Marine biological survey to establish the distribution and status of ...

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COMMISSIONED REPORT Summary Marine biological survey to establish the distribution and status of fan mussels Atrina fragilis and other Marine Protected Area (MPA) search features within the Sound of Canna, Inner Hebrides Commissioned Report No. 438 (iBids no. 10769) Contractor: Aquatic Survey & Monitoring Ltd. Year of publication: 2012 Background Scotland’s seas are of outstanding scenic, historic and cultural value and are part of the national identity at home and abroad. The Marine (Scotland) Act 2010 and the UK Marine and Coastal Access Act 2009 include new powers and duties to ensure that our seas are managed sustainably for future generations, integrating the economic growth of marine industries with the need to protect these assets. Measures to conserve our marine natural heritage will be based on a three pillar approach, with action at the wider seas level (e.g. marine planning or sectoral controls); specific species conservation measures (e.g. improved protection for seals); and through site protection measures - the identification of new Marine Protected Areas (MPAs). To help target action under each of the three pillars, Scottish Natural Heritage (SNH) and the Joint Nature Conservation Committee (JNCC) have generated a focused list of habitats and species of marine conservation importance for which it would be appropriate to use both area based and non-area based mechanisms to achieve better protection - the Priority Marine Features (PMFs). A subset of these, together with the black guillemot and a number of functionally significant large-scale features will drive the selection of Nature Conservation MPAs (MPA search features). The Sound of Canna in the Small Isles has, until recently, attracted relatively little attention from marine biologists. The discovery in 2009 of a population of the rare and fragile fan mussel Atrina fragilis, an MPA search feature, during monitoring studies of a dredge spoil ground has focused attention on the diversity of this small area. The purpose of the current study was to undertake a detailed survey of seabed MPA search features within the sound, with particular emphasis on the population of Atrina fragilis. The survey programme was designed to generate sufficient information on the distribution, quality and extent of the MPA search features present (and other PMFs) to enable a preliminary assessment of this area against the Scottish MPA Selection Guidelines. i
The Sound of Canna is a steep-sided channel over-deepened by glaciation that separates the islands of Canna and Rum. It is just over 3 km wide at its narrowest point and has a deep channel running along its centre which reaches a maximum depth of 275 m. A gravel shelf at about 25 m depth extends along the whole of the north-west coast of Rum. This possible drowned wave-cut platform gives way to a steep sediment slope down to the floor of the channel which supports extensive areas of burrowed mud and mixed muddy sands and gravels. On the western side of the sound, steep sublittoral rock walls along the south and east coast of Sanday (a small island connected to Canna by a road and sandbanks at low tide) drop to a sediment floor in over 60 m of water. Glacial moraines are a significant feature of the sound with small rock walls and boulder piles scattered throughout the deep channel. Along the coastal margins there are also sheltered mud and sand habitats. Survey work during 2010 involved a drop-down video survey to capture footage of the seabed habitats (121 stations); detailed in-situ, diver observations, which included mapping the extent of seagrass beds and the collection of infaunal core samples (15 stations); and grab sampling (23 stations) within benthic sediment communities. The primary objective of the work was to provide an assessment of the current status of the fan mussels. The survey plans were developed from previous sampling records, Admiralty charts, and predictive habitat distribution maps covering this area (Foster-Smith, 2010). Main Findings • The Sound of Canna is a small but unusually biologically diverse area supporting ten MPA search features and two additional PMFs (the white cluster anemone Parazoanthus anguicomus and the burrowing sea anemone Arachnanthus sarsi). • The aggregation of fan mussels Atrina fragilis was of particular significance, covering an area of at least 170 ha, representing the largest known area of fan mussels in UK waters. The northern extremity of the aggregation was not found in 2010. The underwater video footage showed that the fan mussels occurred as scattered individuals or in small clumps. Diverse assemblages of other species were observed living on, or in close association with these large semi-buried bivalves. • A preliminary assessment of the relative nature conservation importance of the 2010 survey area concluded that the Sound of Canna could make an important contribution to the evolving Scottish MPA network. Four of the MPA search features present were considered to be of sufficient quality to have a particular bearing on the outcomes of the assessment, namely: fan mussel aggregations; burrowed mud; northern sea fan and sponge communities; and, northern feather star aggregations on mixed substrata. • The report recommends that further work be carried out to clarify the full extent of the fan mussel aggregation, and the maerl bed at Guirdil Bay on Rum; and also, to ascertain the wider distribution of northern sea fan and sponge communities within the sound. • Survey work to describe the seabed habitats present within areas used regularly by basking sharks to the west and south-west of Canna (Speedie et al., 2009) is also recommended. For further information on this project contact: Laura Clark, Scottish Natural Heritage, Great Glen House, Inverness, IV3 8NW Tel: 01463 725237 For further information on the SNH Research & Technical Support Programme contact: DSU (Policy and Advice Directorate), Scottish Natural Heritage, Great Glen House, Inverness, IV3 8NW Tel: 01463 725000 or pads@snh.gov.uk ii
Page 1 and 2: Scottish Natural Heritage Commissio
Page 6 and 7: Contents Page no. 1 Introduction ..
