transmitted speeds to develop rules to discriminate fishing from non-fishing locations. Speed frequency distributions showed that 1–6 knots appeared to correspond with fishing activity of otter trawlers and scallop dredgers, although beam trawlers appeared to fish at speeds of 2–8 knots.
Having filtered VMS locations on the basis of speed, trawling and dredging lines were created by joining consecutive fishing locations with straight-line paths. To estimate the spatial extent of fishing activity, track lines were buffered using assumed gear width dimensions: 24 m for beam trawlers (2 x 12 m wide beams), 4 m for otter trawlers (2 x 2 m scour tracks left by trawl doors), and 20.4 m for shellfish dredgers (24 x 0.85 m wide dredges) (ICES, 2000; Dinmore et al., 2003; D. Palmer, Cefas, unpublished data). Track areas for the three types of fishing activity were then amalgamated, and boundaries between overlapping tracks were dissolved to estimate the total area of seabed affected by demersal trawling.
The above method is likely to arrive at an underestimate because vessels could potentially deviate from straight-line paths when fishing. We therefore estimated the potential difference between our estimates of fishing extent, which can be considered a minimum estimate, and the extent of fishing on the basis of deviations from straight-line paths. Assuming a constant trawling speed, there is a maximum distance (dmax) a vessel could potentially deviate from a straight line drawn between two consecutive positions separated by distance d. We calculated dmax by assuming that vessels travelled at a constant speed (speed over ground), determined by averaging the speed at consecutive positions. Where speeds were missing (transmission of speed information was not mandatory in 2004), we assumed that vessels were fishing at a constant speed equivalent to the 95% upper limit from the fishing speed frequency distribution for that vessel type. These constant speeds were 6.7 knots for beam trawling, 4.5 knots for otter trawling, and 4 knots for scallop dredging. The extent of deviation from a straight-line path was estimated by comparing the difference between dmax and the straight-line distance d used to estimate the minimum trawled distance between consecutive positions.
A second source of error in our estimate of the spatial extent of demersal trawling is that the VMS database only contains positions for > 18-m vessels. To estimate the total number of individual vessels fishing in E&W waters in 2004, we used data on vessel sightings made by British Fishery Protection aircraft and patrol vessels and compared the total number of vessels observed with the total number of vessels recorded in the VMS database for the same year. This figure provided a broad estimate of the proportion of the entire fleet that contributed to our estimate of the spatial extent of demersal trawling.
Wrecks
Geospatial data for all historical and contemporary wrecks were extracted from a comprehensive database maintained by the UKHO. The database currently contains records for more than 10 000 wrecks in E&W waters. Wreck positions in the database are represented as point locations. We therefore attributed a nominal spatial extent of 962 m2 to each individual wreck, which represented the average size of a wreck on the basis of all available length and beam (width) measurements (3500). This area was represented by a buffer of 35.1-m diameter.
Pressure assessment
After converting all human activity pressures into representations of their spatial distribution in 2004, they were assigned to categories and types (Table 1). The first category is the physical loss of seabed. This can be caused by the presence of a physical obstruction, such as an oil and gas platform, pipeline, cable, or windfarm, or through smothering with sediments or other materials uncharacteristic of the area, such as by waste disposal or the release of drill cuttings from oil and gas exploration (Kingston et al., 1987; Breuer et al., 2004). The second pressure category is physical damage to the seabed, resulting from abrasion, extraction, and siltation. Pressure types such as these arise from aggregate dredging in the form of direct removal (extraction) and plume dispersal (siltation). Sediment abrasion can be caused, for example, by the development of scour pits around the base of windfarm turbines (Gill, 2005; Rees, 2005). The third pressure category is biological disturbance and is primarily the result of selective extraction of target and non-target species in commercial fisheries (Jennings and Kaiser, 1998; Hall, 1999; Kaiser et al., 2002).
Overlapping spatial features for the same pressure type were merged and boundaries dissolved to produce a single complex polygon representing the spatial extent of the pressure type. The area covered by each pressure type was then calculated within all E&W waters, within and outside the 12 nautical mile territorial limit (defining inshore and offshore waters, respectively), and within the UK Government regional sea RAs (Defra, 2005). To represent the pressures visually and to allow more detailed comparison of the relative spatial distribution of pressures across sea regions, we also computed the proportion of seabed affected by each pressure type within grid cells of 2 x 2 nautical mile resolution.
