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Sea Caves of St Kilda & North Rona

In the summer of 2015 I joined my friend/colleague Richard Shucksmith, along with a team of researchers from Heriot Watt University, on a 3-week expedition to some of Britain’s most remote islands, St. Kilda to the west of the Hebrides and North Rona to the north. I’d spent a week or so diving North Rona a few years ago but never had the chance to visit St. Kilda. There are few places on the UK dive map with such a reputation. Remote and often inaccessible, the distant archipelago has attained an iconic status among UK divers, its name often spoken with reverence by the lucky few who have had the opportunity to visit.

Sitting out in the Atlantic just over 40 miles west of The Uists in the Outer Hebrides (and 110 miles from the mainland), the St. Kilda archipelago is the most remote part of the British Isles, made up of several islands and sea stacs created by ancient volcanic activity which dominated the west coast of Scotland over 60 million years ago. Its isolation from mainland UK and any significant sources of pollution, run-off and sedimentation results in exceptionally clear, blue oceanic water. This, combined with its dramatic underwater topography, provide conditions which support a level of marine flora and fauna not commonly seen around the British Isles.

St Kilda silhouetted in the setting sun

Adding to its mystique, St. Kilda also has a fascinating human history. Despite being isolated and perpetually battered by hostile conditions, people thrived here for centuries until relatively recently, surviving largely by scaling sheer cliff faces in order to catch the plentiful seabirds. Unfortunately, in 1930 numerous factors led to the evacuation of what remained of the population.

In 1987, the archipelago’s unique beauty, along with its biological and cultural importance, saw St. Kilda receive dual World Heritage status, making it one of only two-dozen global locations to be recognised for both natural and cultural significance.

St. Kilda’s remoteness has bestowed upon it a number of unique physical and biological properties which have been the focus of numerous scientific research projects for over half a century. It is one of the most important breeding sites in the British Isles for numerous seabirds and also has a number of endemic species of flora and fauna.

Stac Lee
Stac Lee, Stac an Armin & Boreray (left to right)

In 1992, as a result of the Biodiversity Convention at the Rio Earth Summit, the European Community implemented the Habitats Directive. This is the foundation of Europe’s nature conservation policy and was formed to protect its most valuable and threatened species and habitats. Central to the Habitats Directive is the Natura 2000 network, a series of protected marine and terrestrial areas stretching right across Europe. One of the classifications for protected sites is the Special Area of Conservation (SAC). SAC status is assigned to specific habitats and associated wildlife listed under Annex I and II of the Habitats Directive.

Of the 189 habitat types identified by the Directive, 78 are believed to occur in the UK, 13 of which are classified as coastal or marine. Unsurprisingly, with over 6000 miles of coastline, Scotland features a good number of these habitats, including a number of sites and associated species which have received SAC designation. The provision of data from within these areas is generally frequent, but some sites are known better than others; one coastal habitat in particular remains largely unknown, due mainly to the inherent difficulties involved in getting to them. Described in Annex I as “submerged sea caves and also partially submerged caves which are only exposed to the sea at high tide”, many are situated around some of the most remote places in the British Isles, and records are typically sparse as a result.

Additional images

Monitoring marine flora & fauna at a cave entrance

Sea caves are relatively numerous, dotted throughout Europe’s rocky coastlines, and the UK has some of the most varied and extensive examples. As a habitat, however, they are comparatively scarce, making their unique environment of particular value to researchers and conservationists. Due to the relative inaccessibility of many sea cave systems, particularly around some of the outer Scottish islands, there is still much we don’t know about these habitats. What we do know, however, is that they are almost always home to unusual and, very often, rare species.

The heavily indented, fjord-like coastline of Scotland, including the Hebrides and the outer islands, has arguably the highest concentration of sea caves in the British Isles. Of particular interest, and perhaps the least known, are those found around the more remote of the outer islands, and you can’t get much more remote than St. Kilda and North Rona. Although a few biological records do exist from a number of sea caves around St. Kilda, there is nothing published about the North Rona caves. Furthermore, there has never been any formal survey work conducted at either site. In order to rectify this and essentially get the research moving, Scottish Natural Heritage contracted researchers from Heriot-Watt University to establish a monitoring program in order to assess the current status of the major sea cave features, providing a baseline for future assessments.

A scouting mission on the zodiac to check out a potential survey site

The survey was planned over roughly three weeks in the mid-summer of 2015, the primary objectives of course being the caves around St. Kilda and North Rona. The survey vessel was the ever-reliable M/V Halton, a rugged 21m converted trawler built to withstand the worst weather. It has a modern navigation system, spacious galley (including an oil Rayburn cooker) and six comfortable twin-berth cabins. Skipper Bob Anderson is a seasoned and experienced operator and diver around the outer Scottish islands.

