Extent of physical damage to predominant seafloor habitats
Trawling of the seafloor by commercial fisheries is widely distributed. Five out of the eight subdivisions assessed were below target, with high disturbance in 31% to 75% of the cells assessed across the UK sub-regional seas.
Background
UK target on physical damage
This indicator is used to assess progress against the following qualitative target set for seafloor habitats in the Marine Strategy Part One (HM Government, 2012): the level of exposure to pressure at the level of the Marine Strategy Framework Directive sub-regions should not result in more than Moderate Impact/vulnerability of the habitat (dependent on the sensitivity of the habitat to this pressure).
Key pressures and impacts
There is a huge diversity of life inhabiting the seafloor in UK waters. These benthic habitats are subjected to physical disturbance by human activities such as fishing, sand extraction, and offshore construction. Some habitats, such as Maerl beds (calcareous algae) from South West England are highly sensitive to disturbance and can be easily damaged, whereas other habitats, like mobile sediments, are more resilient and are less affected by human disturbance.
Measures taken to address the impacts
Existing measures set out in the Marine Strategy Part Three (HM Government, 2015) include the management of Marine Protected Areas, Common Fisheries Policy regulations, Scallop Fishing Orders, fisheries measures such as trawl bans, and OSPAR recommendations for the protection of threatened and declining habitats.
Monitoring, assessment and regional co-operation
Areas that have been assessed
Progress against the UK target was assessed in sub-divisions of each UK Marine Strategy Framework Directive sub-region (the Celtic Seas and Greater North Sea) and in UK waters within adjacent OSPAR regions (see Table 1).
Table 1. Percentage of seafloor predicted to be subjected to higher levels of disturbance (category 5-9) by fishing.
|
Assessment area |
% area of cells predicted to be exposed to disturbance categories 5-9 |
Assessment against target (<15%) |
|
|
UK Marine Strategy Framework Directive sub-region |
sub-division |
||
|
Celtic Seas |
Southern Celtic Seas |
75 |
Not achieved |
|
Northern Celtic Seas |
31 |
Not achieved |
|
|
Greater North Sea |
Southern North Sea |
48 |
Not achieved |
|
Northern North Sea |
50 |
Not achieved |
|
|
English Channel |
75 |
Not achieved |
|
|
(Other UK waters) |
OSPAR Region I (Arctic Waters) |
0 |
Unknown |
|
OSPAR Region IV (Bay of Biscay and Iberian Coast) |
0 |
Unknown |
|
|
OSPAR Region V (Wider Atlantic) |
12 |
Achieved |
|
Monitoring and assessment methods
The current indicator is focussed on fisheries. Other human activities will be included in later rounds of assessments. The indicator uses best available information from existing programmes:
- the distribution and sensitivity of habitats (resilience and resistance)
- the distribution and intensity of fishing activity by vessels over 12 metres long.
These two sources of information (pressure and sensitivity) are combined to calculate the potential damage to seafloor habitats.
Further information
In many areas of the UK Maritime Area, a shift in benthic community composition has been reported where large and long-lived species have been replaced by small and fast-growing opportunistic species including scavengers. These opportunistic species and scavengers profit from physical damage and the availability of dead organisms resulting from the physical disturbance (Jennings and others, 1999; OSPAR Commission, 2010). The impact of bottom trawling on the seafloor is considered to be the most widespread physical impact. Other activities are equally or more intense but spatially more limited (Halpern and others, 2008; OSPAR Commission, 2010; Foden and others, 2011; Jennings and others, 2012).
A combination of data on pressures from human activities and information on the sensitivity of habitats to those pressures are the main components of this indicator. Activity data are not directly used as the presence of an activity does not always indicate the presence of a pressure. It is the way the activity operates that defines whether it is generating an impacting pressure. The duration, intensity, and the elements or physical parts used for the activities all play a part in defining the nature of the pressure (Korpinen and others ,2012).
To assess the degree of disturbance on the seafloor, data on pressures alone are not sufficient. Data on the sensitivity of the habitats to those pressures also needs to be evaluated to determine the degree of disturbance that could be occurring. For this assessment, disturbance is the effect of the different combinations of the pressure varying in intensity and duration on features with variations in natural sensitivities. The specific combination of pressure, based on the presence and intensity of activities within each grid cell, and sensitivity of the habitats present is used to assess an overall disturbance (damage) for each habitat.
To distinguish between pressures induced by differing fishing gears on seafloor habitats, the penetration depth of different gear components was assessed as either surface abrasion or subsurface abrasion (Johnson, 2011; Church and others, 2016). Surface abrasion pressure damages species and communities on the surface and top layers of sediment, for example, calcareous algae growing on the surface of gravels, and is caused by fishing gears such as trawling nets. Subsurface abrasion pressure is defined as the penetration of the substrate to more than 3cm below the surface, damaging species and communities living within the sediment, such as burrowing bivalves, and is caused by, for example, the ground gear of beam trawls. It should be noted that subsurface abrasion includes elements of surface abrasion as the physical effects will affect both surface and subsurface parts of the sediment.
The current results are based only on abrasion from fishing vessels over 12m during the period 2010 to 2015, although data from vessels between 12m and 15m in length only became progressively more available from 2013 onwards. This may result in an underestimation of fishing effort in the early years.
The potential area of habitat disturbed was calculated for each sub-regional sea, to compare against a threshold of 15%, which was set as a potential indicator target in 2012. These calculations rely on the assumption that fishing activity is evenly distributed across and within all habitats within each grid cell.
Additional disturbance from smaller vessels and information on other human activities causing physical damage will be considered in the next round of the Marine Strategy Framework Directive assessments
Assessment method
The extent of physical damage indicator uses two types of information: i) the distribution and sensitivity of habitats (resilience and resistance), and ii) information on the distribution and intensity of human activities and pressures that cause physical damage, such as mobile bottom gear fisheries, sediment extraction and, offshore constructions, although only fisheries is covered in this assessment. These two sources of information (pressure and sensitivity) are combined to calculate the potential damage to a given seafloor habitat, and the trends across the 6-year period (Figure 1). The datasets included in the assessments are mainly assessing the 2010 to 2015 period, therefore, data on habitat damage and modification which took place before this period is not included.