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Results |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Estimates of the spatial extent of direct, physical pressure caused by a single sector activity operating in E&W offshore waters in 2004 are given in Table 2. Four of the pressure types were caused by a single sector activity, whereas both obstruction and smothering were caused by human activities from more than one sector. Confidence ratings on a scale of 1–3 were attributed to all human activities, 1 indicating the least confidence rating and 3 the highest. Only three activities—marine mineral dredging, windfarm monopiles, and pipeline laying—were assigned the highest rating. For these activities, both location and extent were known. The spatial extent of all other human activities was estimated from known locations, and hence these were assigned a medium confidence rating. Selective extraction was assigned the lowest confidence rating, because both location and spatial extent had to be estimated between consecutive VMS transmissions.
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Four of the six pressure types affected < 1% of the seabed in 2004, whereas siltation and selective extraction affected between 1.2% and 5.4%. Selective extraction by demersal trawling was estimated to have affected a larger area of seabed than all other pressure types combined. At first, this estimate seems rather low for what appears to be a widespread activity (Figure 2, left panel). However, the complexity and sheer volume of trawling lines makes them difficult to represent visually, and at small map scales (larger area, lower resolution), the impression is gained that 50% or more of the seabed is affected. At larger map scales (smaller area, higher resolution), gaps between trawling lines are more easily detectable (Figure 2, right panel), highlighting how areas that appear to be comprehensively trawled at small map scales are in reality quite patchy.
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As described earlier, estimates of the spatial extent of demersal trawling were based on straight-line distances (d) between consecutive positions. The median length of d was
6.8 km (Figure 3), whereas the median length of dmax was 14.8 km, giving a ratio of dmax : d of 2.2. This suggests that our estimate of the spatial extent of demersal trawling in 2004 could increase from a minimum of 5.4% to a maximum of 12.9% if vessel deviation was taken into account. The relationship between d and dmax is unlikely to be directly proportional to the area of seabed trawled, because the number of overlaps between trawl tracks will become increasingly frequent as activity increases. Therefore, in terms of trawled area, the total spatial extent would be unlikely to reach the maximum predicted estimate.
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Our estimate of the spatial extent of demersal trawling will also be affected by the degree to which the VMS database under-represents the total number of vessels of the same type operating in E&W waters in 2004. Physical sightings by British Fishery Protection vessels and aircraft suggest that a total of 1113 registered beam trawlers, otter trawlers, and scallop dredgers fished in E&W waters in 2004. VMS data for the same area in 2004 contained position records for just 669 vessels. This suggests that the total fleet size may be greater by a factor of 1.66. The spatial extent of demersal trawling may therefore be much higher than estimated here, with the estimate of the minimum spatial extent increasing from 5.4% to 9.0% and the maximum estimate from 12.9% to 21.4%.
Siltation caused by screening plumes from marine mineral dredging was estimated to affect 1.2% of E&W waters (Table 2). Smothering was estimated to be the next most widespread pressure at 0.4%, of which the largest contributing factor was cuttings piles produced by the oil and gas industry. Of the three categories of pressure, the spatial extent of biological disturbance in 2004 (using minimum estimates) exceeded physical damage by a factor of 4.5 and physical loss by a factor of 13. The geographic dominance of the only pressure type associated with biological disturbance, namely selective extraction, is shown in Figure 4. Demersal trawling took place throughout much of E&W waters, although some regions clearly experienced less pressure than others. In inshore waters, fishing covered 1.3% of the total area, compared with 6.8% for offshore waters, and fishing pressure was lowest in RA1 at 0.3% (Table 1). These estimates may increase if the two main sources of error described earlier could be taken into account. The total area of seabed affected by fishing pressure compares with its geographic coverage, both being relatively high. In contrast, the total area of seabed affected by pressure from obstruction ranked fifth at 13.5 km2, representing < 0.1% of all E&W waters, and yet was the most geographically widespread pressure after fishing (Figure 4). Conversely, estimates of siltation pressure from dredging plumes ranked second in terms of the total area of seabed affected, but the overall geographic distribution was relatively limited. Visualizing the pressures in geographic space clearly demonstrates the lack of a clear relationship between the total area of seabed affected by each of the six pressures and their overall geographic coverage.
