Outer Hebrides SMR

Location and physical characteristics



Figure 1: Outer Hebrides Scottish Marine Region. The thicker white line delineates the extent of the Outer Hebrides SMR. For a map of all SMRs and OMRs, see Figure 5 here

Coastline length (km) 3,915
Sea area (km2)     20,848
Deepest point (m) 268
Shallowest point (m)  coastline
Average depth (m)  88
Tides (m)       1.2 – 4.6
Salinity       34.77 – 34.99
Sea surface temperature (°C) 8.4 – 13.8



The Outer Hebrides SMR borders the Minch on its eastern side and the shelf edge on the western side (Figure 1). It includes the archipelagos of St Kilda and Flannan. The SMR has only a small local freshwater input. On the western side of the Hebridean islands, conditions are influenced by those of the adjacent OMRs, while those in the Minch (the eastern side) are influenced by the outflows from the west coast sea lochs. Intensified tidal currents occur where the topography constrains the flow. The residual northward flowing Scottish Coastal Current transports water through the Minch and west of the Outer Hebrides. Seasonal variation exists in circulation in both strength and positioning, due to changing wind patterns and water column conditions. The surface wave climate in the western part of this region is influenced by conditions in the North Atlantic where the fetch is long enough to establish large, regular waves and on occasions extreme waves. The east coast of the SMR within the Minch is more sheltered.

Sea-bed sediments

The waters west of the Outer Hebrides SMR are supplied with little sediment from onshore or by tidal currents. The seabed sediment is very thin and typically a carbonate (shell fragment) sand with some gravel. For some 40 km westwards (to a depth of about 120 m off North and South Uist the seabed is predominantly rocky knolls, with limited sediment cover in the intervening lows. West of this rocky area, in the broad depression south of St Kilda (the St Kilda Basin), sands form a continuous sheet less than one metre thick. Present-day sedimentation in this area is negligible. The Minch and the Sea of the Hebrides serve as a sink for a limited volume of sediment, which mainly originates from mainland Scotland to the east. Seabed sediments are largely derived from reworked glacial deposits and shell fragments. Shell-rich sands may contain fragments of bivalves, barnacles, gastropods, tubeworm tubes and sea urchins.

Productive

The Productive Assessment has been undertaken, with a focus on 2014 – 2018, on a sectoral basis. For a number of Sectors, including renewables, oil and gas, carbon capture and storage and aggregates, there was no activity within the Outer Hebrides SMR during the period 2014 – 2018.

However, for many sectors, including aquaculture (Atlantic salmon and mussels), subsea cables, fishing and marine transport there were changes over the period 2014 – 2018. Some saw increases, while others decreases (Figure 2).

Marine aquaculture is especially important to the Outer Hebrides SMR where 30,668 tonnes of Atlantic salmon was produced in 2018. This compares with 23,221 tonnes in 2009.



Figure 2: Changes that have taken place in the Outer Hebrides SMR by Sector.Although the period 2014 – 2018 inclusive has been used where possible, there are some entries when a slightly different time period has been used.

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 Outer Hebrides SMR 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)
  • Fishing - Bottom otter trawling and pair trawls (OTB, OTT, PTB, TB, TBN)
  • Fishing - Creeling and potting (FPO)
  • Fishing - Dive fisheries (not including hydraulic dredging) (HF, MIS)
  • Fishing - Line fishing (hand and mechanized line and longlining) (LHP, LHM, LL, LLS)
  • Fishing - Pelagic trawling & purse seining (OTM, PTM, TM, PS, PS1, PS2)
  • Fishing - Recreational Fishing
  • Fishing - Scallop dredging (DRB)
  • Seaweed harvesting
2 Removal of non-target species (including lethal)
  • Aquaculture - Finfish
  • Fishing - Bottom otter trawling and pair trawls (OTB, OTT, PTB, TB, TBN)
  • Fishing - Creeling and potting (FPO)
  • Fishing - Intertidal fishing (e.g. bait digging but not bivalves).
  • Fishing - Line fishing (hand and mechanized line and longlining) (LHP, LHM, LL, LLS)
  • Fishing - Pelagic trawling & purse seining (OTM, PTM, TM, PS, PS1, PS2)
  • Fishing - Recreational Fishing
  • Fishing - Scallop dredging (DRB)
  • Seaweed harvesting
3 Surface abrasion
  • Fishing - Bottom otter trawling and pair trawls (OTB, OTT, PTB, TB, TBN)
  • Fishing - Creeling and potting (FPO)
  • Fishing - Scallop dredging (DRB)
4 Sub-surface abrasion/penetration
  • Fishing - Bottom otter trawling and pair trawls (OTB, OTT, PTB, TB, TBN)
  • Fishing - Scallop dredging (DRB)
5 Underwater noise
  • Aquaculture - Finfish
  • Military activities – Sea surface activity
  • Military activities – Sonar use
  • Shipping