Figure 1. Schematic showing outline of data inputs, processes and outputs of the physical damage indicator
The components of the analysis are:
- A composite habitat map showing the extent and distribution of habitats (based on observational and modelled data), including the mapped extent of any relevant features such as records and distribution of particular species and biotopes like EUNIS Level 5 habitats or other biological characteristics. For this assessment, a biotope is defined as ‘the combination of an abiotic habitat and its associated community of species’ (Connor and others, 2004). All habitat data were combined at EUNIS level 3.
- Tables relating benthic habitat types to habitat sensitivity scores based on their resistance and resilience (recoverability) (Tillin and others, 2010; Tillin and others, 2010; BioConsult, 2013; Tillin and Tyler-Walters, 2014). The sensitivity is assessed at species, biotope and EUNIS level 3 level (broad-scale habitat classification), depending on what habitat mapping information is available within each grid cell.
- Distribution and intensity of pressures causing physical damage. This analysis focussed on surface and subsurface abrasion caused by bottom trawling (for fishing from vessels greater than 12m only) within 0.05° grid cells. Grid cell size is given in degrees due to the curvature of the earth and allowing comparison between geographical areas. Due to variances in latitude, the extent of the 0.05º grid cell differs in total size across UK water, from just under 14km2 in North East Scottish waters, to 21km2 in the South West English waters. The method assumes even fishing within the grid cell due to the sensitivity of data and restrictions in data access (Johnson, 2011; ICES, 2015; Church and others, 2016)
- Distribution of levels of disturbance per habitat type within years: calculation of disturbance based on the intensity and duration of pressures and habitat sensitivity per pressure type. Please note that the pressures of abrasion (non-fisheries, as well as fishing by vessels <12m in length), siltation and selective extraction, are not currently included in the assessment, but will be incorporated in future developments of the indicator
Data generated by all the elements above are combined using a step-wise approach to calculate the total area of different levels of predicted disturbance and a combination of those, across the sub-region, per habitat type. The results are used to calculate the levels of variability of fishing intensity and identify trends in disturbance per year and across a 6-year period.
The spatial assessment of this indicator is summarised per EUNIS level 3 polygon and has been prepared by combining the sensitivity and exposure to pressure data for habitats, biotopes, and species within the EUNIS level 3 habitat polygons. For this assessment, the OSPAR Regions have been subdivided following biogeographic boundaries (OSPAR Commission, 2016) into Southern North Sea, Northern North Sea, Southern Celtic Seas, Northern Celtic Seas, English Channel, and Bay of Biscay/Iberian Peninsula. Please note that for region IV (Bay of Biscay and Iberian Coast), only a partial assessment has been possible in this assessment, as at present insufficient habitat and sensitivity data on the deep seafloor areas are available.
The indicator method is based on a series of analytical steps to combine the distribution and intensity of physical pressures with the distribution and range of habitats and their sensitivities. The indicator will use an additive approach for future inclusion of other multiple pressures. These pressures will be added at the resolution relevant to the availability each pressure dataset and will be combined at the EUNIS level 3 level (or another scale if special habitats are being assessed).
Step 1: Extent of habitats
An important component of this indicator is the production of a composite habitat map showing the extent and distribution of predominant and special habitats and their associated sensitivities. This map is produced using a combination of benthic survey data and modelled habitat maps. As a basis for the assessment, a full coverage EUNIS level 3 habitat map has been produced for the OSPAR Maritime Area, integrating maps from surveys and broad-scale models (Figure 2). Please note that at the time of writing, the new EUNIS classification has not been taken into consideration as it has not been published, therefore, EUNIS classification version 2007-11 has been used throughout this assessment.
Figure 2. UK full-coverage EUNIS level 3 benthic habitat map, integrating maps from surveys and broad-scale models. Figure shows GSHHG world shore line, available under GNU Lesser General Public License. Full legend explanation for benthic habitat codes can be found in Table 2.
Table 2. Legend explanation for benthic habitat codes, shown in Figure 2. EUNIS is the European Nature Information System.
|
EUNIS Code |
Habitat Description |
|
A3.1 |
High energy infra littoral rock |
|
A3.2 |
Moderate energy infra littoral rock |
|
A3.3 |
Low energy infra littoral rock |
|
A3.7 |
Features of infra littoral rock |
|
A4.1 |
High energy circalittoral rock |
|
A4.2 |
Moderate energy circalittoral rock |
|
A4.3 |
Low energy circalittoral rock |
|
A4.7 |
Features of circalittoral rock |
|
A5.1 |
Subtidal coarse sediment |
|
A5.2 |
Subtidal sand |
|
A5.3 |
Subtidal mud |
|
A5.4 |
Subtidal mixed sediments |
|
A5.5 |
Subtidal macrophyte-dominated sediment |
|
A5.6 |
Subtidal biogenic reefs |
|
A5.7 |
Features of sublittoral sediments |
|
A6.1 |
Deep-sea rock and artificial head substrata |
|
A6.2 |
Deep-sea mixed substrata |
|
A6.3 |
Deep-sea sand |
|
A6.4 |
Deep-sea muddy sand |
|
A6.5 |
Deep-sea mud |
|
A6.6 |
Deep-sea bioherms |
|
A6.8 |
Raised features of the deep-sea bed |
The EUNIS habitats are mapped at different levels of detail depending on the information available, from level 3 physical habitats to level 6 biological communities and then aggregated to EUNIS level 3. The majority of the habitat maps were obtained from the European Marine Observation and Data Network Seabed Habitats portal, including the broad-scale physical habitat map known as EUSeaMap (EMODnet, 2010, Seabed Habitats Map Viewer) and more detailed habitat maps created from survey data available through the European Marine Observation and Data Network Seabed Habitats portal, or as part of OSPAR habitat data calls. The information on the coverage and type of data have been taken into account for the calculation of confidence maps (see step 6). The specification for a habitat map for the assessment of this indicator included the following conditions:
- To contain information on the relevant EUNIS habitat/biotope type at any level between levels 3 and 6
- To refer data on biotopes to Level 3 of the EUNIS habitat classification system
- To use the broad-scale modelled map, EUSeaMap at EUNIS level 3 when higher resolution maps from surveys are not available
- To use the best available evidence on habitat data
- To cover the greatest possible area of the OSPAR North-East Atlantic Region
- To contain no overlaps
Mapping rules were established to decide objectively which of the overlapping datasets would be the sole occupant in the overlapping area. Where a EUNIS habitat map developed from survey data overlapped with the EMODnet broad-scale habitat map, a threshold confidence score of 58% was used as a simple rule for deciding whether or not to favour the habitat map from the survey. This threshold was based on the Mapping European Seabed Habitats protocol (EMODnet, 2010). Within the Mapping European Seabed Habitats scoring system, for any map to have a score greater than 58%, the survey techniques must have used a combination of remote sensing and ground-truthing to derive the habitat types, hence physical and biological elements are included for its production. Therefore, 58% was deemed to be the lower threshold at which an overlapping survey map is considered to be of higher quality than the broad-scale predictive map. Pre-processing conditions and rules for the combining of data are available in the Co-ordinated Environmental Monitoring Programme guidelines.
