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

1. Introduction

1.1 Biodiversity of the Small Isles Marine Protected Area

The Small Isles Marine Protected Area (SMI MPA; Fig. 1) is one of Scotland's biggest inshore MPAs, covering more than 800 km² and containing a complex mosaic of habitats. The SMI MPA includes the Sound of Canna, a submerged valley with depths exceeding 250 m. The seabed habitats within the SMI MPA are some of the most diverse in Scotland and support a wide range of seafloor (benthic) species that have been identified nationally and internationally as needing protection from damaging activities.

Some of the habitats and species found in the SMI MPA are categorised as Priority Marine Features (PMFs). The list of PMFs developed by NatureScot, formerly Scottish Natural Heritage, and the Joint Nature Conservation Committee (JNCC) is intended to inform marine conservation in Scotland (Tyler-Walters et al., 2016; NatureScot, 2020b). During the SMI MPA surveys summarised in this report, four shellfish and other invertebrate species on the PMF list were encountered alongside multiple biotopes or species components from three intertidal and continental shelf habitats: burrowed mud, horse mussel beds and northern sea fan and sponge communities (NatureScot, 2020b; Table 1). Additional PMF species and habitat components were encountered during subsequent coastal surveys in shallower water as described by O'Dell et al. (2021).

Many, but not all, of the PMF species or habitat components occurring in the SMI MPA are listed as protected features for the MPA (see section 1.2) and the majority also occur on related conservation lists or have legal protection. Atrina fragilis, Arachnanthus sarsi, Funiculina quadrangularis, Pachycerianthus multiplicatus and Swiftia pallida are all included in the Scottish Biodiversity List, with 'conservation action needed' for all but A. fragilis; this species is listed as 'avoid negative impacts' (NatureScot 2020c). Atrina fragilis is afforded legal protection under the Wildlife and Countryside Act 1981 and is included on the UKBAP list. F. quadrangularis, P. multiplicatus and S. pallida are also included on the UKBAP list and have each seen declines in Scotland greater than 25%^[1]. Arachnanthus sarsi is considered a rare species in Scottish Waters^[2]. Modiolus modiolus beds as a habitat is included on the Scottish Biodiversity List, where conservation action is needed, and negative impacts should be avoided; M. modiolus beds are also included on the UKBAP list. Both M. modiolus beds and Seapen and burrowing megafauna communities are considered threatened and declining habitats by OSPAR (OSPAR Commission, 2008).

Table 1

Priority Marine Feature (PMF - NatureScot, 2020b) species and habitat components found within the Small Isles MPA.

PMF type: Shellfish and other invertebrate species

PMF species or habitat:

• Arachnanthus sarsi (burrowing cerianthid anemone)
• Atrina fragilis (Fan mussel)
• Leptometra celtica ­­(Northern feather star)
• Parazoanthus anguicomus (White cluster anemone)

PMF type: Intertidal and continental shelf habitats

PMF species or habitat:

• Burrowed mud
• Horse mussel (Modiolus modiolus) beds
• Northern sea fan (Swiftia pallida) and sponge communities

The Sound of Canna within the SMI MPA contains one of the largest aggregations of A. fragilis in the UK (Howson et al., 2012; Stirling et al., 2016, 2018) and the deepest M. modiolus bed in Scotland, at 120 – 180 m water depth (Moore and Roberts, 2011). The rocky sides and reefs of the Sound of Canna also support PMF species or habitat components, such as S. pallida, L. celtica and P. anguicomus. In other areas of the SMI MPA, expanses of flat seabed are covered by mixed muddy sediments, providing habitat for seapens (Pennatula phosphorea, Virgularia mirabilis and F. quadrangularis: O'Dell et al., 2021) and burrowing megafauna, such as Nephrops norvegicus. These muds are heavily bioturbated by burrowing megafauna, with burrows and mounds typically forming a prominent feature of the sediment surface. The bioturbation of the muddy sediment by burrowing megafauna increases structural complexity and oxygen penetration, enhancing the survival of other species and increasing biodiversity in what would otherwise be a low diversity habitat (Widdicombe et al., 2004). The burrows also provide a source of refuge for smaller invertebrates and fish (Hughes, 1998).

1.2 Marine protection in the SMI MPA

The SMI MPA was designated by Scottish Ministers on 7^th August 2014 (Scottish Government, 2014). The Conservation Objectives for the MPA are:

"5. – (1) The conservation objectives of the Small Isles MPA are that the protected features –

a) so far as already in favourable condition, remain in such condition; and

b) so far as not already in favourable condition, be brought into such condition, and remain in such condition.

(2) In paragraph (1) "favourable condition" with respect to a marine habitat, means that –

a) its extent is stable or increasing; and

b) its structures and functions, its quality, and the composition of its characteristic biological communities are such as to ensure that it is in a condition which is healthy and not deteriorating.

