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)Â Â Â | 20,848 |
| Deepest point (m) | 268 |
| Shallowest point (m)Â | coastline |
| Average depth (m) | Â 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.