Step 2: The assessment of habitat sensitivity
The sensitivity of benthic habitats is determined based on a combination of the resistance (tolerance) and resilience (recoverability) of key structural, functional and characterising species of the habitat in relation to a defined intensity of each pressure (Tillin and others, 2010; BioConsult, 2013; Tillin and Tyler Walters, 2014). Due to data limitations, the sensitivity scores are defined using a categorical scoring approach (Tillin and others, 2010). Sensitivity assessments for ecological groups have also been undertaken using Bray-Curtis cluster similarity analysis and Multidimensional Scaling, where resistance and resilience scores are assigned to groups of species with similar biological traits, such as burrowers (Tillin and Tyler-Walters, 2014).
The sensitivity map is created with three steps, using best available evidence where present:
- Species records from survey data, that match a list of species assigned to a specific ecological group, are mapped using their maximum sensitivity value (based on the combination of resilience and resistance). Data are applied to the intersection between the habitat polygon and a 05° grid. Maximum values are selected as a precaution to capture the most sensitive species when the values are aggregated together.
- If there is a high enough density of species recorded per habitat area and agreement between the species and the underlying habitat they are within, the sensitivity from the same species records used in step one are used to assign a modal sensitivity to the surrounding habitat polygon. For example, if data coverage is sufficient, and the substrate and habitat type in the surrounding polygons are the same, then the same sensitivities are applied to these areas
- Finally, to act as a background map, and to fill in areas not covered by the first two steps, the habitat map outlined in Step 1 is used to assign EUNIS level 3 benthic habitat sensitivities to the whole area. The sensitivities used in this step are often a range (from very low sensitivity to very high), in which case the maximum sensitivity is selected.
The maps are then combined geographically to show the highest confidence information across all regions (Figures 3 and 4). For a more detailed explanation of the method, please refer to the Co-ordinated Environmental Monitoring Programme guidelines.
Figure 3. Extent and distribution in Southern Celtic Seas sub-region of habitat and benthic species sensitivities (based on resilience and resistance) to surface abrasion combined within EUNIS Level 3 benthic habitat types. Sensitivity is expressed in categories from 1-5, where 1 is the least sensitive and 5 is the most sensitive
Figure 4. Extent and distribution in Southern Celtic Seas sub-region of habitat and benthic species sensitivities (based on resilience and resistance) to sub-surface abrasion combined within EUNIS Level 3 benthic habitat types. Sensitivity is expressed in categories from 1-5, where 1 is the least sensitive and 5 is the most sensitive
Step 3: The assessment of the extent and distribution of physical damage pressures
For annual assessments: The first step is to determine the relevant human activities causing physical pressures and their spatial and temporal extent. Bottom trawling is known to be affecting a large area of the seafloor (Dinmore and others, 2003; Eastwood and others, 2007; Foden and others, 2010; 2011; Johnson, 2011; Jennings and others, 2012) so the assessment method currently focuses on the corresponding pressures, surface abrasion (damage to seafloor surface features) (Figure 5) and subsurface abrasion (penetration and/or disturbance of the substrate below the surface of the seafloor) (Figure 6).
Figure 5. Distribution of the aggregated surface abrasion for 2010-2015 period. The hatched area around the UK denotes high levels of fishing activity by vessels smaller than 12m.
Figure 6. Aggregated subsurface abrasion pressure using 2010-2015 data series. Pressure unit is swept area ratio (the proportion of grid cell swept by fishing gear). The hatched area around the UK showing the areas where inshore fisheries activity from vessels < 12m is higher than those >12m
Pre-processed aggregated Vessel Monitoring System fishing data are used to calculate the ‘swept area’ of a specific group of fishing gears (métiers if this level of information is available). The swept area is calculated using the parts of the fishing gear in contact with the seabed, and it is calculated on the width of fishing gear (in metres) multiplied by the average vessel speed (in knots) and the time fished. This calculation is undertaken on a cell-by-cell (grids or c-squares) basis per gear and per year, using data covering 6 years from 2010 to 2015. See ICES Working Group Spatial Fisheries Data and Co-ordinated Environmental Monitoring Programme guidelines for the detailed method. Only the part of gear in contact with the seafloor is used for the analysis, and as a result, all the gears have been classified according to type and/or metier group (Eigaard and others, 2015; Church and others, 2016). The swept area ratio (proportion of cell area swept per year) is then calculated by dividing the swept area by the grid cell area. The trawling effort is classified with an intensity scale ranging from ‘none’ to ‘very high’ (cell area swept more than 300% or 3 times per year). A change of three categories or more on the fishing pressure in the same grid across years is considered to be highly variable (Figure 7).
The maximum or 95-percentile was not chosen to avoid overestimating the pressure. For a more detailed explanation of the method, please refer to the Co-ordinated Environmental Monitoring Programme guidelines.
Figure 7. Spatial distribution of variable and consistent fishing using 2010-2015 data series. Variable fishing is defined when a grid has a change of three or more pressure categories.