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Relative differences between the six pressure types become more apparent when summarized in terms of geographic coverage and the proportion of seabed affected within 2 x 2 nautical mile cells (Figure 5). Selective extraction affected up to half of the total area of seabed in E&W waters at this resolution of analysis, but for most of its geographic coverage was at relatively low levels in terms of the proportion of seabed affected within 2 x 2 nautical mile cells. Activities that caused obstruction were equally widespread and were also of limited spatial extent, such that no 2 x 2 nautical mile cells were affected by > 10% of their area. With the exception of siltation, all pressures were widespread but characterized by relatively low spatial extents, so the proportion of seabed affected at 2 x 2 nautical mile resolution was relatively small. The proportion of seabed affected by siltation remained relatively constant throughout its geographic range, with grid cell areas affected at low spatial extents generally equal in number to areas affected at higher levels. Only pressures from smothering and selective extraction were estimated to be greater in offshore waters than in inshore waters. Of the five regional RAs, those in the southeast (RA2 and RA3) were generally associated with the greatest number of pressures at the highest levels of spatial coverage (Table 2).
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Discussion |
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TopIntroductionMaterial and methodsResultsDiscussionReferences |
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Higher levels of pressure from a greater number of sources were found in inshore waters of E&W and also in the southern North Sea and the eastern English Channel, compared with other regional seas. The estimates of spatial extent derived here are similar to those published elsewhere. For example, within the UK Continental Shelf, the spatial extent of cuttings piles has been estimated at between 1056 and 1605 km2 (UKOOA, 2002b; OSPAR, 2005), which differs little from our estimate of 924 km2 for E&W waters. Choosing a generic spatial extent for all cuttings piles will clearly introduce errors, but in the absence of detailed site-specific information for the current array of oil and gas infrastructure, there are no alternative methods. The spatial extent of aggregate dredging and pipelines in E&W waters can be estimated with a relatively high degree of precision and accuracy because detailed digital data for these human activities are available. However, deriving estimates of the spatial extent of fishing with demersal gear is more problematic, despite the recent availability of high-resolution data for tracking fishing vessel movements. As noted earlier, VMS coverage is limited to larger vessels that tend to operate farther from shore, which explains partly why our estimate of fishing pressure inshore is relatively low. Applying generic rules for differentiating between fishing and non-fishing locations can also introduce errors and potentially reduce the overall accuracy of estimates of the spatial extent of fishing (Mills et al., 2007).
Several of the pressures described here will act at different intensities on the seabed. For example, the intensity of trawling activity will vary depending on the density of local fish resources, and the intensity of siltation will be related partly to the distance from the source of the pressure. As already noted, we made no attempt to base our estimates on the spatial intensity of the pressures because of a lack of suitable data for some pressure types and complications in deriving a common metric able to express intensity across all pressure types. For example, to estimate pressure intensity from sediment plumes caused by marine mineral dredging would require site-specific studies to quantify spatial changes in the concentration of suspended sediments above background levels. Unfortunately, there are too few studies of the factors that can influence suspended sediment concentration levels, such as changes in dredging activity over short time scales in relation to the prevailing environmental conditions and changes in the composition of seabed sediments. Siltation pressure will also be confounded by variability in natural background turbidity and go undetected where background levels continually exceed those caused by dredging. This pressure type therefore needs more detailed consideration in future pressure assessments if intensity is to be estimated reliably.
All pressures considered in this assessment not only varied in their spatial extent during 2004, but also in their temporal extent. For example, all fixed infrastructure can be expected to exert a pressure over the entire year, whereas pressure from marine mineral dredging and demersal trawling would be over smaller time scales (hours to weeks). We made no attempt to introduce a temporal component to our pressure assessment owing to the complications of developing a common metric that could accommodate this across all pressure types. Future work needs to consider this issue in more detail.
