Harries, D.B., Purcell, A., Moore, C.G., Lyndon A.R., Porter, J.S., Saunders, G.R. and Hume, C.L. (Heriot-Watt University)

Background

The aim of the current study was to create an inventory of the location, extent and nature of sea caves around Fair Isle. Additionally, the study set out to gather detailed biological data from a series of selected caves. The purpose of this was to gather information relevant to the distribution and quality of the Annex I habitat ‘submerged or partially submerged sea caves’ (as defined by the Habitats Directive) around Scotland. This information is also relevant to Fair Isle’s designation as a Demonstration and Research MPA. 

Due to the extreme wave exposure of Fair Isle, it was necessary to consider a series of contingency survey targets in the event of weather conditions preventing access to the primary target. Weather conditions severely limited time available for surveying the primary target so some attention was diverted to the contingency targets of seagrass beds, flame shell beds and horse mussel beds around Orkney (not included in this report).

Main findings

A total of 175 potential cave sites (including arches and tunnels) have been identified on Fair Isle based on a combination of historical records and direct observation. All available information on caves has been collated into a cave inventory. 

• It was estimated that approximately 60 of these sites are likely to have sufficiently reduced light levels to qualify as the Annex I habitat ‘submerged or partially submerged sea caves’.
• Some caves were extensive (~200 m in one case) and passages tended to be aligned with the rock bedding of the island.
• Four caves were subject to a detailed biological assessment. Three were on the east coast and one on the west coast.
• Cave biota tended to be heavily modified by the effects of scour which was concentrated at or near the floor of the caves. In shallower caves the effect of the scour tended to extend the entire wall height. At more extreme levels the biota was very impoverished with sparse spirorbins and mobile biota. At less extreme levels there was a community characterised by a turf of silty tubes constructed by sabellid polychaetes and / or tube dwelling amphipods.
• Where scour levels were reduced (deeper cave floor) a community characterised by Dendrodoa grossularia and Clathrina coriacea occurred in a number of caves.
• The upper walls of caves with a deep cave floor can support a more diverse and variable community characterised by crisid bryozoan turfs, aggregations of anemones, Tubularia indivisa and Alcyonium digitatum.
• Littoral and supralittoral zones appeared impoverished (relative to the open coast) within the caves.
• The biota recorded was broadly consistent with prior data recorded from Fair Isle caves and broadly similar to that recorded from comparable cave sites elsewhere in Scotland.
• No evidence of anthropogenic impacts or of anthropogenic activities with potential to impact the cave features was observed.
• Based on available data it should be concluded that the cave features were in good condition.