Step 4: The combination of pressure intensity and habitat sensitivity
The degree of disturbance of a habitat is a prediction based on the predicted spatial and temporal overlap of its sensitivity and exposure to a specific pressure. Sensitivity and pressure are combined via a matrix, producing 10 categories of disturbance (0-9, where 0 is no disturbance, and 9 is the greatest amount of disturbance possible). The matrix was created from the result of previous studies that looked at the impacts of pressures on sensitive species and habitats when applied at different levels of intensities (Schroeder and others, 2008; BioConsult, 2013). The matrix is used to calculate the disturbance per cell for each surface and subsurface abrasion per year. These disturbance maps are then combined by selecting, for each cell the highest disturbance category from the two maps pressure layers. The disturbance from surface and subsurface abrasion produce disturbance that overlaps between pressure types. The maximum is, therefore, chosen to prevent double counting (Figure 8). For a more detailed explanation of the method, please refer to the Co-ordinated Environmental Monitoring Programme guidelines.
Figure 8. Spatial distribution of aggregated surface and sub-surface disturbance during 2010-2015 across UK sub-regional seas. Disturbance categories 0-9, with 0= no disturbance and 9= highest disturbance.
At present other activities, which could cause physical damage pressures, are not included. It is anticipated that due to the different nature of the pressures ‘selective extraction’, ‘abrasion’ and ‘changes in siltation’, separate disturbance matrices or algorithms will be required, which will take into account the spatial distribution of pressures and the temporal effects. This information is not currently available and will be included in the next round of assessments for an overall calculation of disturbance caused by physical damage.
Figure 9. Example of distribution of disturbance categories for the Subtidal mud habitat for sub-regions.
Step 5 Disturbance aggregation method and trend analysis
Disturbance values across years are combined using the aggregated fishing pressure spatial layers, developed in Step 3 above. Results are used to calculate trends between years in those grid cells or c-squares identified as variable. This allows the variation of disturbance across years per habitat type to be assessed. The trend analyses are simple plots over the 6-year period, rather than a linear regression, which was not possible due to the small number of years assessed.
The disturbance categories were aggregated into two groups:
- disturbance categories 0 to 4, representing lower levels of disturbance
- disturbance categories 5-9, representing higher levels of disturbance
This scale is the result of a combination of the variation in fishing activity and the range of sensitivity values. The percentage of areas with vulnerable or potentially damaged habitats can be calculated based on an assessment of moderate or greater vulnerability for each sub-regional sea (disturbance categories 5-9), as well as by habitat type. Percentage values were calculated for each sub-regional sea, to compare against a threshold of 15%, which was set as a potential indicator target in 2012 (HM Government, 2012). For a more detailed explanation of the method, please refer to the Co-ordinated Environmental Monitoring Programme guidelines.
Step 6 Confidence assessments
To spatially represent confidence in the available data, a numeric method of calculating confidence was adapted from OSPAR Commission (2015). The method multiplies relative measures of confidence on a scale of 0 to 1, where there is a difference in confidence between categories or classes used in a data layer.
A numerical score (0.33, 0.66 or 1) was manually assigned by the assessor to each of the different attributes used to create the sensitivity layer. A high confidence score was given a numeric value of 1, medium 0.66 and low 0.33. The different methods used to create the sensitivity layer were taken in turn and a numeric confidence score was assigned to each of the attributes: confidence based on underlying data, confidence within data source (such as Mapping European Seabed Habitats confidence for habitats), and confidence in the sensitivity of the habitat to a pressure (Figure 10). For a more detailed explanation of the method, please refer to Co-ordinated Environmental Monitoring Programme guidelines.
Figure 10. Relative confidence scores based on habitats and sensitivity data Please note the white hatched areas show inshore areas where fishing data is incomplete.
Results
Findings from the 2012 UK Initial Assessment
In 2012, a method using expert judgement found that the spatial extent of damage from bottom fisheries was considered to far outweigh contributions from other sources of this pressure (HM Government, 2012).
Latest findings
Status Assessment
Figure 5 shows the distribution of surface abrasion, including areas where no fishing occurs, from low to high surface pressure. The level of pressure across the 6 years was not always constant: 71% of the cells experienced a consistent level of fishing pressure between years, and 29% showed high levels of variability in the fishing pressure (a change of 3 categories or more on the fishing pressure in the same cell across years is considered to be highly variable). The data for surface abrasion (Figure 5), was aggregated with subsurface abrasion data and then combined with habitat sensitivity data, to produce disturbance values for 2010 to 2015 (Figure 8).
The target is achieved in the UK portion of OSPAR Region V (Wider Atlantic), although the disturbance in some areas around Rockall Bank and the shelf break was at the highest level (category 9) due to a combination of sensitive habitats and regular fishing activity. Incomplete data made it difficult to assess fishing impacts in the UK portions of OSPAR Region IV (Bay of Biscay and Iberian Coast) and OSPAR Region I (Arctic Waters).
A confidence analysis of the sources of habitat and sensitivity data available to support the results shows a high degree of spatial variability (Figure 10). The highest level of confidence is around the highly-surveyed areas (such as Marine Protected Areas), for example, off the Northumberland coast, where point data available was used to produce a sensitivity layer. The lowest confidence is observed where modelled habitat data have been used to calculate sensitivity, including large areas of deep-sea, West of Scotland. The most widespread habitat types in the UK, are sublittoral soft sediments such as sublittoral coarse sediment, sand, mud, and mixed sediment. The habitat types were used to examine spatial variability (Figure 11).
Figure 11. Percentage of total area of each habitat that is predicted to be subjected to higher levels of disturbance (category 5-9) in each of the UK Sub-Regional Seas.
Some areas assessed (Southern Celtic Seas and English Channel) showed higher levels of disturbance across all habitat types (>50% of habitat area subject to higher disturbance), while other areas showed a wider range in disturbance levels.
Trend Assessment
Unknown: The indicator was not considered as part of the 2012 Initial Assessment. The analysis of variation in disturbance during 2010 to 2015, did not show any clear trends, mainly due to the limited temporal data available for this assessment.