Our assessment of pressure is distinct from an assessment of impact, which would require information on ecosystem components and attributes and how they respond to varying pressures and their intensities. Instead, we focused attention on estimating pressure, and not impact, as it was in 2004. For fixed obstructions, including historical wrecks, pressures were considered to be constant throughout their lifetime; hence our use of data in these categories prior to 2004. For pressures on shorter time scales, such as marine mineral dredging, demersal trawling, waste disposal, and smothering by oil and gas drill cuttings, historical data were not used because their inclusion would require knowledge of the longevity of the impact associated with the pressures and move the focus of the assessment away from pressure and onto impact. For example, marine mineral dredging and demersal trawling can lead to modifications (impacts) in certain benthic environments lasting longer than a year (Newell et al., 1998; Kaiser et al., 2002; Boyd et al., 2005). Recovery of benthic environments depends on biological sensitivity to pressure type and intensity, but will also be affected by cumulative pressures from a single source and in-combination pressures from multiple sources. The lasting effects of pressures (i.e. their impact) beyond the time period of the assessment will be difficult to estimate in the absence of comprehensive spatial data on community types linked to empirical observations of community responses to pressures of varying type, intensity, and longevity.
Mapping the seabed pressures, and quantifying the pressures within spatial units of fixed resolution, provided a more thorough understanding of the spatial distribution of the pressures relative to the seabed area. In this way, we were able to clearly demonstrate how selective extraction by demersal trawling affected large areas of seabed and was also geographically extensive, how obstruction pressure only affects small areas but is geographically widespread, and how sediment plumes from marine mineral dredging affected large areas of seabed but were contained within relatively limited geographic areas. These differences between the extent of seabed affected and the geographic occurrence of the pressures have important implications for the spatial management of human activities, particularly if linked to information on the sensitivity and recovery rates of benthic communities.
Our estimates of direct, physical pressure on the seabed are not comprehensive, but they do cover all major offshore human activities. Other types of fishing in addition to those considered here, such as potting and the use of certain fixed nets, will also cause direct, physical pressure on the seabed. Unfortunately, information on the sites of these activities is difficult to derive from quantitative sources. Oil spills will cause direct, physical pressure, but again spatial data are not commonly available. In addition to physical pressures, human activities cause a range of direct and indirect chemical and biological pressures on the seabed (Table 1). Many of these are more problematic to assess, but are essential for a more complete assessment of human pressure in E&W waters.
To our knowledge, the only other comprehensive assessment of pressure on the seabed from multiple human activities was carried out almost a decade ago at a North Sea scale, using statistics compiled during the mid-1980s from unpublished and grey literature sources (de Groot, 1996a). It would be inappropriate to attempt a detailed comparison between the two sets of pressure estimates, given that the studies focused on different sea regions and relied on different data. However, it is interesting that the estimates presented by de Groot (1996a) indicate that fishing occupied 50% of the North Sea and that cables and pipelines, the activity with the next largest spatial extent, were estimated to cover 3% of the total area of the North Sea. Our estimates for the E&W sector of the North Sea are considerably lower, fishing affecting between 4.6% and 9.2% when taking into account estimation errors (see above), and where all sources of obstruction amounted to no more than 0.02%. It is unlikely that these differences are primarily a result of the discrepancy between the geographic scale of analysis for the two assessments, or from changes in the level of these human activities over time, because although demersal fisheries have declined over the past two decades, oil and gas exploration has increased (de Groot, 1996b; ICES, 2005a). Instead, differences may be due in part to the lack of digital data for offshore human activities in the mid-1980s and also to the methods used to estimate the spatial extent of associated pressures. By describing our methods in detail, we hope to encourage debate over their validity and accuracy in order to reach agreement over time as to the most suitable methods for estimating pressures on the seabed from multiple human activities. We also call for better access to accurate and validated geospatial data on the location and extent of international human activities. Pressure assessments will depend upon improvements in data access and agreed standards for data processing if they are to be used to set future management objectives.
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VMS data were provided by the UK's Department for Environment, Food and Rural Affairs (Defra) in raw, uninterpreted form. The Secretary of State for the Environment, Food and Rural Affairs does not accept any liability whatsoever as to the interpretation of the data or any reliance placed thereon. We gratefully acknowledge the supply of aggregate dredging data from The Crown Estate, and wrecks data from the United Kingdom Hydrographic Office through SeaZone Solutions Ltd. Comments from two anonymous referees helped improve the manuscript considerably. The work was funded by Defra (AE1148 and MF0731).
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