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

5. Summary

The Marine Scotland survey programme builds on previous datasets, especially further offshore, and strengthens the growing body of evidence available to determine the biological status of the Small Isles Marine Protected Area (SMI MPA) and to inform future management of the MPA. In combination, these new and pre-existing datasets enable 1) the preliminary identification of potential changes to priority marine feature (PMF) species or habitat components over time and space, and 2) initial investigations into the effect of fishing pressure.

This study indicates that specific areas within the SMI MPA appear to be important to certain PMF species or habitat components, with some species only recorded from a small number of locations. The Sound of Canna has some of the highest recorded densities of the Northern seafan Swiftia pallida, White cluster anemone Parazoanthus anguicomus, and the Northern featherstar Leptometra celtica across the MPA. The Sound of Canna also provides the only known records of the Horse mussel Modiolus modiolus and Fan mussel Atrina fragilis within the SMI MPA. These results suggest that the Sound of Canna and neighbouring areas of seabed may benefit from tailored management measures for the conservation and recovery of PMF species and habitat components located there, with the aim of contributing to the regional health of these features.

A few PMF species or PMF habitat components appear to be showing decreases in density within the SMI MPA and the wider area over time. The Tall seapen 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. Although these two boxes occurred where fishing activity was apparent, causal links between the significant decrease in F. quadrangularis density and fishing activity could not be tested. 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 fishing activity despite the stratified design. Considering the wider literature, it is likely that greater spatial and temporal resolution of fishing pressure data is needed to determine conclusively if bottom-contacting towed gear causes distributional changes in F. quadrangularis.

The two other species that appeared to show a decrease in density over time, A. fragilis and M. modiolus, were only encountered at a few sites and in generally low numbers, making it difficult to assess if these decreases were real, a result of natural variation, or if they reflected challenges in surveying these species consistently using standard benthic ecology survey methods. The quantitative assessment of changes in S. pallida over time for two survey boxes in the south of the Sound of Canna did not detect any significant change in S. pallida density between 2015, 2016 and 2017. Other species, such as A. sarsi, P. multiplicatus, P. anguicomus and L. celtica, did not appear to demonstrate consistent patterns in density across boxes or years. Elucidating spatial and temporal trends for species occurring in low densities, in aggregations, or in habitats with complex patchy distributions presents different challenges compared to species with relatively homogeneous distributions.

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. However, where species have biological traits that make them difficult to survey repeatedly over time, sentinel hypothesis-based monitoring may be less effective. One potential alternative could be to focus monitoring efforts on the pressure-causing activities at high spatiotemporal resolution rather than attempting to consistently monitor small-scale patchily distributed biological features. Different management measures and monitoring strategies may be required to effectively conserve PMF species and habitat components with different distribution patterns.

Continued survey of the SMI MPA and surrounding region will be needed to monitor changes to PMF species or habitat components over space and time, and to assess the effectiveness of any future management measures. To ensure efficient processing and analysis of the extensive imagery data generated from continued survey efforts, new methods or approaches may need to be implemented in line with current best practice, for example the use of image annotation software, or even automated processing using artificial intelligence. Existing data archiving infrastructure will need to be maintained to ensure the underlying data continue to be stored and shared in line with the Scottish Government's Open Data Strategy, thus making these data available for further analyses of trends in PMF species and habitat components.