Further information
Composite habitat map
An overview of the method used to run the indicator assessment as provided in this assessment sheet can be seen in Figure 1. The first step of this assessment produces a composite map of habitat data, using the methods described previously. The output provides a spatial overview of the extent and distribution of broad-scale physical habitat types across the UK maritime area (Figure 2). This composite layer contains information at different levels of detail (equivalent to different levels in the EUNIS habitat classification hierarchy) but can be summarised at a consistent level equivalent to EUNIS level 3.
The UK has a large variety of habitats with the most widespread being soft sediments. The broad-scale habitat with the largest coverage is Sublittoral Sand in both the North Sea and the Celtic Sea. The deepest habitats can be found in Region IV (Bay of Biscay and Iberian Coast) in the area known as the Canyons, with depths of over 3800m. The UK also has large areas of complex heterogeneity with habitats with different physical structure being found within some areas such as rocky areas with a thin veneer of sediments located in the English Channel.
Abrasion pressures and disturbance
The extent of the subsurface abrasion pressure, caused by parts of fishing gears that penetrate deep into sediment (such as the doors of otter trawls), is generally lower (Figure 6) than the extent of surface abrasion pressure (Figure 5). The results from 2010 to 2015 showed pressure and disturbance caused by fishing activities to be widespread, occurring to some degree in 57% of the cells (categories 1 to 9) for all habitats within UK waters. Highest intensities of subsurface abrasion are found in the offshore waters of South West England, the eastern part of Northern Ireland, English Channel and eastern parts of England. To a lesser extent, scattered areas of high intensity can also be found around Scottish waters. The data from the subsurface abrasion contributes to calculating the overall pressure, see below.
The results presented here are assuming an even distribution of fishing pressure within each grid cell, which may be incorrect and could have a substantial effect on the overall estimate of the area of habitat disturbance. Additionally, some data assigned as fishing activity could instead be vessels that are transiting at a speed within the ranges considered to be fishing, which in turn could overestimate the amount of abrasion of vessels >12m in areas close to the shore. It should be noted that the surface and subsurface pressures maps (Figures 5 and 6) do not capture all fishing activity. A gap exists around the activities of smaller vessels (<12m) fishing mostly in coastal waters that are not equipped with a Vessel Monitoring System recorder. It is estimated that inshore fleets (<12m length) represent 86% of the active EU vessels in the OSPAR area. However, many of these vessels do not use mobile bottom-contacting gear, and in general, they are not as active individually as larger (>12m) vessels. The lack of inshore fishing data causing surface and subsurface pressure is an important issue for countries where small vessels represent the largest proportion of the fleet operating within inshore waters (less than 12 nautical miles). Some countries' fleet data are also missing from the Swept Area Ratio calculations as Vessel Monitoring System and logbook data were not submitted to the International Council for the Exploration of the Sea as requested in data calls which is likely to impact the estimates of fishing pressure within UK waters. Also, data submitted by other countries have some incomplete entries, and it is expected that some data are missing or potentially misallocated due to different approaches to extracting data from the national databases. The balance between the overestimation in some cells and underestimation in others will be addressed as the indicator is further refined in the next round of assessment and a more complete dataset of fishing data becomes available.
For the current analysis at an OSPAR level only aggregated Vessel Monitoring System and logbook data per grid cell are available (logbook data from other countries is not available to UK Agencies) and grouped by gear type such as ‘otter trawl’. As a result, there is a likely overestimation of the results for both surface and subsurface abrasion in some areas, as each grid cell has a unique Swept Area Ratio value that assumes a homogenous distribution of fishing. However, this can be partially addressed if the existing information from gear types at level 6 (metiers) are made available for this analysis, which will help to refine the analysis of disturbance by matching the type of metiers against the distribution of relevant habitat type within the cell or c-square.
Pressure variability across years
To aggregate the pressure maps for the assessment period 2010 to 2015 and to detect trends in pressure intensity, cells with consistent fishing pressure were distinguished from those with high fishing variability (Figure 7 and Table 3). The calculations to determine variability have been undertaken using an analysis of variance (trend analyses were not statistically viable due to the limited number of years available). Overall 29% of c-squares grids across the UK experienced highly variable fishing for the period 2010 to 2015 (Table 3). An area with especially high fishing variability is the Northern North Sea (for example, the Fladen grounds), whereas, in other assessment areas, most of the area experiences consistent levels of fishing intensity. For example, some of the grids in the offshore eastern parts of the English Channel are under the same level of fishing pressures across all years, all above a Swept Area Ratio of more than 3. Areas off the west coast of Scotland around the Outer Hebrides showed changes in the pressure categories from a Swept Area Ratio of 3.6 in 2010 to no fishing in 2011, 2014, and 2015. Others, such as those found off the east coast of Scotland, had little or no fishing occurring during 2010 to 2012, followed by an increase to categories 3 and 4 in subsequent years up to 2015.
|
Fishing Variability |
Number of frid cells (c-squares) |
Area (km2) |
Percentage of grids (c-squares) |
|
Highly Variable Fishing |
8,864 |
147,200 |
29 % |
|
Constant Fishing |
21,486 |
369,000 |
71 % |
Table 3. Summary statistics showing the percentages and km2 of seafloor with consistent and variable fishing pressure
Habitat sensitivity results
The distribution of habitat sensitivity to surface and subsurface abrasion has been calculated using the method outlined in the previous section (please note, for the calculation of sensitivity both resistance and resilience are being considered). The extent that a habitat is disturbed is affected by its ability to withstand the impacts (resistance) and also its ability to recover after subsequent events (resilience) (Tillin and others, 2010). Therefore, resilience is a key element to assess the temporal effects of pressures. The sensitivity of habitats to surface abrasion and subsurface abrasion, which varies across the regions, are shown in Figures 3 and 4). There are some variations between the surface and subsurface sensitivities, but overall medium range sensitivity (score of 3) is the most common type. Habitats that have been assigned the highest level of sensitivity to abrasion are rocky areas (for example, sponge and anthozoan communities), biogenic reefs (such as Sabellaria spinulosa) and deep-sea habitats (such as coral gardens).
