Scapa Flow Harbours

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Port Environment

Natural Environment of the Port of Scapa Flow

Beyond the savage bulk of Cape Wrath there was an empty ocean until in the summer dawn they saw the Orkneys like sleeping whales … (George Mackay Brown)

A succinct description of the natural environment of Orkney, with particular reference to the marine environment , is given in the JNCC (1999) publication Marine Nature Conservation Review Sector 2. Orkney: area summaries. This states “Orkney is an archipelago surrounded by clear, relatively shallow water. The islands have a complex coastline, varying from high cliffs on Hoy(4) and west mainland to extensive sand beaches and dune systems on Sanday(5). The channels between the islands are relatively shallow and consequently cause an increase in tidal streams, with some areas such as Hoy Sound having tidal races exceeding 7 knots.(1) Orkney is subject to severe winds, and some of the coastline is thus very exposed to wave action.(2) The complexity of the coastline affords shelter for some areas such as Scapa Flow where shelter and limited tidal movements provide calmer conditions and a marine environment different from many other parts of Orkney(3). Orkney has no major rivers so true estuarine habitats, if any, are few and small.” The Loch of Stenness has recently been designated as a saline lagoon SAC (Special Area of Conservation), the largest such lagoon in the UK.

The remainder of this section will concentrate on Scapa Flow with reference to other parts of Orkney as appropriate.

Scapa Flow is a semi-enclosed body of water with entrances to the south (sounds of Switha and Hoxa) and west (Hoy Sound). The main body of Scapa Flow is a glacially scoured basin with an area of approximately 130 square kilometres and is bounded on all sides by fault lines. The main basin is steep sided with a relatively level bottom, mostly between 25 and 35 metres deep. The deepest part of the Flow is the area known as the Bring Deeps which is believed to be an old glacial lake not ye filled in and is over 60 metres deep in parts. Parts of the Pentland Firth falling within the Harbour Area are in excess of 75 metres deep.

The coastline of the area is varied including high cliffs, rocky shoreline, gravel and sandy beaches and a limited amount of saltmarsh habitat. The seabed over the main body of Scapa Flow is characterised by mud, sand and gravel sediments. Areas exposed to strong tidal currents tend to have coarser sediment or even bare rock. The blocking of the eastern entrances to Scapa Flow by the Churchill Barriers has altered the tidal regime and may have affected the sediment distribution, with an increase in the extent of the muddy area of the north-east of Scapa Flow. However, the very limited sediment load of Orkney’s waters means that additional deposition will only be apparent after a long period of time.

The unique physical and hydrological features which make Scapa Flow such a great natural harbour also create an environment for nature like no other.

Scapa Flow is an area of considerable importance to many species of birds. The population and composition of the bird community on and around the Flow varies significantly between the seasons. During the summer months the Flow provides a feeding ground for a variety of auks, terns and gulls and is frequently visited by Red Throated Divers which nest around lochans and lochs in the nearby hills of Hoy and the West Mainland. Flotta and South Walls have breeding colonies of of arctic Tern and Arctic Skua of European importance.

Scapa Flow is however more important for birds in the winter months. As a particularly well sheltered expanse ofsea, in the far north of Scotland, Scapa Flow provides an important wintering location and resting place for birds migrating south from Iceland, the Faeroes, northern Scandinavia, Arctic Siberia, Greenland and possibly mainland Canada. Over 26 species overwinter in Scapa Flow with Long-Tailed Duck, Great Northern Divers and Slavonian Grebes being of particular significance. The distribution of birds varies throughout the winter and according to weather conditions but typically the greatest concentrations are in the north west corner of the Flow, around Graemsay and the Bay of Ireland, and along the Flow’s eastern shores, particularly around Hunda and Widewall Bay.

The sheltered bays around the Flow provide an important habitat for shore birds that feed on various infauna (molluscs, crustaceans and worms). Species of shore bird regularly seen include oystercatcher, turnstone, redshank, curlew, lapwing, ringed plover, purple sandpiper, dunlin and bar-tailed godwit.

The sheltered shores of Scapa Flow provide an ideal haul-out location for Common Seals with around 2000 counted in the area. This represents about 30% of the Orkney total or 7% of the UK total. Grey Seals to occur in the area though they tend to be more timid and their haul-out and breeding sites more isolated. The isles of Switha and Swona and two sites on South Ronaldsay produce significant numbers of Grey Seal pups.

The Harbour Porpoise is the cetacean most commonly seen in Scapa Flow, though a range of other species includingWhite Beaked Dolphin, White Sided Dolphin, Risso’s Dolphin, Minke Whale, Humpbacked Whale, Sperm Whale and Killer Whale have also been observed in recent years. Otters are also found around the Flow though it is more often the markings, or spraints, of these elusive animals which are seen rather than the animals themselves.

The principal commercial species fished in Scapa Flow are lobster, brown, velvet and green crabs, prawns, scallops, queens, razorfish, whelks and winkles. The lobsters, crabs and whelks are taken by creels or pots, the prawns by creels or trawl and the scallops, queens and razorfish by dredge or diving. The winkles are hand-collected from the intertidal zone.

The Bring Deeps area has seen championship size Skate and Halibut caught by rod and line in recent years and there are historical accounts of commercial catches of Haddock, Cod, Whiting, Coalfish, Ling, Eel, Turbot, Flounder, Mackerel, Herring, Skate and Dogfish from the waters of Scapa Flow. Mackerel shoals are still evident in some summers and Coalfish and Dogfish occur around certain ‘marks’ but few other species have been reported in fishable quantities since the closure of the Churchill Barriers. It has been suggested that the two are linked but this is difficult to test.

As well as the resident and migratory populations of fish species around Orkney and in Scapa Flow in the autumn of many years strange ‘visitors’ are reported. These are often members of the ‘Lusitanian’ plankton assemblage which has its origins around the entrance to the Mediterranean Sea and can include Ray’s Bream, Trigger Fish, Greater Forkbeard and the Opah or Moonfish. Also reported from Orkney waters are ocean wanderers like the Sunfish and Basking Shark. Occasionally, at the same time of year, Leatherback Turtles have been reported, sometimes swimming, sometimes stranded, coming to Orkney across the Atlantic following the jellyfish swarms that form their main food source.

Other rarities in recent years have included catches of Sea Bass and Red Mullet. There is some evidence that these fish were extending their range northwards but whether this was a reflection of a ‘warm’ year or an indication of an overall warming trend is still unclear.

The study of the status of the marine environment around Orkney has been stimulated by the Flotta oil terminal, fish farming developments and the potential for renewable energy installations. However, man’s influence can also introduce pressures and risks which need very careful management if they are not to affect the natural balances in a dynamic environment. Conservation of an asset in all senses.

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