Outer Hebrides SMR

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.

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.  

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

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.

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’.