Assessment by sub-region
The spatial distribution of disturbance aggregated across the six years has been split by assessment area (Figure 8). The results have been used in combination with habitat distribution in Figure 2 to explore the predicted proportions of broad-scale habitats affected by different levels of disturbance, and the trend analyses undertaken as part of the OSPAR Assessments (OSPAR Commission, 2017) (Figure 12).
Figure 12. Trend analyses for a selection of habitats (Sublittoral Coarse Sediment, Sublittoral Sand, Sublittoral Mud and Sublittoral Mixed Sediments) per sub region for area (km2) with disturbance in categories 5-9. Only cells with high variability in fishing pressure are shown.
Celtic Seas UK Marine Strategy Framework Directive sub-region
Southern and Northern Celtic Seas (OSPAR Region III)
The majority of the Southern Celtic Seas grid cells have some level of disturbance value (Figure 8). The highest disturbance occurs mostly on coarse sediments followed by mud habitats, whereas in the Northern Celtic Seas, disturbance distribution is patchier, the highest levels occur mostly on areas of sublittoral mud off the east coast of Northern Ireland and the west coast of Scotland, and deep-sea mud from the west of Ireland. Both assessment areas show temporal variations across years (Figure 12).
Greater North Sea UK Marine Strategy Framework Directive sub-region
Southern and Northern North Sea (OSPAR Region II)
In the Northern North Sea, some areas show very low or no levels of disturbance which can be attributed to low levels of fishing activity in these areas (for example, central North Sea). Habitats with the highest proportions of disturbance are sand and mud habitats, in particular, those along the northerly shelf edge of the UK. It should be noted that in most cases the pattern of disturbance follows the distribution of predominant habitats and depth contours along the shelf.
Channel (OSPAR Region II)
The English Channel has the highest proportion of disturbance on coarse sediment off the south-west of England, although very low disturbance is indicated around the Isle of Wight and coastal areas of Sussex and Devon. The trend analysis shows a slight decrease in this assessment area for disturbance between 2012 and 2015 (Figure 12).
Other UK Waters
UK waters of OSPAR Region I (Arctic Waters)
Two small areas of UK waters are within OSPAR Region I, with a small amount of disturbance present at the southern end of each area. At present, it is not possible to undertake further analysis due to the limitations of habitat sensitivity data available.
UK waters of OSPAR Region IV (Bay of Biscay and Iberian Coast)
One small area of OSPAR Region IV is present in the deeper waters off the south-west of the UK. No disturbance is shown with our calculations, however, this could be due to incomplete data sets submitted to the International Council for the Exploration of the Sea.
UK waters of OSPAR Region V (Wider Atlantic)
There was little disturbance in the UK waters of OSPAR Region V, with exceptions to the eastern end and around Rockall bank, and the shelf break where disturbance values were very high.
Assessment by habitat type
The predicted level of disturbance within a habitat type is not constant, as demonstrated in Figure 9, showing the distribution of disturbance categories for sublittoral mud. This variability is mainly driven by the differences in fishing pressure within a habitat across the regions. For example, in the Celtic Seas, the distribution of fishing effort on sub-littoral mud is constrained by the patchy and localised distribution of this habitat type within that region, the Northern Celtic Seas has the highest level of disturbance categories 8 and 9, and within the Southern Celtic Seas the bulk of this habitat type is above the disturbance category 4.
The percentages of habitat area with predicted higher levels of disturbance (categories 5 to 9) for all EUNIS level 3 habitats per assessment area included in the assessment are shown in table 4. Nearly all deep-sea habitats that are subject to fishing pressure in these areas are experiencing high disturbance, as the sensitivity of these habitats is determined as very high, although proportionally the areas under high disturbance are small. The region showing the highest level of disturbance is Southern Celtic Seas with more than 74% of the main soft sediments (sand, mud, coarse and mixed sediments) showing disturbance of categories 5 and above. Sublittoral sands have the highest percentage of disturbance (above 60%) in the Southern Celtic Seas, Southern North Sea and English Channel. By contrast, infralittoral habitats generally experience lower disturbance. Overall, the results for sublittoral soft sediments are highly variable across regions.
Table 4. Total area and percentage of habitat area with disturbance values falling in categories 5-9 per habitat and OSPAR sub-region. Areas with high disturbance for commonly occurring habitats are marked with a *. Region 4 is not included in the table as the area of disturbance falling in categories 5-9 was 0km2 for all habitats. N=northern, S=Southern, CS=Celtic Sea, NS=North Sea, Channel=English Channel.
