Location and physical characteristics
Figure 1: Hebrides Shelf Offshore Marine Region.The thicker white line delineates the extent of the Hebrides Shelf OMR. For a map of all SMRs and OMRs, see Figure 5 here
| Sea area (km2) | 30,437 |
| Deepest point (m) | 1,359 |
| Shallowest point (m) | 19 |
| Average depth (m) | 241 |
| Tides (m) | 1.6 – 3.4 |
| Salinity | 34.65 – 35.32 |
| Sea surface temperature (°C) | 8.9 – 14.1 |
The Hebrides Shelf OMR is situated between the Bailey and Rockall OMRs to the west and the Outer Hebrides SMR to the east (Figure 1). It is a relatively shallow area, extending from coastal areas close to the Hebridean islands to the margin of the deep waters of the Rockall Trough.
In the Hebrides Shelf OMR the residual flow is northwards. Seasonal variations in circulation in both strength and positioning occur. This is due to both changes in wind patterns, and to the occurrence of frontal jets in summer-time when the water column stratifies due to seasonal heating at the sea surface. Where there is a transition between permanently mixed and these seasonally stratified regions, frontal jets exist. The Slope Current flows northwards along the edge of the continental slope (at about the 400-500 m contour) at speeds of up to 0.15 – 0.3 m/s.
The overall wave climate is influenced by conditions in the North Atlantic, with large swell developing at times due to the long fetch.
The Hebrides Shelf OMR comprises a diverse variety of sediments, such as gravels, sands and mud as well as rocky reefs. Sedimentary habitats vary from coarser to finer sediments moving down the slope into the deeper, calmer water. A wide range of geological features are represented in the area. Slide deposits and scars are found in the Geikie Slide Geodiversity area together with an example of deep-water sediment failure on the slope to the west of the Isle of Lewis. The Barra Fan and the Peaches Slide Complex have a number of geomorphological features, such as an iceberg ploughmark field, prograding wedges, continental slope turbidite canyons, slide deposits, scour moat, continental slope, and the Hebrides Terrace Seamount.
Productive
The Hebrides Shelf OMR has no aquaculture or aggregates extractions associated with it, nor any oil and gas activity or renewable energy developments. In addition, the Hebrides Shelf OMR includes a portion of a substantial military exercise area off the west coast of Scotland.
There are some demersal and pelagic fish landings from the Hebrides Shelf OMR, mainly from the areas bordering Bailey OMR and Rockall OMR.
The Productive Assessment was undertaken, with a focus on 2014 – 2018, on a sectoral basis. However, for the fishing sector, there were changes over the period 2014 – 2018 (Figure 2).
Figure 2: Changes that have taken place in the Hebrides Shelf OMR for the fishing Sector.
Pressures from human activities
As part of SMA 2020, an assessment of the main pressures from human activities in each of the Scottish Marine Regions and Offshore Marine Regions was undertaken through a MASTS-led workshop. The process and outcomes are presented in detail in the Pressure from Activities section. Five main pressures identified for the Hebride Shelf OMR ordered as per the MASTS-led Pressure Assessment Workshop were:
| Priority [1] | Pressure (FeAST classification) [2] | Main healthy and biologically diverse components affected [3] | Main contributing FeAST activity /activities to pressure [4] | Associated productive assessments [5] |
|---|---|---|---|---|
| 1 | Removal of target species (including lethal) |
|
||
| 2 | Removal of non-target species (including lethal) |
|
||
| 3 | Surface abrasion |
|
||
| 4 | Sub-surface abrasion/penetration |
|
Clean and safe
The assessments cover eutrophication, hazardous substances, marine litter, noise and microbiology and algal toxins which have the potential to impact on habitats and species as well as being a consequence of human activity. Although sources of litter or contaminants may be local, there are cases when the source is some distance from the impacted area. The main findings for the Hebrides Shelf OMR are:
Hazardous substances
Hazardous substances (polycyclic aromatic hydrocarbons (PAHs), polychlorinated biphenyls (PCBs), polybrominated diphenyl ethers (PBDEs) and heavy metals (Hg, Cd and Pb)) assessments in sediment and biota (fish and shellfish) were undertaken at the scale of the five Scottish biogeographic regions: Atlantic North-West Approaches, Irish Sea (Clyde and Solway), Minches and Western Scotland, Scottish Continental Shelf and Northern North Sea. Hebrides Shelf OMR is in the Scottish Continental Shelf biogeographic region. There are few sediment and biota sites in the Scottish Continental Shelf biogeographic region, however, none of these sites are in the Hebrides Shelf OMR. Concentrations of hazardous substances at the few sediment and biota sites in the Scottish Continental Shelf biogeographic region were generally above background but below concentrations where adverse effects could occur at these sites. In addition, concentrations were stable or declining for all hazardous substances measured.
