Priority marine feature surveys within the Small Isles MPA and surrounding waters

Executive summary

The Small Isles Marine Protected Area (SMI MPA) is one of Scotland's biggest inshore MPAs, covering more than 800 km² and containing a complex mosaic of habitats. Multiple Priority Marine Feature (PMF) shellfish and other invertebrate species, and components of PMF habitats, occur within the SMI MPA. This report focusses on eight seafloor invertebrate species that are either PMF species, components of PMF habitats, or components of MPA protected features, and provides detailed spatial and temporal information about where these occur within the SMI MPA.

The six-year survey programme (2012 – 2017) conducted by Marine Scotland, a Directorate within the Scottish Government, used a non-destructive approach that collected imagery of the seafloor. During the study period, 9,374 digital still image quadrats and 3,690 minutes of quantifiable high definition video footage were collected and analysed to determine baseline densities of the eight seafloor species. The impact that bottom-contacting towed gear may have on the distribution of the Tall seapen (Funiculina quadrangularis) in the wider area of the Minch, Inner Sound and Sea of Hebrides was also assessed using information on fishing vessel activity, environmental variables, and seapen abundance.

The results indicate that specific areas within the SMI MPA are important to certain PMF species or habitat components, with some species only recorded from a small number of locations. The Sound of Canna recorded some of the highest densities of the Northern seafan Swiftia pallida, white cluster anemone Parazoanthus anguicomus, and the Northern featherstar Leptometra celtica during the study, and this location provided the only records of the Horse mussel Modiolus modiolus and Fan mussel Atrina fragilis from within the SMI MPA during the survey period.

A few species recorded decreases in density within the SMI MPA and the wider area over the baseline study period - F. quadrangularis, A. fragilis and M. modiolus - although the latter two species were encountered rarely from a limited number of locations and were not the focus of repeated sampling. Funiculina quadrangularis was consistently recorded at lower densities within the SMI MPA after 2014. Statistically significant decreases in F. quadrangularis density between 2014 and all subsequent survey years (2015, 2016, 2017) were detected for two survey boxes in the south of the Sound of Canna. In a follow up study, the assessment of F. quadrangularis density and distribution across the Minch, Inner Sound, and Sea of Hebrides was unable to establish linkages between density of F. quadrangularis and the occurrence of towed bottom-contacting fishing.

Some species did not show any consistent patterns in density over space and time, such as the tube-dwelling anemones Arachnanthus sarsi and Pachycerianthus multiplicatus and P. anguicomus and L. celtica. There was no statistically significant change in density over time for S. pallida in two survey boxes in the south of the Sound of Canna (2015, 2016, 2017). Although no density changes were expected for A. sarsi, P. multiplicatus, S. pallida, P. anguicomus and L. celtica, their association with harder substrate patches within the SMI MPA means they were less likely to be consistently encountered during individual survey tows, thereby reducing the likelihood that any changes over space and time would be detected.

In summary, the six-year Marine Scotland survey programme presented in this report provides a biological baseline record of the SMI MPA at a spatial coverage and temporal resolution previously unavailable. The data analysed strengthens the evidence base underpinning the Scottish MPA network and can help to inform the establishment of management measures to support an ecologically coherent MPA network. The results from this study raise the following points for consideration:

• Density information for the eight seafloor invertebrate species indicates that the Sound of Canna provides important habitat within the SMI MPA. This location may benefit from tailored management measures for the conservation and recovery of the species located there with the aim of contributing to the regional health of these features.
• Continued survey of the SMI MPA and surrounding region is suggested to monitor changes in species density over space and time, and to assess the effectiveness of any future management measures. Higher spatial and temporal resolution of biological survey data, potentially using new technology, would help to determine trends in species associated with the wide range of habitats occurring within the SMI MPA.
• Where species occur in low densities, in aggregations, or are reliant on habitats with patchy distributions that are difficult to survey repeatedly over time, the resolution of the data collected may not be sufficient to detect a change or deterioration in status over time or space, thus sentinel hypothesis-based monitoring may be less effective for some species.
• Greater spatial and temporal resolution of fishing pressure data is needed to better assess the impacts of fishing activity, given that there was insufficient evidence to determine if observed decreases in species density were linked to fishing activity.

The full SMI MPA dataset is archived and stored in line with the Scottish Government's Open Data Strategy and data relating to PMFs were provided to the Geodatabase of Marine features adjacent to Scotland (GeMS) curated by NatureScot.