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 Outer Hebrides SMR are:

Eutrophication

There was no evidence of eutrophication as a consequence of nutrient enrichment. However, there was a statistically significant increasing trend in nutrient inputs although loads were an order of magnitude lower than most other SMRs. This increase was attributed to an increase in marine farmed fish biomass in the region. Winter nutrient concentrations and chlorophyll concentrations were below assessment criteria and relatively stable. In addition, dissolved oxygen concentrations are above levels required to maintain healthy marine ecosystems.

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. The Outer Hebrides SMR is in the Minches and Western Scotland and Scottish Continental Shelf biogeographic regions. There are limited biota and sediment sites in the Scottish Continental Shelf biogeographic region but more sites in the Minches and Western Scotland biogeographic region, with one biota site being in the Outer Hebrides SMR. Contaminant concentrations at this site were typical of the Scottish Continental Shelf and Minches and Western Scotland biogeographic regions, being generally above background but below concentrations where adverse effects could occur. In addition, there were no increasing trends.

A number of biological effects were also measured, and assessments undertaken at the scale of the five Scottish biogeographic regions. Assessment of imposex in dog whelks, an indicator of TBT contamination, included one site in the Outer Hebrides SMR. Imposex at this site indicated that concentrations of TBT in the marine environment are causing significant harm (> Environmental Assessment Criteria). One site in Outer Hebrides SMR was assessed for EROD, and was at background levels.

External fish disease, a general measure of fish health, was assessed at two sites in the Outer Hebrides SMR, and showed that the fish health status was satisfactory.

Marine litter

Due to the lack of assessment criteria for marine litter, beach litter and microplastic, status assessments were not possible. However, litter and microplastics are present in all SMRs, including the Outer Hebrides SMR. The Outer Hebrides SMR has a relatively low concentration of microplastics in surface water (< 5,000 microplastics per km2 of sea surface) at most sites.

Seafloor litter was assessed at the scale of the biogeographic regions; The Outer Hebrides SMR is in the Minches and Western Scotland and Scottish Continental Shelf biogeographic regions. The evidence indicates that there are apparent decreases in sea-floor litter density over time between 2012 to 2018 for the Scottish Continental Shelf biogeographic region, but no trend in the Minches and West Scotland biogeographic region.

Beach litter data were not available for the Outer Hebrides SMR.

Noise

There are limited noise data for the Outer Hebrides SMR; no continuous noise data were collected and there are very few impulsive noise data for this SMR. It is not possible to do a status assessment as there are no assessment criteria to say what levels of noise are harmful, and not enough years of data to carry out a trend assessment.

Microbiology and algal toxins

Escherichia coli is monitored in shellfish as a proxy of the microbiological quality of the water from shellfish production areas. Classifications are awarded according to the Food Standards Scotland (FSS) Protocol for Classification and Management of Escherichia coli. A site can be designated A, B, C, A/B or B/C, with Class A products able to go direct for human consumption. In the Outer Hebrides SMR the majority of production areas were in the highest class (A, 79%), and no areas had prohibited levels of Escherichia coli.