|
|
Region 1 |
Region 5 |
N CS |
S CS |
Channel |
S NS |
N NS |
|||||||
|
|
km2 |
% |
km2 |
% |
km2 |
% |
km2 |
% |
km2 |
% |
km2 |
% |
km2 |
% |
|
A3.1 |
0 |
- |
|
|
43 |
3 |
0 |
0 |
9 |
13 |
0 |
- |
35 |
8 |
|
A3.2 |
0 |
- |
|
|
41 |
19 |
1 |
3 |
41 |
41 |
0.01 |
4 |
25 |
6 |
|
A3.3 |
0 |
- |
|
|
32 |
22 |
0 |
- |
5 |
20 |
0 |
- |
3 |
3 |
|
A3.9 |
0 |
- |
|
|
0 |
- |
0 |
- |
142 |
61 |
0 |
- |
0 |
- |
|
A4.1 |
0 |
- |
|
|
1710 |
24 |
386 |
28 |
1178 |
69 |
20 |
40 |
1493 |
65 |
|
A4.2 |
0 |
- |
74 |
100 |
2175 |
44 |
559 |
54 |
352 |
73 |
53 |
20 |
5390 |
79 |
|
A4.3 |
0 |
- |
41 |
25 |
590 |
16 |
100 |
48 |
1 |
33 |
0 |
- |
3118 |
49 |
|
A4.7 |
0 |
- |
0 |
- |
0 |
- |
0 |
- |
0 |
- |
0 |
- |
0 |
- |
|
A5.1 |
0 |
- |
1570 |
66 |
7582 |
21 |
22221* |
75* |
16792* |
81* |
8113 |
44 |
14859 |
43 |
|
A5.2 |
0 |
- |
266 |
43 |
7007 |
25 |
22933* |
74* |
3494* |
70* |
22135* |
60* |
54506 |
46 |
|
A5.3 |
0 |
- |
2417 |
55 |
11527* |
76* |
7575* |
95* |
294* |
53* |
72 |
1 |
24883* |
89* |
|
A5.4 |
0 |
- |
13 |
12 |
1554 |
28 |
915* |
79* |
2308* |
64* |
1267 |
33 |
1118 |
38 |
|
A5.5 |
0 |
- |
0 |
- |
6 |
5 |
0 |
- |
1 |
3 |
0 |
- |
3 |
10 |
|
A5.6 |
0 |
- |
0 |
- |
0 |
- |
2 |
11 |
21 |
77 |
37 |
8 |
10 |
17 |
|
A6.1 |
11 |
77 |
837 |
13 |
0 |
- |
0 |
- |
0 |
- |
|
|
13 |
100 |
|
A6.2 |
758 |
18 |
4043 |
12 |
3 |
84 |
8 |
90 |
0 |
- |
|
|
4255 |
51 |
|
A6.3 |
890 |
29 |
12576 |
22 |
92 |
100 |
21 |
100 |
0 |
- |
|
|
5095 |
86 |
|
A6.5 |
116 |
2 |
15 |
50 |
0 |
- |
0 |
- |
0 |
- |
|
|
71 |
1 |
|
A6.5 |
3 |
0 |
10857 |
8 |
0 |
- |
0 |
- |
0 |
- |
|
|
300 |
20 |
|
A6.6 |
0 |
- |
2 |
11 |
0 |
- |
0 |
- |
0 |
- |
|
|
0 |
- |
An analysis of disturbance values across years for the most widespread habitat types (only grid squares with high fishing variability) shows temporal and regional variations (Figure 12). Sublittoral sand and mud habitats in the Northern North Sea, Northern Celtic Seas and the Southern Celtic Seas are especially subject to high variations in fishing pressure (Figure 12). In 2011, a strong decrease in disturbance of sublittoral sand and mud occurred in these assessment areas, followed by an increase again in 2012. A similar, but less pronounced trend for sublittoral sand is also visible in the Southern North Sea, whereas generally there are no large changes in levels of disturbance in this assessment area for other habitats. In the English Channel there appears to be a decrease in fishing pressure on sublittoral coarse sediment, which is the habitat with the largest area by far. However, the disturbance of sublittoral sand has increased in the Channel.
Levels of confidence
The resulting disturbance values for benthic habitats must be viewed alongside the confidence map (Figure 10). This map indicates the type and source of habitat and includes sensitivity data that underpins the results. Confidence in the sensitivity scores appear to be influencing the broad patterns of spatial variability in the overall confidence map. The sensitivity scores are based on the level of data used and the quality of information available on the resistance and resilience of habitats and species to physical damage pressures - sensitivity scores derived from experimental or field survey studies have high confidence, whereas scores based on expert judgments have low confidence. Many of the areas with the highest confidence scores in the habitat data are from habitats classified as sublittoral sediment. Please note that the confidence is a combination of the
habitat data models/point data, and the associated sensitivity information on the resilience and resistance of the habitats and is not confidence in the presence of a sensitivity feature at a particular site.
Based on the levels of disturbance per habitat type across the region a ‘Physical Damage Index’ value for each benthic habitat or geographical area should be calculated. This index is still under development and testing, and thus results cannot be displayed within this assessment but will be available in the future (see assessment method). It will still be required to include additional data during the next cycle to fully understand the extent of damage across the regions.
Conclusions
The UK targets for the Greater North Sea and Celtic Seas have not been met (Table 1). The results from 2010 to 2015 showed pressure and disturbance caused by fishing activities to be widespread, occurring to some degree in 57% of the cells within UK waters. Only the UK portion of OSPAR Region V (Wider Atlantic) is within the agreed disturbance targets. The assessed areas with the highest levels of disturbance were the Southern Celtic Seas and English Channel, with around 75% of cells showing higher levels of disturbance. The habitat identified as being subject to the highest disturbance was sublittoral mud, with more than 75% of the total habitat area identified as subject to high disturbance in the Southern Celtic Seas, Northern Celtic Seas and Northern North Sea.
There is an underestimation of disturbance in some areas due to the lack of data prior to this assessment and from small inshore fishing vessels. However, some areas might have an overestimation as pressure data is evenly distributed within a cell.
Further information
These results estimate the current distribution and extent of habitat sensitivity, the overlapping fishing pressures, and the resulting habitat disturbance. These results allow the distinction to be made between seafloor habitats of varying sensitivity that are under pressure from fishing activities. Some areas under high levels of surface, and in particular, subsurface abrasion show low disturbance. This could be caused by either the habitats containing naturally resistant or resilient species or ongoing pressure modifying the habitats with sensitive species being replaced by opportunistic and less sensitive species.
The UK portion of Region V (Wider Atlantic) is at present the only sub-region where predicted disturbance levels are above the indicator quantitative target. However some areas within this region, such as the shelf break, has the highest level (category 9). Incomplete data submissions hindered our ability to interpret the fishing impacts in regions I (Arctic Waters) and IV (Bay of Biscay and Iberian Coast). An analysis of disturbance values across years for the most widespread habitat types (only grids with high fishing variability) shows temporal and regional variations. High levels of temporal variability can be found in the Northern North Sea (for example, the Fladden grounds), whereas in other assessment areas, most of the area experiences consistent levels of fishing intensity.
At present, there are limitations due to data availability and accessibility for assessing habitat extent and distribution and their associated sensitivity. The combination of sample data with modelled outputs is thus providing an alternative approach in the absence of detailed survey data.
Some areas have already lost some of the sensitive species/biotopes due to past human activities that cannot be assessed by this indicator and could result in a lower disturbance score. To address these, the outputs of this indicator will be validated using the assessment results from other benthic condition indicators, such as the Condition of Benthic Habitats Communities, including the development of reference conditions, as they become available.