Page 8 and 9: Page no. Figure 28 Basking shark ho
Page 10 and 11: 1 INTRODUCTION 1.1 Background The S
Page 12 and 13: Rum has a far more complex structur
Page 14 and 15: In 1993 an SNH team conducted remot
Page 16 and 17: Richardson et al. (2004) showed tha
Page 18 and 19: 2 METHODS 2.1 Sampling strategy The
Page 20 and 21: Figure 6 Shallow water video statio
Page 22 and 23: Figure 8 Video stations surveyed in
Page 24 and 25: Each discrete grab sample was empti
Page 26 and 27: Following the survey, the infauna w
Page 28 and 29: Table 2 Biotopes identified in the
Page 30 and 31: Figure 11 2010 survey coverage off
Page 32 and 33: Circalittoral sandy mud (SS.SMu.CSa
Page 34 and 35: Description The distribution of the
Page 36 and 37: Figure 16 Division of the megafauna
Page 38 and 39: Some L. saccharina forest (IR.LIR.K
Page 40 and 41: Rhodophyllis werneri and Scinaia in
Page 42 and 43: Plate 8 A northern sea fan Swiftia
Page 44 and 45: Zostera marina/angustifolia beds on
Page 46 and 47: This is consistent with infaunal co
Page 48 and 49: the fan mussels ranged from an esti
Page 50 and 51: Plate 13 A small basking shark in C
Page 52 and 53: Distribution Depth range: 72 - 21
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Figure 31 Other species of interest
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coverage predictive biotope distrib
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4 DISCUSSION This section presents
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Table 4 Summary of the assessment o
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The burrowed mud supported langoust
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in areas where it is protected from
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Table 6 Summary of a preliminary as
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5.2 Future data review and biologic
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Ingle, R.W. (1996). Shallow-water c
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Appendix 1 Drop-down video and dive
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Date Station Time Latitude Longitud
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Table A1.2 Summary notes (biologica
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Tow Id no Tow Data point Date Tape
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Tow Id no Tow Data point Date A123
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Table A1.3 Summary of sampling unde
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Site No. Date Habitat Id (Site No:H
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Species MCS Code D D 1 _ 2 D D 1 _
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Species MCS Code Suberites ficus C2
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Species MCS Code Cryptopleura ramos
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Species MCS Code Scrupocellaria Y83
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Table A1.6 Species recorded (matrix
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Species MCS Code A 1 2 _ 1 A 1 2 _
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Table A1.7 Species recorded (matrix
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Species MCS Code Dive 1.1 Dive 2.1
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Table A2.1 Positional and depth inf
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Table A2.3 List of taxa identified
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Taxa MCS Code G 01 G 02 G 03 G 04 G
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Taxa MCS Code G 01 G 02 G 03 Diasty
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Taxa MCS Code G 01 Ascidiella sp. Z
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Figure A2.1 Bray-Curtis similarity
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Appendix 3 Macrofaunal and sediment
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Table A3.3 Results of univariate an
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Appendix 4 Photo and video logs - 2
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File Name Subject Site Name Island
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Table A4.2 Drop-down video log, 201
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File Name File size Folder Site No
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File Name File size Folder Site No
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