A number of marine algal species produce biotoxins which, by accumulation in bivalve molluscs such as mussels and oysters, can cause human illness when these shellfish are eaten. Both biotoxins and phytoplankton are routinely monitored in classified shellfish production areas under Regulation (EU) 2017/625. Such monitoring takes place at several sites within the Outer Hebrides SMR. Concentrations of diarrhetic shellfish toxins (DSTs) exceeded regulatory limits (RL) in every year between 2010 and 2018. Paralytic shellfish toxins above RL resulted in site closures in the Outer Hebrides SMR in 2012, 2013, 2015, 2017 and 2018. Amnesic shellfish toxins above regulatory limits were found in cockles from the Outer Hebrides SMR in August 2013, and in razor clams in June 2016.

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 Outer Hebrides SMR, including changes in distribution and extent.

Marine Protected Areas

Progress in developing the Scottish MPA network

There are 39 MPAs in the Outer Hebrides SMR 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 SMRs / OMRs.  Where this is the case, these MPAs have been counted as contributing to the MPA network in all of the SMRs/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 Outer Hebrides SMR that contribute to the Scottish MPA network, including number of new MPAs introduced since 2012.

Type of MPA

Abbr.

Total no. of MPAs

No. of new MPAs 2012-2018

Nature Conservation MPA

MPA

1

1

Ramsar

-

4

0

Site of Special Scientific Interest

SSSI

16

0

Special Area of Conservation

SAC

9

2

Special Protection Area

SPA

9

0

Note that in December 2020 there were another three Nature Conservation MPAs designated in this region (Sea of the Hebrides, North-east Lewis, and Shiant East Bank) and one SPA (West coast of the Outer Hebrides).

Highlights from the various MPAs include:          

The MPA network protects features such as breeding seabirds, benthic and intertidal habitats, and geomorphology.  The Sound of Barra SAC comprises a mixture of islands, extensive rocky reefs, sandbanks and shallow channels between the southern end of South Uist and the north-eastern shore of Barra. The range of subtidal sandbank habitat biotopes reflects the environmental conditions within the Sound. The area is highly exposed in the west (with highly mobile, impoverished sands), tide swept but reduced exposure in the mid-channel (with increased diversity of fauna with some maerl) with deeper more sheltered areas to the east (with stable fine sand with a diverse infaunal community).  St Kilda SAC supports one of the most extensive sea cave systems in the UK. Throughout the island group basalt and dolerite dykes have eroded to form caves and tunnels above and below the water. The communities these support are diverse and reflect the degree of surge to which they are exposed. In shallow water in the extremes of surge the cave walls are blanketed by the sponge Myxilla incrustans. With a reduction in surge, species such as the northern anemone Phellia gausapata are common, and thin encrusting sponges, bryozoans and the anemones Corynactis viridis and Sagartia elegans are abundant. Microhabitats in the deeper caves show a wave exposure gradient, with species usually found in more sheltered conditions, such as the fan-worm Sabella pavonina and the burrowing anemone Cerianthus lloydii, present in the inner regions. Rarely recorded nocturnal species have also been found in the inner caves, most notably the crab Bathynectes longipes and the anemone Arachnanthus sarsi.  East Mingulay SAC is currently the only known area with extensive cold-water coral reefs within UK territorial waters. Most of the East Mingulay Lophelia pertusa reefs form typical biogenic masses that host a large variety of associated species. The remaining reef areas within the site boundary include a rich mix of habitats and species developed on dead coral, boulders, and rocky and cobbly reef structures.  The Inner Hebrides and the Minches SAC covering 1.38 million hectares has been designated for harbour porpoise and is the only protected site for this species in Scotland.  The Monach Isles SAC, hold the largest breeding colony of grey seals in the UK, contributing over 20% of annual UK pup production.  The Outer Hebrides SMR has internationally important populations of a number of breeding seabird species.  The North Rona and Sula Sgeir SPA supports large numbers of petrels, auks, gulls and northern gannets which feed in the waters off the north coast of Scotland and is one of only seven known nesting localities in the EU for Leach’s petrel.

Progress in managing MPAs

The progress in implementing management for MPAs in the Outer Hebrides SMR 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, whether by statutory bodies or through citizen science.

Table 2. Summary of progress in managing Marine Protected Areas in the Outer Hebrides SMR. Note that the spatial measures listed in the table are in addition to the protection provided as a result of consideration of activities/developments through licensing and consenting processes. Also, the monitoring of birds, mammals and habitats within SSSIs and SACs has been split out to reflect the different programmes of work.  These figures cannot be added together to provide a total number of SSSIs and SACs in which monitoring took place because of overlaps in coverage.  

