A team of scientists from Heriot-Watt University, National Museums Scotland, and SNH carried out site condition monitoring within the Mousa SAC in order to assess the condition of the designated rocky reefs and sea caves. They conducted a systematic search of the coastline of Mousa to compile a comprehensive cave inventory, and SCUBA divers entered five selected cave locations and three rocky reefs to conduct detailed biological and physical surveys.
A total of 20 caves have been recorded at Mousa. The majority had a significant subtidal component and tended to be heavily influenced by scour. Most were <30 m in length but one was ~40 m and two were in excess of 80 m in length. There was no evidence of change in the extent or number of caves in the area since 2008 and the distribution and composition of biotopes in the surveyed caves was essentially unchanged since 2008.
Change in the distribution of biotopes was recorded along all three reef transects studied in 2008 and 2016 and at 38 of the 100 dropdown video sites examined in both years. Temporal differences in camera tracking and variation in natural environmental factors such as hydrodynamism were considered most likely to be responsible for the observed changes. MNCR phase 2 surveys along three reef transects and quadrat quantification of the biota at two of these reef sites revealed no temporal decline in species richness between the 2008 and 2016 surveys. The quadrat and transect surveys revealed changes in the abundance of several species, but these were considered to be consistent with natural temporal variation.
There was no evidence of anthropogenic activities having caused any deterioration in the condition of the reefs or sea caves features since the establishment of site condition monitoring in 2008. Based on the available evidence it is recommended that the reefs and sea caves features should be assigned to the condition category "Favourable Maintained".
The team also collected information on the distribution of sand eel habitat and carbonate rich sediments (protected features of the Mousa to Boddam MPA), through a drop-down video survey and an assessment of a maerl bed in the northern part of Mousa Sound by divers.
The distribution of carbonate rich sediments, based on the presence of visually evident carbonate material along the 120 video runs, mirrored the distribution of sandeel habitat, although clean, rippled sands in the northern region of Mousa Sound did not show clear visual evidence of a carbonate origin. Carbonate material was generally manifest visually as shell gravel and larger shell material, with maerl gravel becoming a significant component in the northern part of Mousa Sound.
The patchy maerl bed in the northern region of Mousa Sound occupied an area of 13.0 ha over a depth range of 25.4 - 28.4 m. Within this area the bed formed a mosaic with reef biotopes supporting dense Alcyonium digitatum. Live Phymatolithon calcareum was abundant (>40% cover) over much of the bed and supported dense Clavelina lepadiformis but an otherwise sparse sessile community.