Awaiting divers at the entrance to a cave
Coming out of a cave, looking up at the zodiac

Although three weeks sounded like plenty of time to survey what appeared – at least on paper – to be a handful of sites, we were all aware of the potential difficulties in getting out to these remote islands as well as all the possible complications that diving inside caves can present. In rough weather, getting to the Outer Hebrides can be tricky enough. Although not a great deal further, the additional hop to St. Kilda means venturing out into the open Atlantic and, more often than not, this stretch of water is dominated by strong incoming winds and exceptionally large swells. The leg out to North Rona can be equally unforgiving and is generally subject to the same conditions; if the weather wasn’t favourable for getting to one location it would be unlikely we would be able to get to the other. Needless to say the forecast for the next week didn’t look ideal as we set off from Stromness.

Heading down the dramatic west coast of Hoy to mainland Scotland, we aimed to get as far west as possible on the first day.  As we progressed in a moderately uncomfortable swell it quickly became apparent from incoming weather reports that heading straight to St Kilda may not be the best idea.


Kitted up ready to dive

Contingency plans for bad weather were put into action by Dr Dan Harries (Heriot Watt lead scientist) and Dr Lisa Kamphausen (SNH marine biologist), and so we headed to Loch Eriboll to refine survey methods and logistics.

Awaiting pickup by the Halton in Village Bay
The Team

The rugged coastline along the north-west coast of Scotland supports numerous sea caves, many of which remain unexplored. The poor conditions further to the west, which was creeping towards us, meant we had to find a few spots to shelter along the way. Fortunately there are plenty of sea lochs, effectively fjords, some of which extend many miles inland which provide ideal shelter from poor sea conditions.

The shelter of the lochs gave the team a perfect opportunity to get organised and put some of the survey methods into action. Although essentially a Plan B, the surveys undertaken on the Scottish mainland were a useful addition and yielded some valuable data. We ended up spending over a week diving in and around Loch Eriboll, Loch Laxford and briefly at Loch nam Madadh on North Uist before we finally made the dash out to St. Kilda.

Additional images

Looking out of a cave through a kelp forest to the surface

We arrived in the early hours and a wave of enthusiasm and motivation swept around the boat as everyone appeared on deck to see the sun rise over Village Bay. Remarkably the weather had cleared completely and, as the sun came up, we all gazed up at the blue sky above the steep cliffs of Hirta to see clouds of puffins circling overhead. Conditions were about as good as we could hope – it was time to survey the sea caves of St Kilda.

The scouting team headed off in the zodiac and, after an hour or so, returned having identified a number of potential sites. By now the survey routine had been polished to military precision so we were kitted up, on the zodiac and in the water in no time at all. The visibility was predictably very good and, the marine life was striking enough to make you realise you were diving somewhere quite unlike anywhere else in the UK. On a simplistic level it’s the vivid colours that provide the most characteristic feature of diving around St. Kilda. Intense pinks, greens and oranges offset by that rich turquoise backdrop. Although similar underwater environments can be found elsewhere in the UK there’s something about these islands that sets them apart. Maybe there’s a psychological factor involved here, the knowledge that you are diving in a remote place on the very edge of Europe. It gives you a sense that everything is wild and untamed, a feeling you perhaps don’t get when you are closer to civilisation.

Diving Dùn Arch (photo: Richard Shucksmith)
Entering a sea cave (photo: Richard Shucksmith)

In total we surveyed three full cave systems. The cave entrances were marked by dense kelp forests and carpets of iridescent jewel anemones, literally hundreds of thousands packed onto the cliff faces and boulders. This changed from kelp to animal turfs as the light became less the further into the cave you went. The caves tended to narrow as we got further inside, looking up our trapped air bubbles looked like liquid metal as they danced across the cave ceiling. Most of the caves ascended toward the rear. Turning around and swimming slowly back toward the cave entrances always provided a spectacular sight, a window looking out into the open ocean with the rays of light dancing through the clear, blue water.

Our final dive on St. Kilda was a rocky reef survey underneath the natural archway on Dùn. Separated from Hirta by a narrow channel, Dùn provides Village Bay with protection from prevailing south westerlies. Essentially a narrow, vertical crack in the island the archway can be subject to some fierce tidal surges. This results in some spectacular marine life all placed to take advantage of the never-ending food supply made available in the passing currents. Jewel anemones were so numerous the sight of entire cliff faces covered in every colour of the rainbow was breath-taking. The walls at the entrance were covered with incredibly dense thickets of hydroids and even the hydroids themselves provided shelter for what must have been literally millions of tiny mysid shrimps. Dùn Arch was a suitably spectacular final dive on St. Kilda.


Sun setting through the Sound of Harris