Type of MPA

Spatial measures in place pre-2012

New spatial measures in place 2012-2018

Spatial  measures in discussion 2012-2018

No. of MPAs monitored by statutory bodies 2012-2018

No. of MPAs monitored via citizen science 2012-2018

Nature Conservation MPA

N/A

0

1

1

0

Ramsar

0

0

0

3

0

Site of Special Scientific Interest

0

 

0

Birds

1

Birds

0

Mammals

3

Mammals

0

Habitats

10

Habitats

0

Special Area of Conservation

1

2

4

Mammals

5

Mammals

1

Habitats

6

Habitats

1

Special Protection Area

0

0

0

9

0

Within the Outer Hebrides SMR there are Fisheries Orders in place for Loch Roag Lagoons, St Kilda SAC and East Mingulay SAC. These management measures mainly focus on restrictions to demersal mobile gear but there are also static gear restrictions within part of East Mingulay SAC because of the highly sensitive Lophelia reefs (cold water coral). Further management measures for fishing activities are under discussion for Sound of Barra SAC, Monach Isles SAC/MPA, North Rona SAC and Loch nam Madadh SAC.

During this assessment period the majority of the MPAs have been monitored at least once by the statutory bodies.  Monitoring has covered seabirds, wintering waterbirds, intertidal and subtidal habitats, and mammals.  There has been a significant amount of monitoring of different types of sea lochs and lagoons led by NatureScot, reflecting the importance of the Outer Hebrides SMR for these habitats.  Monitoring of seals continues via the collaboration between the Sea Mammal Research Unit and NatureScot.

Seasearch surveys contribute to the monitoring of the reefs in the Loch nam Madadh SAC. Sightings data of harbour porpoise in the Inner Hebrides and the Minches SAC are recorded by the Hebridian Whale and Dolphin Trust (HWDT) and the Whale and Dolphin Conservation Shorewatch Sightings programme.

Information on MPA boundaries can be viewed in Marine Scotland’s NMPi. To find out more about specific MPAs, please visit NatureScot’s SiteLINK. Detailed reports on habitat monitoring are referenced in Further reading – seabed habitat monitoring reports.

Intertidal and continental shelf habitats

SMA2020 contains three relevant habitat assessments: intertidal seagrass beds, subtidal biogenic habitats and predicted extent of physical disturbance to the seafloor.  The biogenic habitats assessment considered the status and trend of subtidal seagrass beds within this SMR. The assessment aimed to include beds of maerl, blue mussels, horse mussels and flame shells, as well as serpulid aggregations. Flame shell beds and serpulid aggregations have not been reported from this SMR, and there were insufficient data for maerl, blue mussel and horse mussel beds for their inclusion in the assessment.  Modelling work was also carried out to predict the extent of physical disturbance to the seafloor more generally.  Assessment of the status of intertidal seagrass beds could not be carried out in this region due to lack of relevant data.

Biogenic habitats

For the Outer Hebrides SMR biogenic habitats the status category assigned for SMA2020 was ‘Some concerns’ on the basis of a reduction in habitat extent of seagrass beds.

The most extensive known seagrass beds are located in the Sound of Barra and the Sound of Harris.  Based on 2006 survey work in the Sound of Barra (Harries et al., 2007), 2005 survey work in the Sound of Harris (Malthus et al., 2006) and a 1996 survey of the small beds in Loch nam Madadh (Entec, 1996), the total known subtidal seagrass extent for the SMR was estimated as 665 ha.  An overall trend of thinning of the beds in the Sound of Eriskay (located within the Sound of Barra) between 2001 and 2006 was identified by Harries et al. (2007), although some local increase in seagrass density was also noted. Hydrological changes resulting from the construction of the Eriskay causeway were identified as a possible driver of the change, although natural temporal fluctuation in density may also have been a significant factor.