The method used has been thoroughly tested and reviewed by OSPAR and the Healthy Biologically Diverse Seas Education Group experts and through dedicated workshops. It represents a realistic approach for assessing the distribution of physical disturbance across the UK based on current knowledge and using all available evidence. However, it is important to note that the strength of any assessment is dependent on the quality of the input data and the approach to its analysis, and this will, in turn, dictate the power and utility of the resulting information. As it stands the method proposed gives a partial assessment of physical damage at a UK scale focused only on physical abrasion and as a result of bottom fishing by vessels greater than 12m in length. This assessment probably underestimated the amount of benthic habitat that has been damaged by fishing, because it does not include all areas that are still impacted by fishing activities before 2010, and it excludes activity of small vessels in inshore fisheries as well as Spanish registered vessels. However, the results presented are also predicated on an assumption of even distribution of fishing pressure within each grid cell, which may often be incorrect and could have a substantial effect on the overall estimate of the area of habitat disturbance.
It is also expected that disturbance matrices and the final algorithm will be calibrated in the future and, if required, modified using the outputs from site-scale condition indicators, not only for fishery information but also for other human activities. The use of condition indicators such as the Condition of Benthic Habitats Communities, which is based on the Water Framework Directive multimetric indicators, and the typical species will help to improve the confidence and accuracy of the results on the distribution and levels of disturbance from the Extent of Physical Damage indicator.
Knowledge gaps
Research is required to develop a fully quantitative and cost efficient method in collaboration with agencies both across and outside of the UK (including OSPAR and the International Council for the Exploration of the Sea) to address:
- the lack of data from small vessel/inshore fisheries operating around the UK and from other activities causing physical damage such as offshore construction
- the development of reference conditions
- the review of biogeographical assessment units and environmental data
- the review and development of the sensitivity and disturbance methods
- better understanding of the spatial and temporal impacts of different fishing gear types.
Further information
Data access
Additional information from benthic species collected during surveys can be added to improve the quality of the results. The availability of biotope (EUNIS habitat Level 5 and below) survey maps will increase the quality of the composite map being produced for this indicator. Combined, these will improve the evidence available on the distribution of sensitivity information across the regions. This alongside access to the aggregated fisheries Vessel Monitoring Systems data, in particular data on métiers, being collected as part of the ongoing data calls is important to improve the accuracy of results without incurring on additional costs.
Quantified impacts values
Development of quantifiable pressure-impact relationship values are very important to the effective incorporation of information on other activities causing physical damage, and assessing the additive effects in multi-pressure systems. The scale of fishing pressure and matrix underpinning disturbance values needs to be evaluated using experimental and field studies to improve evidence on the pressure-impact-response relationships. Results from the assessment of the Condition of Benthic Habitat Communities indicator will be used to validate and help calibrate impact calculated under this assessment.
Sensitivity and disturbance data
The evidence base can be improved if more information on the resilience and resistance of biotopes, or habitats at EUNIS levels 4 to 6, is made available and/or further studies are undertaken. Sensitivity information used in the production of the surface abrasion and subsurface abrasion sensitivity layers was developed based on studies, literature reviews, and matrices from the UK, France, and Germany. It is, therefore, mainly relevant for OSPAR Regions II (Greater North Sea) and III (Celtic Seas). Some habitat types, in particular, the deep-sea habitats, are based on very limited sensitivity information, and in some cases considered as highly sensitive, mainly based on a precautionary approach. This gap is of particular importance for region IV (Bay of Biscay Iberian Coast) as it is expected that the characterising species and biotopes will be slightly different from those in regions II (Greater North Sea) and III (Celtic Seas). The confidence of the assessment results will benefit greatly from an increase in the availability of more accurate habitat data. As part of the EU co-funded Applying an Ecosystem Approach to Regional Habitat Assessment project, new information has been tested to increase the qualitative aspects of the indicator, in particular for the analyses of sensitivities and disturbance, using case studies in Region IV (Bay of Biscay and Iberian Coast). These results, alongside data and information generated by other EU projects, have been incorporated within the methodology for the next assessment cycle
Additionally, a review of the sensitivity method should be undertaken. In particular, the relevance of incorporating recoverability aspect into the assessment and when it may be more appropriate to factor it into the temporal trend analysis and to management actions (for example, priorities for habitats with very long recovery times).
Calculation of an index per sub-region and habitat type
The final outputs of this model are levels of disturbance per habitat type across a sub-region. These levels of disturbance are amalgamated into a ‘Physical Damage Index’ value for each benthic habitat. However, during indicator development, a formula for calculating this Index was tested and found not to deliver satisfactory results to capture temporal changes in fisheries pressures. The formula and results are not presented here as they are still under development and further testing, which will include the revision of the sensitivity/pressure matrix to incorporate new data from fishing impact curves and condition indicators.
Further understanding of the cumulative effects of historical damage from some human activities, and in particular the effects that the continued loss of sensitive habitats could cause on the integrity and ecological function of benthic habitats, need to be investigated to allow the definition and development of reference values for the assessment of this indicator. Physical damage by other human activities than fisheries will be calculated for the next round of assessments, such as the OSPAR Quality Status Report.
Effects of other gear types
A better understanding of the effects of differing and innovative gear types (such as SumWing trawls), which may result in a lower abrasion pressure.
References
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Acknowledgements
Assessment Metadata
Please contact marinestrategy@defra.gov.uk for metadata information
Recommended reference for this indicator assessment
Cristina Vina-Herbon1, Bryony Meakins1, Anita Carter1, Dan Edwards1, Graeme Duncan1, Helen Lillis1, Laura Pettit1, Laura Robson1, Gemma Singleton1 Mike Young3, Karen Robison2, Annika Clements6, Tim Mackie4, Jane Hawkridge1, David Vaughan1, Phil Boulcott6 and Graham Phillips7 2018. Extent of Physical damage to predominant seafloor habitats*. UK Marine Online Assessment Tool, available at: https://moat.cefas.co.uk/biodiversity-food-webs-and-marine-protected-areas/benthic-habitats/physical-damage/
* Adapted from OSPAR Intermediate Assessment 2017 on Physical Damage Assessment
1Joint Nature Conservation Committees
2Natural Resources Wales
3Natural England
4Department of Environment, Agriculture & Rural Affairs, Northern Ireland
5Marine Scotland
6Agri-Food & Biosciences Institute
7Environment Agency