Of the biological effects measurements included in the assessment, there are no sites in the Hebrides Shelf OMR and also limited data at the biogeographic region scale for the Scottish Continental Shelf. Therefore, contaminant-specific biological effects (PAH bile metabolites and 7- ethoxyresorufin O-deethylase (EROD) activity) cannot be commented on. External fish disease, a general measure of fish health, was assessed at one site in the Scottish Continental Shelf biogeographic region, and showed that the fish health status was satisfactory.
Marine litter
Due to the lack of assessment criteria for marine litter, status assessments were not possible. Seafloor litter was assessed at the scale of the biogeographic regions; Hebrides Shelf OMR is included in the Scottish Continental Shelf biogeographic region. The evidence indicates that there are apparent decreases in seafloor litter density over time between 2012 to 2018.
Healthy and biologically diverse
This section summarises the information from the Marine Protected Areas (MPAs) and intertidal and continental shelf habitats assessments from SMA2020. It also provides information from the relevant case studies relating to Priority Marine Features (PMFs), with a focus on habitats. Further work is required to enable assessment at a regional scale for most species; this will be included in Scotland’s next marine assessment.
At a regional scale for MPAs the focus is on the number of new MPAs, MPAs with new spatial management measures, and MPAs in which spatial management measures are in discussion, as well as recognising monitoring that has been undertaken between 2012-2018. For the marine habitats, the focus is on interpreting the relevant intertidal and continental shelf habitat assessments – biogenic habitats, predicted extent of physical disturbance to the seafloor (BH3) and intertidal seagrass beds. For PMFs, a summary is provided of the changes in our understanding of the habitats of most relevance to the Hebrides Shelf OMR, including changes in distribution and extent.
Marine Protected Areas
Progress in developing the Scottish MPA network
There are 3 MPAs in the Hebrides Shelf OMR that contribute to the Scottish MPA network (see Table 1).
Some of these MPAs overlap completely or partially in terms of their spatial coverage and/or the features (habitats, species etc.) they were set up to help conserve. They are counted as separate MPAs because they have been established under different legislation which influences the way in which they are managed. Also note that there are MPAs that straddle the boundaries between different OMRs or in some cases overlap each other. Where this is the case, these MPAs have been counted as contributing to the MPA network in all of the OMRs in which they are present. This means that the total number of MPAs in Scotland cannot be calculated through combining the SMR / OMR totals. Please see the Marine Protected Area assessment which contains statistics for the Scottish MPA network as a whole.
Table 1. Numbers of types of MPAs in the Hebrides Shelf OMR that contribute to the Scottish MPA network, including the number of new MPAs introduced since 2012.
|
Type of MPA |
Abbr. |
Total no. of MPAs |
No. of new MPAs 2012-2018 |
|
Special Area of Conservation |
SAC |
1 |
0 |
|
Nature Conservation MPAs |
MPA |
2 |
2 |
Note: The Seas off St Kilda SPA and the West of Scotland MPA were also established in 2020 (though only 1% of the West of Scotland MPA lies within this OMR).
Progress in Managing MPAs
The progress in implementing management measures for MPAs is summarised in Table 2. This includes information on where spatial management measures are in place and where they are under discussion. It also includes information on the number of MPAs that have been monitored by statutory bodies.
Table 2. Summary of progress in managing Marine Protected Areas in the Hebrides Shelf OMR .
|
Type of MPA |
No. of MPAs with spatial measures in place pre-2012 |
No. of MPAs with new spatial measures in place 2012-2018 |
No. of MPAs with spatial measures under discussion 2012-2018 |
No. of MPAs monitored by statutory bodies 2012-2018 |
No. of MPAs monitored by citizen scientists 2012-2018 |
|
Special Area of Conservation |
0 |
0 |
1 |
1 |
0 |
|
Nature Conservation MPA |
0 |
0 |
2 |
0 |
0 |
Progress is ongoing with fisheries management options being developed in the three MPAs (Stanton Banks SAC, The Barra Fan and Hebrides Terrace Seamount MPA, and the Geikie Slide and Hebridean Slope MPA).
Progress in Monitoring MPAs
Information on the evidence base used to characterise the MPAs in the Hebrides Shelf OMR and any subsequent monitoring is given in the Site Information Centre web page ( https://jncc.gov.uk/our-work/offshore-mpas/ ) for the following MPAs:
• The Barra Fan and Hebrides Terrace Seamount MPA,
• Stanton Banks SAC,
• Geikie Slide and Hebridean Slope MPA.
For links to the MPA Surveys and monitoring reports in the Geikie Slide and Hebridean Slope MPA, see MPA Monitoring Survey Reports | JNCC - Adviser to Government on Nature Conservation
Priority Marine Features
The assessments focus on individual/ grouped habitats and species with a number of case studies reflecting more detailed research and monitoring as outlined in ‘What is assessed’. A key component of an OMR is the number and type of Priority Marine Features (PMFs) present in the region and the associated protected areas. In addition, there is concern about invasive non-native species and the impact that they are having in any particular region. With respect to these three aspects, the principal findings of SMA 2020 that are most relevant to the Hebrides Shelf OMR are summarised below.