Analysis of satellite imagery from 2005 to 2017 suggests that the process of seagrass thinning has continued.  No quantitative assessments of eelgrass loss have been carried out but the visual impression from the satellite imagery is that several of the major beds (totalling c.360 ha in the Sound of Barra) have possibly lost in the order of 50% of their cover between 2001 and 2017. This figure includes both thinning and peripheral contraction of beds and could represent up to a 27% reduction in the known extent of the habitat in the SMR.  Satellite imagery from 2019 indicates a similar picture to 2017.  Drivers of seagrass decline clearly extend beyond causeway construction as marked thinning and contraction of beds can also be identified up to 8 km away from the Eriskay causeway.

Confidence in the assessment is regarded as low.  Good quantitative data on habitat loss are lacking and the extent to which natural factors contribute to the estimated loss is unknown. 

In addition to the studies noted above, knowledge of Sound of Barra seagrass beds has been enhanced by a number of recent surveys and habitat extent assessments.  The GeMS database contains seagrass coverage estimates for both Sound of Barra and Sound of Harris beds, based on digitisation of aerial imagery from respectively 2014 (largely) (SNH, 2017) and 2018 (SNH, 2018).  The 2014 Sound of Barra estimate of 245 ha is markedly less than the 360 ha figure recorded for 2006 (Harries et al., 2007).  The 2018 Sound of Harris estimate of 661 ha is markedly greater than the 280 ha figure recorded for 2005 (Malthus et al., 2006).  However, no conclusions regarding temporal trends can be derived from these data in view of the great difference in methods that were employed and the geographical difference in areas examined by the 2006 and 2014 Sound of Barra assessments.

The 2015 SNH diving (Bunker et al., 2018) and drop-down video (Moore, 2017) surveys have significantly increased the number of seagrass habitat records, although they have not modified understanding of habitat distribution or extent.  The diving survey established baseline transects for future condition monitoring.

Predicted physical disturbance to the seafloor

To assess physical disturbance to seafloor habitats SMA2020 employed a modelling approach which generates a map of predicted relative disturbance levels from demersal fishing activity on a scale of 0 (zero) to 9 (severe).  The map was produced by the combination of information on the distribution of habitats, the sensitivity of the habitats (and species present to varying degrees) and the fishing pressure from demersal trawling, dredging and seine netting.  Fishing pressure information was derived from Vessel Monitoring System (VMS) data from 2012 - 2016 and was categorised as either surface abrasion (disturbance of surface and upper layers of sediment) or sub-surface abrasion (disturbance to a depth of >3 cm).  The final predicted disturbance index utilises the greater of these two pressure values and for descriptive purposes has been categorised as no disturbance (0), low disturbance (1-4) and high disturbance (5-9).

It should be emphasised that this method does not measure disturbance to seabed habitats, but predicts relative levels of disturbance. These relative levels are dependent upon the accuracy of habitat data and sensitivity assessments.  Many of the habitat data are derived from modelling and there is a low level of confidence in its accuracy.  Geographical variation in the accuracy of the sensitivity information employed is likely to be great, being dependent upon the level and quality of information used locally.  A significant limitation of the method is that during the assessment period pressure data were only available for vessels >12 m, which has probably resulted in an underestimation of disturbance for the SMRs.

Predicted habitat disturbance for the Outer Hebrides SMR is the second lowest in terms of the proportion of the seabed with high disturbance (33%, compared to an average of 50% for all SMRs).  The area predicted to experience no disturbance is by far the highest of all SMRs (36%, with an average 12% for all SMRs) and covers hard grounds throughout much of the area west of the Outer Hebrides SMR (Figure 3).  Heavier disturbance is concentrated in the North Minch east of Lewis and Harris and around the south of the island chain, particularly in areas of mud, where the predicted disturbance level closely reflects the degree of surface and subsurface abrasion pressure.  The demersal fisheries prohibitions within the relevant MPAs commenced in 2016 and so will have had little effect on the assessment, which covered the period 2012 - 2016.

Figure 3.  Predicted physical disturbance to the seafloor in the Outer Hebrides SMR and prohibition areas for all mobile demersal fishing.  (Note that the prediction of physical disturbance covered the period 2012-2016 and so the fisheries prohibition relating to the relevant MPAs will have had little effect on this assessment.)

