Assessment of the appropriateness of areas closed to protect priority marine features from scallop dredging around Shetland.

Horse mussel (Modiolus modiolus) and maerl beds are ‘priority’ habitats that can be threatened by human activity and as such require conservation under the UK Biodiversity Action Plan (UK BAP). During 2010 the Shetland Shellfish Management Organisation (SSMO), the body responsible for the management of all shellfish stocks within six nautical miles of Shetland, proposed to voluntarily close 24 areas to scallop dredging vessels in order to help conserve these priority habitats. Initially, the boundary of each closed area was defined from data extracted primarily from the Shetland Marine Spatial Plan but also through consultation with local maritime users. Some data were of limited quality, extent, or based on non-validated reports several decades old. In several cases fishermen actively involved in the development of the closed areas questioned whether the priority feature was present in the area at all, based on their local knowledge. Despite such reservations, the science-industry partnership involved agreed to adopt an iterative, precautionary approach whereby areas would be closed pending detailed survey and assessment, and the legalisation of the voluntary closed areas came into force in 2011. The primary aim of this report was to define the effectiveness of the current closed areas based on up-to-date, high resolution survey data so that the SSMO can validate and refine its spatial management plan.

Closed areas to scallop dredging are widely distributed around Shetland, amounting to just under 26 km2 of sea surface area. The areas vary in size from 0.003 km2 to 14.6 km2, the largest accounting for 56.6% of the total area closed around Shetland. This variation is primarily due to the type of externally sourced data (i.e. point data or predicted species bed) used to originally define the extent of the feature. Historically, predicted species beds had been derived from a number of sources and various agencies from a combination of point data and localised bathymetry information. The current study surveyed existing closed areas using a hull mounted multibeam system and groundtruthed with an underwater camera system. Information was imported to GIS in order to create a georeferenced map of the presence of the species of interest, and in particular where they occur in sufficient abundance to consitute a ‘priority feature’ or habitat. Although it is generally accepted that the presence of the species does not inidcate a priority feature at low density and/or small patch size, some debate remains as to what criteria should be met to define the presence of a significant feature; a suggested criteria appropriate to the survey methodology was defined for this study.

For the 20 sites surveyed in detail the predicted species beds, based on or derived from historical data, were not found to be representative of the current distribution of priority features. As a result the corresponding closed areas originally established by the SSMO were either not fully encompassing the UK BAP habitats located at the sites or were not protecting any UK BAP habitats (none present). Of the 20 sites surveyed, 12 were found to have either M. modiolus, maerl, or both; nine of these contained a priority feature but only two had a closed area which completely encapsulated the full extent of the feature. Alteration or removal of the boundaries of the existing closed areas is therefore appropriate and recommendations for this process are included. Further refinement of boundary areas may be warranted after consultation and additional focussed surveys.

Although the surveys illustrate the need for validation of closed areas with high quality acoustic and visual survey data, the exercise provides a practical and successful industry-science approach to the establishment and incremental development of local spatial management plans. In the absence of detailed maps for entire sea areas, which rarely if ever exist, the use of historical data and local knowledge provides a basis for the focussing of survey resources. The use of acoustic surveying technology, such as multibeam, proved highly cost effective in mapping priority habitats. Multibeam surveys cover large areas of seabed relatively quickly, producing good quality maps to a high degree of accuracy, and as with this example, this is ideally suited to management plans based on ‘physical’ features (biogenic reefs) rather than biotopes. However, broad scale mapping of this type may inform the likely presence of biotopes, which can be confirmed by more cost and labour intensive methods if required.

Although the science-industry partners acknowledge that the methodology of using historical data and local knowledge may not result in absolute protection of priority features during the first iteration, as is the case here, it was deemed preferable to establish a locally agreed framework, dialogue, and options for further refinement toward this goal than to leave the conservation of priority features unaddressed or awaiting investment in large-scale, holistic marine surveys. Direct stakeholder involvement in the process installed belief that clear and unambiguous science would rectify issues at a later date, and therefore fishermen agreed to voluntary closures of these areas before they were made statutory. As the Scottish Government currently empowers the SSMO to make changes to the spatial management plan for shellfisheries in the waters around Shetland by means of a Regulating Order, further surveys and consultation will increment toward more fine-scale improvements to the appropriateness of the plan well into the future.