Number of Priority Marine Features and birds (non-PMF) recorded
The Hebrides Shelf OMR is the location for a range of PMFs Table 3.
Table 3. Summary of Priority Marine Features in the Hebrides Shelf OMR .
|
PMFs – grouped habitats and species |
No. of species/habitats recorded |
|
Intertidal and continental shelf habitats |
5 |
|
Fish |
26 |
|
Mammals (regularly occurring) |
11 |
|
Shellfish & other invertebrates |
2 |
|
Seabirds (non-PMF) - breeding |
0 |
|
Seaducks, grebes & divers (non-PMF) – non-breeding |
0 |
- There are 44 PMFs recorded in this region, including shellfish, fish species, marine mammals and a range of habitats.
- Subtidal sand and gravel communities are found on the continental slope.
- Burrowed mud occurs at the base of the continental slope, characterised by sea pens and burrowing megafauna communities which are an OSPAR threatened and/or declining habitat.
- Numerous feeding tracks, mounds and burrows found in offshore deep-sea muds indicate the presence of large decapods such as the squat lobster Munida tenuimania. Deep-sea muds provide a habitat for a diverse range of sea life including sea urchins, sea spiders, deep-sea bristleworms and xenophyophores.
- The designated reef area in this region is particularly deep and wave exposed. The associated community includes encrusting flora and fauna such as red coralline algae, barnacles and serpulid worms, sponges, robust hydroids and mobile fauna such as brittlestars and featherstars.
Climate change
There is good evidence that climate change is driving changes in the physical, chemical and biological conditions of the marine environment but the current evidence base limits the ability to draw conclusions at the scale of the individual marine regions, including Hebrides Shelf OMR. This is a combination of the lack of comprehensive spatial coverage of key monitoring programmes, the relatively short time series, and the complex linkages of climate change impacts in the marine environment.
Increasing concentrations of atmospheric greenhouse gases have caused more energy to be trapped within the Earth’s atmosphere, land and ocean. Approximately 90% of this excess energy has been absorbed by the ocean, resulting in warming ocean temperatures (see Temperature assessment and Climate change Sea temperature assessment).
The increasing concentration of carbon dioxide, one of these greenhouse gases, has the additional consequence of driving a reduction in the pH of the ocean, a process known as ocean acidification (see Ocean acidification assessment and Climate change Ocean acidification assessment).
Mean sea level is rising due to increased contributions of freshwater from melting of land-based ice (glaciers and the polar ice sheets) and due to thermal expansion of water (see Sea level and tides assessment and Climate change Sea level assessment).
The warming temperatures also result in lower oxygen concentrations due to fact that warm water holds less oxygen and changes in stratification further influence oxygen concentrations (see Dissolved oxygen assessment and Climate change Dissolved oxygen assessment). Together with increased metabolic rates in organisms resulting in increased respiration, oxygen depletion has a severe impact on marine organisms due to the impact on metabolic processes.
These changes in the physical environment are also having an impact on marine life, such as changes to their metabolism, changes in seasonality and the timing of events in natural cycles, and changes in their distribution. These changes have consequences for the growth, survival and abundance of species, including those of commercial importance or critical to conservation objectives.
At present, most of these impacts are assessed at scales greater than marine regions. The Community Temperature Index combines species temperature affinity and their abundances. This index has the potential to inform how communities change due to climate change. An example of changes in the Community Temperature Index from bottom-living fishes can be found in the Fish section within Biological Impacts of Climate Change, where more information on other impacts in marine food webs can be found (such as seabirds and marine mammals) on large regional scales in Scottish waters.
Sea surface temperature in the Hebrides Shelf region has increased since 1870 by 0.03 °C per decade on average. The rate of increase has not been constant, and in the last 30 years (1988-2017), the rate of change in temperature was +0.13 °C per decade.
Summary
The Hebrides Shelf OMR has seen a 59% decrease in the value of the fisheries catch over the five years from 2014-2018.
The four main pressures affecting the OMR are Removal of target species, Removal of non-target species, Surface abrasion, Sub-surface abrasion/penetration. Other pressures identified are Death or injury by collision below water, Introduction or spread of non-indigenous species, Physical change, Synthetic compound contamination, Underwater noise and Visual disturbance.
No contaminant (i.e. PAHs, PCBs, PBDEs and heavy metals) samples were collected from the OMR. In the wider Scottish Continental Shelf biogeographic region the evidence shows there is a decrease in sea-floor litter density between 2012-2018.
Two new MPAs were designated between 2012-2018, and no new spatial management measures were put in place. Spatial fisheries management measures are under discussion for three MPAs. One MPA was monitored by statutory bodies during the period 2012-2018.
In the last 30 years sea temperature has risen by 0.13 °C per decade.