Priority Marine Features and birds (non-PMF)

Overview of recorded PMFs and birds

The Outer Hebrides SMR supports a range of PMFs and breeding seabirds as well as wintering waterbirds (i.e. waders, estuarine waterfowl, seaduck and coastal water feeding birds) as detailed in Table 3. 

Table 3: Details of PMFs, seabirds, and wintering waterbirds found in the Outer Hebrides SMR

Priority Marine Features (PMFs; grouped habitats and species) and birds

No. of species/

habitats recorded

Intertidal and continental shelf habitats

15

Deep sea habitats

1

Fish[1]

20

Mammals (regularly occurring)

15

Shellfish & other invertebrates

8

Seabirds[2] (non-PMF) – breeding

23

Wintering waterbirds[3] (non-PMF) – non-breeding

14

The Outer Hebrides SMR has 59 PMF species and habitats, and 37 marine bird species.   

Kelp beds are widely distributed in this SMR and are important due to the rich biodiversity they support and the coastal protection they provide.  The SMR holds some of the most extensive subtidal seagrass beds in Scotland with the Sound of Barra bed covering an estimated 360 ha, and the Sound of Harris bed covering an estimated 280 ha.  Herring are abundant in the area west of the Western Isles in the summer months and migrate into the Minch in the winter.  This Outer Hebrides SMR is one of the most important areas in north-west Europe for cetaceans.  The commonest cetacean species in nearshore waters are the harbour porpoise, common dolphin, white-beaked dolphin, Risso’s dolphin, and minke whale.

Progress in understanding intertidal and continental shelf habitats listed as PMFs

Over the last 10 years there has been a change in emphasis of survey work in the region, with much of the earlier work by government agencies focused on obtaining an understanding of the distribution and conservation importance of habitats and species through detailed surveys of the sea lochs.  Over the 2012-2018 assessment period the focus was on the identification and subsequent monitoring of MPAs.  Wider monitoring of Nephrops habitat has continued in areas of suitable habitat.

The temporal sequence of records of all PMF habitats is provided in Table 4, based on inclusion in the Marine Recorder (2021) and GeMS (2021) databases, as well as identified additional sources.  Associated commentary, however, is restricted to PMFs for which the information has the potential to inform regional marine planning.  For example, some PMFs are excluded on the basis that they are very widely distributed and for which the records represent a small proportion of their likely distribution, such as several of the kelp habitats. 

Table 4.  Temporal frequency of PMF habitat records within the Outer Hebrides SMR obtained from GeMS (2021), Marine Recorder (2021) and other sources.  The numbers of All records are given, as well as those from Citizen Science (CS) surveys alone.

PMF

<2012

2012-2018

>2018 

 

All

CS

All

CS

All

CS

Blue mussel beds (subtidal)

1

0

0

0

0

0

Burrowed mud

65

3

79

2

0

0

Cold-water coral reefs

6

0

0

0

0

0

Horse mussel beds

3

0

0

0

0

0

Inshore deep mud with burrowing heart urchins

5

0

0

0

0

0

Intertidal mudflats

9

0

15

0

0

0

Kelp and seaweed communities on sublittoral sediment

140

3

61

1

0

0

Kelp beds

427

65

26

12

5

5

Low or variable salinity habitats

38

0

0

0

0

0

Maerl beds

126

2

347

3

0

0

Maerl or coarse shell gravel with burrowing sea cucumbers

16

0

8

0

1

1

Native oyster

3

0

0

0

0

0

Northern sea fan and sponge communities

148

9

45

13

0

0

Sea loch egg wrack beds

3

0

0

0

0

0

Seagrass beds (intertidal)

0

0

2

0

0

0

Seagrass beds (subtidal)

64

1

36

1

8

8

Tide-swept algal communities

119

1

9

0

1

1

Tide-swept coarse sands with burrowing bivalves

66

0

34

0

0

0

Table 4 shows that the contribution of citizen science records is modest both pre-2012 (7%) and post-2012 (7%).  This is likely to be influenced, at least in part, by the relative inaccessibility of the region. There has, however, been important information furnished, particularly with respect to seagrass beds and northern sea fan and sponge communities.