(3) In paragraph (2)(b) the reference to the composition of the characteristic biological communities of a marine habitat includes a reference to the diversity and abundance of species forming part of, or inhabiting, that habitat."

The protected features of the SMI MPA include many of the PMFs mentioned in section 1.1, alongside additional features (Table 2). It is important to note that some PMF shellfish and other invertebrate species, such as the Fan mussel and Northern feather star, are only considered to be protected features within the SMI MPA where they form aggregations, and for the latter, only on mixed substrata. Horse mussels are only considered to be protected features where they form beds. White cluster anemones are protected features regardless of density or substrata. Specific guidance is available for determining what constitutes an 'aggregation' for Fan mussels and the Northern feather star, and for what constitutes a 'bed' for Horse mussels (NatureScot, 2018a,b,c).

Despite the SMI MPA being designated in 2014, the fisheries management measures have not yet been finalised (as of January 2023) (Fig. 1).

Table 2

Protected features for the Small Isles MPA, taken from the Small Isles Marine Protected Area Order 2014 (Scottish Government, 2014), Schedule 2, Article 4. The terminology for feature type in this table reflects the PMF listing provided by Tyler-Walters et al. (2016).

Protected feature: Black guillemot

Type of feature: Mobile species

Protected feature: Burrowed mud

Type of feature: Habitat

Protected feature: Circalittoral sand and mud communities

Type of feature: Habitat

Protected feature: Fan mussel aggregations

Type of feature: Habitat

Protected feature: Horse mussel beds

Type of feature: Habitat

Protected feature: Northern feather star aggregations on mixed substrata

Type of feature: Low or limited mobility species

Protected feature: Northern sea fan and sponge communities

Type of feature: Habitat

Protected feature: Shelf deeps

Type of feature: Large-scale feature

Protected feature: White cluster anemones

Type of feature: Low or limited mobility species

Protected feature: Quaternary of Scotland – glaciated channels/troughs, glacial lineations, meltwater channels, moraines and streamlined bedforms

Type of feature: Geomorphological

1.3 Pressures arising from human activity in the SMI MPA

Two of the most extensive activities occurring in the SMI MPA are trawling by the Nephrops fishery and, with a smaller footprint, scallop dredging. Within the SMI MPA burrowed mud habitats coincide with high levels of demersal trawling, with scallop dredging targeting harder substrates nearer to the shoreline. Some of the PMFs that occur within the SMI MPA are known to be sensitive to pressures related to fishing activities. For example, in terms of classification, A. fragilis aggregations have high sensitivity to the removal of non-target species and subsurface abrasion or penetration, and medium sensitivity to surface abrasion, whilst Northern featherstar aggregations on mixed substrata have medium sensitivity to both removal of non-target species and surface abrasion (FeAST, 2013). Some PMF species or habitat components within the SMI MPA that are known to be sensitive to abrasion pressure (Tillin et al., 2010) lie outside the footprint of bottom-contacting towed gear (Marine Scotland vessel monitoring data). However, where overlaps occur, the presence of such fishing activity has the potential to affect the distribution of PMF species and habitats (Stirling et al., 2016).

In addition to fishing activity, there is an active seawater finfish aquaculture site northeast of Rum. There are also dredge spoil disposal sites in the SMI MPA, located within complex topography within the Sound of Canna (Fig. 2). This area has not been exposed to high levels of activity from bottom-contacting towed gear historically (Howson et al., 2012) with the dredge spoil disposal site acting as a localised de facto refugium from the action of bottom-contacting towed gear (Shepard et al., 2012; Stirling et al., 2016).

The OSPAR Commission (OSPAR Commission, 2022) has also formally agreed a list of pressures occurring within the North East Atlantic marine environment and their associated definitions. The list, developed by the Intersessional Correspondence Group on Cumulative Effects (ICG-EcoC), includes descriptions of pressures that are linked to certain fishing activities, such as penetration and/or disturbance of the substrate below the surface of the seabed, including abrasion, and the removal of target or non-target species. In the regional context of the SMI MPA, the Scottish Marine Assessment 2020 (Moffat et al., 2020) identifies the removal of target and non-target species and surface and sub-surface abrasion amongst its list of top pressures occurring within the West Highlands Scottish Marine Region (SMR) where the SMI MPA is located. Whilst occurrence of such pressures may not always result in an impact of concern, the Scottish Marine Assessment 2020 concluded that fishing, (including bottom-contacting and pelagic fishing), is the dominant pressure-causing activity across the Scottish Marine Regions due to the size of the geographical footprint of fishing and the nature of the activity.

1.4 Biodiversity status and pressures reporting for the SMI MPA

The aims of this report on the biodiversity status and pressures related to fishing activity in the SMI MPA are twofold:

1. To provide extensive baseline data from the seabed within and around the SMI MPA, with specific focus on benthic PMF species or components of PMF habitats occurring in the study area (see section 2.1).