Nephrops burrow density surveys carried out by Marine Scotland from 2007-2016 indicate that the burrowed mud habitat is widely distributed to the south east and north east of the Hebrides, although there are few historical or recent records of specific burrowed mud biotopes both pre- and post-2012 in these locations (Figure 4).  However, recent surveys (2014 - 2015) by SNH and SEPA have identified its widespread presence in some of the east coast sea lochs, especially Loch nam Madadh (Moore et al., 2016), Loch Shell (SEPA, unpublished) and Loch Seaforth (SEPA, unpublished), the latter dominated by the PMF component biotope SS.SMu.CFiMu.SpnMeg.Fun.

Figure 4  Temporal pattern of records of selected burrowed mud PMF habitat and species components for the Outer Hebrides SMR, as well as mean Nephrops burrow densities from Marine Scotland monitoring surveys.

The intertidal mudflat habitat is not extensively developed in the Outer Hebrides SMR.  Recent SNH site condition monitoring surveys have provided information on the associated fauna and biotopes present in the Loch nam Madadh SAC in North Uist (Moore et al., 2016), and in the Luskentyre Banks and Saltings SSSI in Harris (ASML, 2014) and Tong Saltings SSSI in Lewis (ASML, 2014), where the habitat was also mapped.

The greatest advance in knowledge of PMF habitats in the SMR since 2012 (Figure 5) is for maerl beds, particularly in relation to the establishment and monitoring of the Sound of Barra SAC.  SNH and Marine Scotland diving, grab and drop-down video surveys between 2015 and 2018 have provided considerable additional detail of its distribution and condition in the Sound of Barra and adjacent waters (Moore, 2017, 2019, 2020; Bunker et al., 2018; Allen, 2019).  Site condition monitoring of the Loch nam Madadh SAC in 2015 also provided information on the composition and condition of the maerl beds there (Moore et al., 2016).  A 2016 SNH/Marine Scotland survey also revealed clusters of records of ‘maerl or coarse shell gravel with burrowing sea cucumbers’ straddling the southern boundary of the Sound of Barra SAC (Moore, 2017).  Many new records of ‘tide-swept coarse sands with burrowing bivalves’ have been reported in the same area from grab surveys in 2016 -17 (Allen, 2019; Franco et al., 2017) and it is possible that biotope identification has been strongly influenced by the sampling methodology, with grab sampling unlikely to record the scattered presence of burrowing sea cucumbers.

There are many recent records of northern sea fan and sponge communities in the region, mostly in areas where the habitat is already recognised (e.g. Shiant East Bank MPA and along the eastern coastline of North and South Uist, Lewis and Harris), but also first records for the western coast of the Outer Hebrides off the mouth of Loch Resort, Lewis (Seasearch 2017 survey).

There has been some improvement in understanding of the distribution and extent of seagrass beds in the region as a result of digitisation of 2014 and 2018 aerial imagery of beds in respectively the Sounds of Barra and Harris (SNH 2017, 2018).  Diving and drop-down video surveys of the Sound of Barra in 2015  confirmed the persistence of several beds and provided details of the associated fauna and flora (Bunker et al., 2018; Moore, 2017).

Seagrass Spotter records for 2019 and 2020 have confirmed the continued existence of a subtidal seagrass bed in West Loch Roag, Lewis and have identified the presence of two previously unknown beds in the Sound of Taransay, one of which off the Harris shoreline is clearly discernible on 2005 aerial imagery, extending to perhaps 50 ha.

The SNH 2015 site condition monitoring survey of the Loch nam Madadh SAC (Moore et al., 2016) increased knowledge of the distribution of tide-swept algal communities within the SAC.  The only other recent habitat record is for West Loch Roag, Lewis (2019 Seasearch survey).

There has been little recent progress in knowledge relating to the distribution of several other PMFs (Table 4).  Although currently excluded from the GeMS and Marine Recorder databases, there are 60 records of Lophelia (the cold-water coral reef forming coral) in the region resulting from the mainly experimental studies forming part of the 2012 RRS James Cook ‘Changing Oceans Expedition’ (Roberts, 2013).  These all lie within the East Mingulay SAC and within polygons delineating the habitat in GeMS (2021).

Figure 5.  Temporal pattern of records of selected PMF habitats for the Outer Hebrides SMR.

Status and trend in grey and harbour seals

Grey seal pup production at the major breeding sites including the Monach Isles SAC is monitored biennially and has shown little or no change in numbers and their status is assessed as ‘few or no concerns’.  Harbour seal numbers are monitored on a rolling 4/5year cycle and have shown little or no change over the assessment period and their status is assessed as ‘few or no concerns’. 


[1] Records from International Bottom Trawl Surveys from MSS.

[2] Records of seabirds during breeding season only, from BTO Bird Atlas.

[3] Records of birds during the non-breeding season, from the BTO Bird Atlas

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 Outer Hebrides SMR. 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 Outer Hebrides region has increased since 1870 by 0.04 °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 +0.19 °C per decade.

Tide gauge records from around Scotland’s coast show a high degree of year-to-year change in coastal water levels (typically several centimetres). The long-term average mean sea-level change in the Outer Hebrides region, as estimated from a historical climate model run (UKCP18), was 7 cm (likely range between 5 and 10 cm) higher in 2018 than the 1981-2000 average. For reference, the Scottish average is estimated to be 5 cm (likely range between 3 and 8 cm). By 2100, mean sea level in the Outer Hebrides region is anticipated to be approximately 46 cm for a medium emissions scenario (UKCP18 RCP4.5; see also and Climate change Sea level assessment).

Summary

The Outer Hebrides SMR has seen a 17% increase in passenger numbers over the five years between 2014-2018 whilst the GVA for marine tourism decreased by 64% in the four years from 2014-2017.  The value of fish catch decreased by 59% over the five years between 2014-2018.  Both Atlantic salmon and mussel production values increased by 20% and Pacific oyster production value for West Highlands SMR and North Coast SMR combined increased by 990% over the same period. Rod and line salmon and sea trout catch decreased by 52% over the five years between 2014-2018.  An estimated 13,500 wet tonnes of seaweed were harvested annually.  Other active sectors include water abstraction and subsea cables.

The five main pressures affecting the SMR are Removal of target species, Removal of non-target species, Surface abrasion, Sub-surface abrasion / penetration and Underwater noise.  Other pressures identified are Barriers to species movement, Deoxygenation, Death or injury by collision below water, Local emergence regime changes, Genetic modification and translocation of indigenous species, Hydrocarbon and PAH contamination, Introduction of microbial pathogens, Introduction and spread of non-indigenous species, Nitrogen and phosphorous enrichment, Organic enrichment, Physical change, Physical loss, Synthetic compound contamination and Visual disturbance.

There is no evidence of eutrophication as a consequence of nutrient enrichment in this SMR although there was a statistically significant increasing trend in nutrient inputs but loads were an order of magnitude lower than most SMRs. Contaminant (i.e. PAHs, PCBs, PBDEs and heavy metals) concentrations are generally above background but below levels that might cause adverse biological effects and there were no increasing trends.  Based on imposex samples from one site concentrations of TBT are causing significant harm.  Litter and microplastics are present but microplastic occur at relatively low concentrations in the surface waters of the SMR and no beach litter data were available.  There are few noise data, and it was not possible to do an assessment.  Concentrations of diarrhetic shellfish toxins above regulatory levels in every year between 2010 and 2018, paralytic shellfish toxins concentrations resulted in site closures in 2012, 2013, 2015, 2017 and 2018.  Amnesic shellfish toxins above regulatory limits were found in cockles in 2013 and razor clams in 2016.

Three new MPAs were designated between 2012-2018, and two new spatial management measures were put in place.  Spatial management measures were under discussion for a further five.  (Note that in December 2020, four new MPAs were put in place.)  Subtidal biogenic habitats were assessed as ‘some concerns’, with 33% of the seafloor predicted to have been subject to high physical disturbance and 36% subject to none.  There have been significant improvements in the knowledge of several seabed habitats including maerl beds, burrowed mud and kelp and seaweed communities on sublittoral sediments.    

In the last 30 years sea temperature has risen by 0.19 °C per decade.  Sea level in 2018 is estimated 7 cm higher than the 1981-2000 average.