2. To assess the impact that bottom-contacting towed gear may have on the distribution of PMF species, habitats, or habitat components using information on fishing vessel activity, environmental variables, and the abundance of the Tall seapen (Funiculina quadrangularis) in the wider area of the Minch, Inner Sound and Sea of Hebrides (see section 2.2).

To support monitoring efforts, baseline benthic data were obtained through surveys conducted from the MRV Alba na Mara in and around the SMI MPA over a six-year period (2012 – 2017). The primary objective of these surveys was to establish the "before" time point of a BACI study (Before, After, Control, Impact: Green, 1979) that would have the ability to assess the effectiveness of fisheries management measures within the MPA. BACI studies associated with assessing the impact of fishing within MPAs are intended to detect changes over time by comparing the "before" and "after" status for a "control" area that would remain fished and an "impact" area where fisheries management measures effect a reduction in anthropogenic pressure. Monitoring long-term change and attributing that change to natural or anthropogenic sources is a key aim of such BACI surveys. Baseline and monitoring surveys need to be carefully designed to effectively determine if changes in density or distributions of species or habitats within MPAs result from the removal of fishing or are attributable to other coincidental factors.

As part of the baseline surveys conducted, high-definition video (HDV) and high-definition digital still images (DSI) were recorded outside the SMI MPA boundary, and inside and outside the areas where fisheries management measures have been proposed within the SMI MPA (Fig. 1). The surveys described in this report were conducted over six years, affording the opportunity to obtain extended "before" data.

Since measures are yet to be implemented for the SMI MPA, the surveys from all six years covered the "before" aspect of the BACI design. Thus, this report does not test for the impact of measures on SMI MPA target species or features, as was originally intended. Instead, the report provides an extensive description of the distribution and density of target species and features within and close to the SMI MPA, both over space and time.

Assessing the impact of fishing on F. quadrangularis was considered a priority for this report, in part because the species is of conservation importance within the Greater North Sea and Celtic Sea (OSPAR areas II and III, respectively). The associated "seapen and burrowing megafauna communities" habitat is of key conservation importance as a defined habitat under Annex V of the 1992 Oslo Paris (OSPAR) Convention (OSPAR Commission, 1992, 2010). In addition, existing seapen Vulnerable Marine Ecosystem (VME) closed areas within the Northwest Atlantic Fisheries Organisation (NAFO) regulatory area were assessed as having a high overall risk of significant adverse impacts from bottom fishing activities (NAFO, 2016).

Funiculina quadrangularis is not listed as a "PMF shellfish and other invertebrate species", however it is a component of the 'Tall seapen' biotope within 'burrowed mud PMF habitat', listed under intertidal and continental shelf habitats (NatureScot, 2020b). The burrowed mud PMF habitat (NatureScot, 2020b) broadly aligns with the SMI MPA protected feature of 'Burrowed mud' habitat (Scottish Government, 2014). Thus, according to the Small Isles Marine Protected Area Order 2014, F. quadrangularis could be considered a species component of the SMI MPA burrowed mud habitat protected feature (see section 1.2).

Funiculina quadrangularis is a colonial cnidarian adapted for life on soft muddy or sandy sediments (Manuel, 1988; Hayward and Ryland, 1990). Individual colonies can exceed 200 cm in length, with approximately one-quarter of their structure buried and the rest protruding above the sediment (Ager, 2003). They are found across a wide range of depths, from 20 to more than 200 m, with their European distribution being restricted to the deep muddy areas around the northwest coasts of Ireland and Scotland (Greathead et al., 2007), Norwegian fjords (Rosenberg et al., 1996), and the continental slopes off northern Spain (Serrano et al., 2006). Given their sedentary lifestyle and upright position in the sediment, physical abrasion pressure from demersal fishing activities may have a modifying effect on seapen distribution (Malecha and Stone, 2009; Tyler-Walters et al., 2009), with the action of bottom-contacting towed gear causing in-situ mortality or bringing about their removal, forming unwanted bycatch.

Understanding the environmental preferences of F. quadrangularis and how fishing activities may impact it is an important step towards designing fisheries management measures that can provide effective protection for this species and its associated habitat. Previous work indicates that F. quadrangularis exhibits strong environmental preferences, allowing their distribution to be modelled successfully within a species distribution model (SDM) framework (Greathead et al., 2015; Downie et al., 2021). Correlational methods can also offer powerful prognostic tools to assess the effects of various drivers of ecosystem change, such as the abrasion pressure caused by bottom-contacting towed fishing gears. Thus, in this report we utilised DSI data collected during 2017 surveys, Vessel Monitoring System (VMS) fisheries data, and environmental information in a modelling approach to determine the impact of fishing on F. quadrangularis distribution.